Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2018	Apr. 30, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	Blueprint Medicines Corp
Entity Central Index Key	0001597264
Document Type	10-Q
Document Period End Date	Mar. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		43,837,337
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 134,886	$ 400,304
Investments, available-for-sale	472,439	273,052
Prepaid expenses and other current assets	8,373	12,149
Total current assets	615,698	685,505
Investments, available-for-sale	13,798
Property and equipment, net	29,074	24,363
Restricted cash	4,557	4,555
Other assets	1,341	1,314
Total assets	664,468	715,737
Current liabilities:
Accounts payable	3,288	3,744
Accrued expenses	29,087	30,541
Current portion of deferred revenue	4,803	5,373
Current portion of lease incentive obligation	1,714	1,714
Current portion of term loan payable	1,106	1,518
Total current liabilities	39,998	42,890
Deferred rent, net of current portion	4,389	4,129
Deferred revenue, net of current portion	34,931	30,000
Lease incentive obligation, net of current portion	14,189	14,617
Other long term liabilities	88	131
Commitments (Note 10)
Stockholders’ equity:
Preferred stock, $0.001 par value; 5,000,000 shares authorized; no shares issued and outstanding
Common stock, $0.001 par value; 120,000,000 shares authorized; 43,821,247 and 43,577,526 shares issued at March 31, 2018 and December 31, 2017, respectively; 43,821,247 and 43,577,526 shares outstanding at March 31, 2018 and December 31, 2017, respectively	44	43
Additional paid-in capital	988,872	979,785
Accumulated other comprehensive loss	(591)	(269)
Accumulated deficit	(417,452)	(355,589)
Total stockholders’ equity	570,873	623,970
Total liabilities and stockholders’ equity	$ 664,468	$ 715,737

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2018	Dec. 31, 2017
Preferred Stock Disclosures
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	5,000,000	5,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common Stock Disclosures
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	120,000,000	120,000,000
Common stock, shares issued (in shares)	43,821,247	43,577,526
Common Stock, shares outstanding (in shares)	43,821,247	43,577,526

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenues
Collaboration revenue	$ 954	$ 5,840
Operating expenses:
Research and development	49,954	28,487
General and administrative	9,911	5,683
Total operating expenses	59,865	34,170
Other income (expense):
Other income, net	2,394	425
Interest expense	(32)	(72)
Total other income (expense)	2,362	353
Net loss	(56,549)	(27,977)
Other comprehensive loss:
Unrealized loss on investments	(323)	(114)
Comprehensive loss	$ (56,872)	$ (28,091)
Net loss per share applicable to common stockholders - basic and diluted (in dollars per share)	$ (1.29)	$ (0.84)
Weighted-average number of common shares used in net loss per share applicable to common stockholders — basic and diluted	43,700	33,190

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating activities
Net loss	$ (56,549)	$ (27,977)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	780	398
Noncash interest expense	4	11
Stock-based compensation	5,549	2,240
Accretion of premiums and discounts on investments	(692)	86
Changes in assets and liabilities:
Unbilled accounts receivable		751
Prepaid expenses and other current assets	3,473	(1,972)
Other assets	(27)	14
Accounts payable	(482)	71
Accrued expenses	(525)	(1,552)
Deferred revenue	(954)	(3,015)
Deferred rent	(140)	(135)
Net cash used in operating activities	(49,563)	(31,080)
Investing activities
Purchases of property and equipment	(6,312)	(210)
Purchases of investments	(338,816)	(18,003)
Maturities of investments	126,000	54,000
Net cash (used in) provided by investing activities	(219,128)	35,787
Financing activities
Principal payments on loan payable	(417)	(833)
Payment of offering costs	(164)	(489)
Proceeds from issuance of common stock, net of repurchases	3,643	902
Net cash provided by (used in) financing activities	3,062	(420)
Net (decrease) increase in cash, cash equivalents, and restricted cash	(265,629)	4,287
Cash, cash equivalents, and restricted cash at beginning of period	405,072	53,336
Cash, cash equivalents, and restricted cash at end of period	139,443	57,623
Supplemental cash flow information
Public offering costs incurred but unpaid at period end	166	516
Property and equipment purchases unpaid at period end	3,126	80
Cash paid for interest	$ 18	$ 50

Condensed Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Reconciliation of cash, cash equivalents, and restricted cash
Cash and cash equivalents	$ 134,886	$ 400,304	$ 56,356
Restricted cash	4,557		1,267
Total cash, cash equivalents, and restricted cash shown in condensed consolidated statements of cash flows	$ 139,443	$ 405,072	$ 57,623	$ 53,336

Nature of Business	3 Months Ended
Nature of Business	Mar. 31, 2018
Disclosure Text Block
Nature of Business	1. Nature of Business Blueprint Medicines Corporation (the Company), a Delaware corporation incorporated on October 14, 2008, is a biopharmaceutical company focused on developing potentially transformational medicines to improve the lives of patients with genomically defined cancers and rare diseases. The Company’s approach is to leverage its novel target discovery engine to systematically and reproducibly identify kinases that are drivers of diseases in genomically defined patient populations and to craft highly selective and potent drug candidates that may provide significant and durable clinical responses for patients without adequate treatment options. The Company is devoting substantially all of its efforts to research and development, conducting pre-clinical and clinical development, commencing pre-commercial activities and raising capital. The Company is subject to a number of risks similar to those of other early stage companies, including dependence on key individuals; establishing safety and efficacy in clinical trials for its drug candidates; the need to develop commercially viable drug candidates; competition from other companies, many of which are larger and better capitalized; and the need to obtain adequate additional financing to fund the development of its drug candidates. If the Company is unable to raise capital when needed or on attractive terms, it would be forced to delay, reduce, eliminate or out‑license certain of its research and development programs or future commercialization efforts. On May 5, 2015, the Company completed an initial public offering (IPO) of its common stock, which resulted in the sale of 9,367,708 shares of its common stock at a price to the public of $18.00 per share, including 1,221,874 shares of common stock sold by the Company pursuant to the exercise in full by the underwriters of their option to purchase additional shares in connection with the offering. The Company received net proceeds of $154.8 million, after deducting underwriting discounts and commissions and offering expenses paid by the Company. On December 13, 2016, the Company closed a follow-on public offering of 5,750,000 shares of its common stock at a price to the public of $25.00 per share, including 750,000 shares of common stock sold by the Company pursuant to the exercise in full by the underwriters of their option to purchase additional shares in connection with the offering. The Company received net proceeds of $134.5 million, after deducting underwriting discounts and commissions and offering expenses payable by the Company. On April 4, 2017, the Company closed a follow-on public offering of 5,750,000 shares of its common stock at a price to the public of $40.00 per share, including 750,000 shares of common stock sold by the Company pursuant to the exercise in full by the underwriters of their option to purchase additional shares in connection with the offering (the April 2017 follow-on public offering). The Company received net proceeds of $215.6 million, after deducting underwriting discounts and commissions and offering expenses payable by the Company. On December 15, 2017, the Company closed its underwritten public offering of 4,259,259 shares of its common stock at a price to the public of $81.00 per share, including 555,555 shares of common stock sold by the Company pursuant to the exercise in full by the underwriters of their option to purchase additional shares in connection with the offering. The Company received net proceeds of approximately $325.7 million, after deducting underwriting discounts and commissions and offering expenses payable by the Company. As of March 31, 2018, the Company had cash, cash equivalents and investments of $621.1 million. Based on the Company’s current plans, the Company believes its existing cash, cash equivalents and investments, excluding any potential option fees and milestone payments under its existing collaboration with F. Hoffmann-La Roche Ltd and Hoffmann-La Roche Inc. (collectively, Roche), will be sufficient to enable it to fund its operating expenses and capital expenditure requirements into the middle of 2020.

Summary of Significant Accounting Policies and Recent Accounting Pronouncements	3 Months Ended
	Mar. 31, 2018
Disclosure Text Block
Summary of Significant Accounting Policies and Recent Accounting Pronouncements	2. Summary of Significant Accounting Policies and Recent Accounting Pronouncements Basis of Presentation The unaudited interim condensed consolidated financial statements of the Company included herein have been prepared in accordance with accounting principles generally accepted in the United States (GAAP) as found in the Accounting Standards Codification (ASC), Accounting Standards Update (ASU) of the Financial Accounting Standards Board (FASB) and the rules and regulations of the Securities and Exchange Commission (SEC). Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these financial statements should be read in conjunction with the financial statements as of and for the year ended December 31, 2017 and notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017, filed with the SEC on February 21, 2018. The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited financial statements, except as noted below with respect to the adoption of Accounting Standards Codification (ASC) Topic 606, Revenue from Contracts with Customers (ASC 606), and include the accounts of the Company and its wholly-owned subsidiary, Blueprint Medicines Security Corporation, which is a Massachusetts subsidiary created to buy, sell, and hold securities. All intercompany transactions and balances have been eliminated. In the opinion of the Company’s management, the accompanying unaudited interim condensed consolidated financial statements contain all adjustments which are necessary to present fairly the Company’s financial position as of March 31, 2018 and the results of its operations and cash flows for the three months ended March 31, 2018 and 2017. Such adjustments are of a normal and recurring nature. The results for the three months ended March 31, 2018 are not necessarily indicative of the results for the year ending December 31, 2018, or for any future period. Due to the underwritten public offerings completed on April 4, 2017, and December 15, 2017, there was a significant increase in shares outstanding in the year ended December 31, 2017, which impacts the period-over-period comparability of the Company’s net loss per share calculations. Use of Estimates The preparation of financial statements in conformity with GAAP requires the Company’s management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results could differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and in developing the estimates and assumptions that are used in the preparation of the financial statements. Management must apply significant judgment in this process. Management’s estimation process often may yield a range of potentially reasonable estimates and management must select an amount that falls within that range of reasonable estimates. Estimates are used in the following areas, among others: stock‑based compensation expense; revenue recognition; accrued expenses; and income taxes. Significant Accounting Policies The Company’s critical accounting policies are those policies that require the most significant judgments and estimates in the preparation of our financial statements. Management has determined that the Company’s most critical accounting policies are those relating to revenue recognition, accrued research and development expenses, available-for-sale investments and stock-based compensation. Revenue Recognition Effective January 1, 2018, the Company adopted ASC 606, using the modified retrospective transition method. Under this method, results for reporting periods beginning after January 1, 2018 are presented under ASC 606, while prior period amounts are not adjusted and continue to be reported in accordance with ASC Topic 605, Revenue Recognition (ASC 605). The Company only applied the modified retrospective transition method to contracts that were not completed as of January 1, 2018, the effective date of adoption for ASC 606. This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance, collaboration arrangements and financial instruments. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the entity performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. The Company enters into licensing agreements that are within the scope of ASC 606, under which it may exclusively license rights to research, develop, manufacture and commercialize its product candidates to third parties. The terms of these arrangements typically include payment to the Company of one or more of the following: non-refundable, upfront license fees; reimbursement of certain costs; customer option exercise fees; development, regulatory and commercial milestone payments; and royalties on net sales of licensed products. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under its agreements, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. As part of the accounting for these arrangements, the Company must use significant judgment to determine: (a) the number of performance obligations based on the determination under step (ii) above; (b) the transaction price under step (iii) above; and (c) the stand-alone selling price for each performance obligation identified in the contract for the allocation of transaction price in step (iv) above. The Company uses judgment to determine whether milestones or other variable consideration, except for royalties, should be included in the transaction price as described further below. The transaction price is allocated to each performance obligation on a relative stand-alone selling price basis, for which the Company recognizes revenue as or when the performance obligations under the contract are satisfied. Amounts received prior to revenue recognition are recorded as deferred revenue. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as current portion of deferred revenue in the accompanying consolidated balance sheets. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion. Exclusive Licenses. If the license to the Company’s intellectual property is determined to be distinct from the other promises or performance obligations identified in the arrangement, the Company recognizes revenue from non-refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. In assessing whether a promise or performance obligation is distinct from the other promises, the Company considers factors such as the research, development, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can benefit from a promise for its intended purpose without the receipt of the remaining promise, whether the value of the promise is dependent on the unsatisfied promise, whether there are other vendors that could provide the remaining promise, and whether it is separately identifiable from the remaining promise. For licenses that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The measure of progress, and thereby periods over which revenue should be recognized, are subject to estimates by management and may change over the course of the research and development and licensing agreement. Such a change could have a material impact on the amount of revenue the Company records in future periods. Research and Development Services. The promises under the Company’s collaboration agreements may include research and development services to be performed by the Company on behalf of the partner. Payments or reimbursements resulting from the Company’s research and development efforts are recognized as the services are performed and presented on a gross basis because the Company is the principal for such efforts. Reimbursements from and payments to the partner that are the result of a collaborative relationship with the partner, instead of a customer relationship, such as co-development activities, are recorded as a reduction to research and development expense. Customer Options. If an arrangement is determined to contain customer options that allow the customer to acquire additional goods or services, the goods and services underlying the customer options that are not determined to be material rights are not considered to be performance obligations at the outset of the arrangement, as they are contingent upon option exercise. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. The Company allocates the transaction price to material rights based on the relative standalone selling price, which is determined based on the identified discount and the probability that the customer will exercise the option. Amounts allocated to a material right are not recognized as revenue until, at the earliest, the option is exercised. Milestone Payments. At the inception of each arrangement that includes development milestone payments, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant revenue reversal would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. Royalties. For arrangements that include sales-based royalties, including milestone payments based on a level of sales, which are the result of a customer-vendor relationship and for which the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied or partially satisfied. To date, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements. For a complete discussion of accounting for collaboration revenues, see Note 6, “Collaborations.” There have been no other significant changes to the Company’s critical accounting policies discussed in its Annual Report on Form 10-K for the year ended December 31, 2017 related to accrued research and development expenses, available-for-sale investments and stock-based compensation. Recent Accounting Pronouncements Recently Adopted In May 2014, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2014-09, which amends the guidance for accounting for revenue from contracts with customers. ASU No. 2014-09 superseded the revenue recognition requirements in ASC 605 and created ASC 606 described above. In 2015 and 2016, the FASB issued additional ASUs related to ASC 606 that delayed the effective date of the guidance and clarified various aspects of the new revenue guidance, including principal versus agent considerations, identifying performance obligations, and licensing, and they include other improvements and practical expedients. Effective January 1, 2018, the Company adopted ASC 606 using the modified retrospective transition method. As a result of adopting ASC 606, the Company has recorded a cumulative-effect increase to opening accumulated deficit of $5.3 million as of January 1, 2018 and a corresponding increase to deferred revenue. Total revenue recorded in the three months ended March 31, 2018 under ASC 606 was $0.9 million, as compared to $1.3 million that would have been recorded under ASC 605. Total deferred revenue as of March 31, 2018 was $39.7 million under ASC 606, as compared to $34.0 million that would have been recorded under ASC 605. The most significant change to the Company’s accounting for revenue as a result of the adoption of ASC 606 relates to its revenue recognition pattern under step (v) above for the Company’s collaboration and license agreement with Roche (as amended, Roche Agreement). Under ASC 605, the Company was recognizing the revenue allocated to each unit of accounting on straight‑line basis over the period the Company expected to complete its obligations. Under ASC 606, the Company is recognizing the revenue allocated to each performance obligation measuring progress using an input method over the period the Company expects to complete each performance obligation. ASC 606 also requires disclosure of the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. For further discussion of the adoption of this standard, see Note 6, “Collaborations.” In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash (ASU No. 2016-18). The amendments in ASU No. 2016-18 require an entity to reconcile and explain the period-over-period change in total cash, cash equivalents and restricted cash within its statements of cash flows. ASU No. 2016-18 was effective for the Company on January 1, 2018. The Company adopted ASU No. 2016-18 using a full retrospective approach and it did not have a significant impact on its consolidated financial statements and related disclosures. In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230) (ASU No. 2016-15), which simplifies certain elements of cash flow classification. The new guidance is intended to reduce diversity in practice in how certain transactions are classified in the statement of cash flows. ASU No. 2016-15 was effective for the Company on January 1, 2018, and it did not have a material impact on the Company’s consolidated financial statements. In 2016, the FASB issued ASU No. 2016-01 Financial Instruments (ASU No. 2016-01) related to the recording of financial assets and financial liabilities. Under the amended guidance, equity investments (except those accounted for under the equity method of accounting or those that result in consolidation of the investee) are to be measured at fair value with changes in fair value recognized in net income (loss). However, an entity has the option to either measure equity investments without readily determinable fair values either (i) at fair value or (ii) at cost adjusted for changes in observable prices minus impairment. Changes in measurement under either alternative will be recognized in net income (loss). The amended guidance became effective January 1, 2018. As the Company does not currently hold equity securities, there was no impact on the financial statements at the adoption date. The Company may hold equity securities in the future, at which time the Company will apply the provisions of ASU No. 2016-01 and record changes in the fair value of the equity securities in net income (loss). Not Yet Adopted In February 2016, the FASB issued ASU No. 2016-02, Leases (ASU No. 2016-02), which will change the way the Company recognizes its leased assets. ASU No. 2016-02 will require organizations that lease assets—referred to as “lessees”—to recognize on the balance sheet the assets and liabilities representing the rights and obligations created by those leases. ASU No. 2016-02 will also require disclosures to help investors and other financial statement users better understand the amount, timing, and uncertainty of cash flows arising from leases. The standard is effective for annual reporting periods (including interim reporting periods within those years) beginning after December 15, 2018. Early adoption is permitted. The Company is currently evaluating the methods of adoption allowed by the new standard and the effect that adoption of the standard is expected to have on the Company’s consolidated financial statements and related disclosures.

Cash Equivalents and Investments

3 Months Ended

Mar. 31, 2018

Disclosure Text Block

Cash Equivalents and Investments

3. Cash Equivalents and Investments

Cash equivalents are highly liquid investments that are readily convertible into cash with original maturities of three months or less when purchased. Investments consist of securities with original maturities greater than 90 days when purchased. The Company classifies these investments as available-for-sale and records them at fair value in the accompanying condensed consolidated balance sheets. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment.

Cash equivalents and investments, available-for-sale, consisted of the following at March 31, 2018 and December 31, 2017 (in thousands):


	Average		Amortized		Unrealized		Unrealized		Fair
March 31, 2018	Maturity		Cost		Gain		Losses		Value
Cash equivalents:
Money market funds			$	134,886	$	—	$	—	$	134,886
Investments, available-for-sale:
U.S. treasury obligations		298 Days		486,829		—		(592)		486,237
Total			$	621,715	$	—	$	(592)	$	621,123


	Average		Amortized		Unrealized		Unrealized		Fair
December 31, 2017	Maturity		Cost		Gain		Losses		Value
Cash equivalents:
Money market funds			$	400,304	$	—	$	—	$	400,304
Investments, available-for-sale:
U.S. treasury obligations		348 Days		273,321		—		(269)		273,052
Total			$	673,625	$	—	$	(269)	$	673,356

Although available to be sold to meet operating needs or otherwise, securities are generally held through maturity. The cost of securities sold is determined based on the specific identification method for purposes of recording realized gains and losses. During the three months ended March 31, 2018 and 2017, there were no realized gains or losses on sales of investments, and no investments were adjusted for other than temporary declines in fair value.

At March 31, 2018, the Company held 58 securities that were in an unrealized loss position. The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of March 31, 2018 was $456.3 million. As of March 31, 2018, there were $29.9 million securities held by the Company in an unrealized loss position for more than twelve months. The Company has the intent and ability to hold such securities until recovery. The Company determined that there was no material change in the credit risk of the above investments. As a result, the Company determined it did not hold any investments with an other-than-temporary impairment as of March 31, 2018.

Fair Value of Financial Instruments

3 Months Ended

Mar. 31, 2018

Disclosure Text Block

Fair Value of Financial Instruments

4. Fair Value of Financial Instruments

The fair value hierarchy prioritizes the inputs to valuation techniques used to measure fair value into three broad levels as follows:

Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities.

Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly.

Level 3 inputs are unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the asset or liability. Financial assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement.

Financial instruments measured at fair value as of March 31, 2018 are classified below based on the fair value hierarchy described above:


			Active		Observable		Unobservable
	March 31,		Markets		Inputs		Inputs
Description	2018		(Level 1)		(Level 2)		(Level 3)
Financial Assets
Cash equivalents:
Money market funds	$	134,886	$	134,886	$	—	$	—
Investments, available-for-sale:
U.S Treasury obligations		486,237		486,237		—		—
Total	$	621,123	$	621,123	$	—	$	—

Financial instruments measured at fair value as of December 31, 2017 are classified below based on the fair value hierarchy described above:


			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2017		(Level 1)		(Level 2)		(Level 3)
Financial Assets
Cash equivalents:
Money market funds	$	400,304	$	400,304	$	—	$	—
Investments, available-for-sale:
U.S Treasury obligations		273,052		273,052		—		—
Total	$	673,356	$	673,356	$	—	$	—

The fair value of the Company’s term loan payable is determined using current applicable rates for similar instruments as of the balance sheet date. The carrying value of the Company’s term loan payable approximates fair value because the Company’s interest rate yield approximates current market rates. The Company’s term loan payable is a Level 3 liability within the fair value hierarchy.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Collaborations	3 Months Ended
Collaborations	Mar. 31, 2018
Disclosure Text Block
Collaborations	6. Collaborations Roche In March 2016, the Company and Roche entered into the Roche agreement, which provides for the discovery, development and commercialization of up to five small molecule therapeutics targeting kinases believed to be important in cancer immunotherapy, as single products or possibly in combination with other therapeutics. The parties are currently conducting activities for up to five programs under the collaboration, including up to two collaboration programs leveraging the Company’s novel target discovery engine and proprietary compound library to select potential targets. Under the Roche agreement, Roche is granted up to five option rights to obtain an exclusive license to exploit products derived from the collaboration programs in the field of cancer immunotherapy. Such option rights are triggered upon the achievement of Phase 1 proof-of-concept. For up to three of the five collaboration programs, if Roche exercises its option, Roche will receive worldwide, exclusive commercialization rights for the licensed products. For up to two of the five collaboration programs, if Roche exercises its option, the Company will retain commercialization rights in the United States for the licensed products, and Roche will receive commercialization rights outside of the United States for the licensed products. The Company will also retain worldwide rights to any products for which Roche elects not to exercise its applicable option. Prior to Roche’s exercise of an option, the Company will have the lead responsibility for drug discovery and pre-clinical development of all collaboration programs. In addition, the Company will have the lead responsibility for the conduct of all Phase 1 clinical trials other than those Phase 1 clinical trials for any product in combination with Roche’s portfolio of therapeutics, for which Roche will have the right to lead the conduct of such Phase 1 clinical trials. Pursuant to the Roche agreement, the parties will share the costs of Phase 1 development for each collaboration program. In addition, Roche will be responsible for post-Phase 1 development costs for each licensed product for which it retains global commercialization rights, and the Company and Roche will share post-Phase 1 development costs for each licensed product for which the Company retains commercialization rights in the United States. Subject to the terms of the Roche agreement, the Company received an upfront cash payment of $45.0 million and will be eligible to receive up to approximately $965.0 million in contingent option fees and milestone payments related to specified research, pre-clinical, clinical, regulatory and sales-based milestones. Of the total contingent payments, up to approximately $215.0 million are for option fees and milestone payments for research, pre-clinical and clinical development events prior to licensing across all five potential collaboration programs, including contingent milestone payments for initiation of each of the collaboration programs for which the parties will work together to select targets (pre-option exercise milestones). In addition, for any licensed product for which Roche retains worldwide commercialization rights, the Company will be eligible to receive tiered royalties ranging from low double-digits to high-teens on future net sales of the licensed product. For any licensed product for which the Company retains commercialization rights in the United States, the Company and Roche will be eligible to receive tiered royalties ranging from mid-single-digits to low double-digits on future net sales in the other party’s respective territories in which it commercializes the licensed product. The upfront cash payment and any payments for milestones, option fees and royalties are non-refundable, non-creditable and not subject to set-off. The Roche agreement will continue until the date when no royalty or other payment obligations are or will become due, unless earlier terminated in accordance with the terms of the Roche agreement. Prior to its exercise of its first option, Roche may terminate the Roche agreement at will, in whole or on a collaboration target-by-collaboration target basis, upon 120 days’ prior written notice to the Company. Following its exercise of an option, Roche may terminate the Roche agreement at will, in whole, on a collaboration target-by-collaboration target basis, on a collaboration program-by-collaboration program basis or, if a licensed product has been commercially sold, on a country-by-country basis, (i) upon 120 days’ prior written notice if a licensed product has not been commercially sold or (ii) upon 180 days’ prior written notice if a licensed product has been commercially sold. Either party may terminate the Roche agreement for the other party’s uncured material breach or insolvency and in certain other circumstances agreed to by the parties. In certain termination circumstances, the Company is entitled to retain specified licenses to be able to continue to exploit the licensed products. The Company assessed this arrangement in accordance with ASC 606 and concluded that the contract counterparty, Roche, is a customer prior to the exercise, if any, of an option by Roche. The Company identified the following material promises under the arrangement: (1) a non-transferable, sub-licensable and non-exclusive license to use the Company’s intellectual property and collaboration compounds to conduct research activities; (2) research and development activities through Phase 1 clinical trials under the research plan; (3) five option rights for licenses to develop, manufacture, and commercialize the collaboration targets; (4) participation on a joint research committee (JRC) and joint development committee (JDC); and (5) regulatory responsibilities under Phase 1 clinical trials. The Company determined that the license and research and development activities were not distinct from another, as the license has limited value without the performance of the research and development activities. Participation on the JRC and JDC to oversee the research and development activities was determined to be quantitatively and qualitatively immaterial and therefore is excluded from performance obligations. The regulatory responsibilities related to filings and obtaining approvals related to the products that may result from each program do not represent separate performance obligations based on their dependence on the research and development efforts. As such, the Company determined that these promises should be combined into a single performance obligation. The Company evaluated the option rights for licenses to develop, manufacture, and commercialize the collaboration targets to determine whether it provides Roche with any material rights. The Company concluded that the options were not issued at a significant and incremental discount, and therefore do not provide material rights. As such, they are excluded as performance obligations at the outset of the arrangement. Based on these assessments, the Company identified one performance obligation at the outset of the Roche Agreement, which consists of: (1) the non-exclusive license; (2) the research and development activities through Phase 1; and (3) regulatory responsibilities under Phase 1 clinical trials. Under the Roche Agreement, in order to evaluate the appropriate transaction price, the Company determined that the upfront amount of $45.0 million constituted the entirety of the consideration to be included in the transaction price as of the outset of the arrangement, which was allocated to the single performance obligation. The option exercise payments that may be received are excluded from the transaction price until each customer option is exercised as it was determined that the options are not material rights. The potential milestone payments that the Company is eligible to receive prior to the exercise of the options were excluded from the transaction price, as all milestone amounts were fully constrained based on the probability of achievement. The Company will reevaluate the transaction price at the end of each reporting period and as uncertain events are resolved or other changes in circumstances occur, and, if necessary, adjust its estimate of the transaction price. Revenue associated with the performance obligation is being recognized as revenue as the research and development services are provided using an input method, according to the costs incurred as related to the research and development activities on each program and the costs expected to be incurred in the future to satisfy the performance obligation. The transfer of control occurs over this time period and, in management’s judgment, is the best measure of progress towards satisfying the performance obligation. The research and development services related to this performance obligation are expected to be performed over a period of approximately eight years. The revenue recognized during the first quarter of 2018 represents revenue that was previously deferred. The amounts received that have not yet been recognized as revenue are recorded in deferred revenue on the Company’s condensed consolidated balance sheet. Through March 31, 2018, the Company had recognized revenue of $5.3 million as collaboration revenue in the Company’s consolidated statements of operations and comprehensive loss under the Roche Agreement. Deferred revenue related to the Roche Agreement amounted to $39.7 million as of March 31, 2018, of which $4.8 million is included in current liabilities. Alexion In March 2015, the Company entered into a research, development and commercialization agreement (Alexion agreement) with Alexion to research, develop and commercialize one or more drug candidates targeting the ALK2 kinase for the treatment of fibrodysplasia ossificans progressiva (FOP). On July 26, 2017, the Company received written notice from Alexion of its election to terminate the Alexion agreement for convenience, and the termination became effective on October 24, 2017. Since the Alexion agreement terminated prior to January 1, 2018, the Company recognized revenue from the Alexion collaboration in accordance with ASC 605. Pursuant to the Alexion agreement, the Company was responsible for research and pre-clinical development activities related to any drug candidates, and Alexion was responsible for all clinical development, manufacturing and commercialization activities related to any drug candidates. In addition, Alexion was responsible for funding 100% of the Company’s research and development costs incurred under the research plan, including pass through costs and a negotiated yearly rate per full time equivalent for its employees’ time and their associated overhead expenses. As a result of the termination of the Alexion agreement, the Company will not be entitled to receive payment from Alexion for any research and development expenses incurred after the effective date of termination. Prior to termination, the Company had received an aggregate amount of $18.8 million in upfront and milestone payments. The Company received a $15.0 million non-refundable upfront payment in March 2015 upon execution of the Alexion agreement and an aggregate amount of $3.8 million in pre-clinical milestone payments prior to the termination of the Alexion agreement. As a result of the termination, the Company was not entitled to receive payment from Alexion for any additional milestones. The Company determined that there were three deliverables under the former Alexion collaboration: (i) an exclusive license to research, develop, manufacture and commercialize the licensed products and the compounds in the field in the territory, (ii) conducting research and development activities under the research plan and (iii) participation on a joint steering committee (JSC) and joint project team (JPT). The Company determined that the license did not have value to Alexion on a stand-alone basis due to the specialized nature of the research services to be provided by the Company that are not available in the marketplace. Therefore, the deliverables were not separable and, accordingly, the license, undelivered research and development activities and JSC and JPT participation were a single unit of accounting. When multiple deliverables are accounted for as a single unit of accounting, the Company bases its revenue recognition model on the final deliverable. Under the Alexion agreement, the last deliverable to be completed is its research and development activities and participation on the JSC and JPT, which were expected to be delivered over the same performance period. The Company recognized the remaining deferred revenue balance related to the upfront payment and non-substantive milestone payment previously received under the former collaboration with Alexion, utilizing the proportional performance model over the remaining period of performance, which ended October 24, 2017. The Company evaluated whether the milestones that it was eligible to receive in connection with the Alexion agreement were substantive or non-substantive milestones. The Company concluded that the first pre-clinical milestone payment under the former Alexion collaboration was non-substantive due to the certainty at the date the arrangement was entered into that the event will be achieved. In the second quarter of 2015, the Company achieved the first pre-clinical milestone under the former Alexion collaboration and received a $1.8 million payment from Alexion. The Company recognized revenues from the related milestone payment over the remaining period of performance. The remaining non-refundable pre-clinical milestones that the Company was eligible to achieve as a result of the Company’s efforts prior to the termination were considered substantive. The Company has recognized and received an aggregate of $2.0 million in substantive milestones through December 31, 2017. As a result of the termination, the Company did not recognize revenue under the Alexion agreement during the three months ended March 31, 2018. During the three months ended March 31, 2017 the Company recognized revenue under the Alexion agreement of $4.4 million, which represents $2.8 million of reimbursable research and development costs, as well as a portion of the $15.0 million upfront payment and the $1.8 million non-substantive milestone payment previously received under the collaboration.

Term Loan	3 Months Ended
Term Loan	Mar. 31, 2018
Disclosure Text Block
Term Loan	7. Term Loan In May 2013, the Company entered into a loan and security agreement with Silicon Valley Bank, which provided for up to $5.0 million in funding, to be made available in three tranches. Loan advances accrue interest at a fixed rate of 2% above the prime rate. In November 2014, the Company amended the loan to allow the Company to borrow an additional $5.0 million. The Company accounted for the amendment as a modification to the existing 2013 loan. The Company immediately drew the additional $5.0 million and was required to make interest‑only payments until December 1, 2015, and consecutive monthly payments of principal, plus accrued interest, over the remaining term through November 1, 2018. The Company is required to pay a fee of 4% of the total loan advances at the end of the term of the loan. The fee is being accreted to interest expense over the term of the loan. In the event of prepayment, the Company is obligated to pay 1% to 2% of the amount of the outstanding principal depending upon the timing of the prepayment. The term loan is collateralized by a blanket lien on all corporate assets, excluding intellectual property, and by a negative pledge of the Company’s intellectual property. The term loan contains customary default provisions that include material adverse events, as defined therein. The Company has determined that the risk of subjective acceleration under the material adverse events clause is remote and therefore has classified the outstanding principal in current and long‑term liabilities based on scheduled principal payments. The Company assessed all terms and features of the term loan in order to identify any potential embedded features that would require bifurcation. As part of this analysis, the Company assessed the economic characteristics and risks of the term loan, including put and call features. The Company determined that all features of each of the term loan are clearly and closely associated with a debt host and do not require bifurcation as a derivative liability, or the fair value of the feature is immaterial to the Company’s financial statements. The Company will continue to reassess the features on a quarterly basis to determine if they require separate accounting. Future minimum payments, which include principal and interest due under the term loan, are $1.1 million, in the aggregate, for the remainder of 2018.

Stock Awards

3 Months Ended

Mar. 31, 2018

Disclosure Text Block

Stock Awards

8. Stock Awards

2015 Stock Option and Incentive Plan

In 2015, the Company’s board of directors and stockholders approved the 2015 Stock Option and Incentive Plan (the 2015 Plan), which replaced the Company’s 2011 Stock Option and Grant Plan, as amended (the 2011 Plan). The 2015 Plan includes incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock, restricted stock units, unrestricted stock, performance share awards and cash-based awards. The Company initially reserved a total of 1,460,084 shares of common stock for the issuance of awards under the 2015 Plan. The 2015 Plan provides that the number of shares reserved and available for issuance under the 2015 Plan will be cumulatively increased on January 1 of each calendar year by 4% of the number of shares of common stock issued and outstanding on the immediately preceding December 31 or such lesser amount as specified by the compensation committee of the board of directors. For the calendar year beginning January 1, 2018, the number of shares reserved for issuance under the 2015 Plan was increased by 1,743,101 shares. In addition, the total number of shares reserved for issuance is subject to adjustment in the event of a stock split, stock dividend or other change in our capitalization. At March 31, 2018, there were 2,452,704 shares available for future grant under the 2015 Plan.

Awards

Options and restricted stock awards granted by the Company generally vest ratably over four years, with a one‑year cliff for new employee awards, and are exercisable from the date of grant for a period of ten years.

The Company did not have any unvested restricted stock as of March 31, 2018 and December 31, 2017. There were no restricted stock awards that vested during the three months ended March 31, 2018. The total fair value of restricted stock that vested during the three months ended March 31, 2017 was $0.1 million.

A summary of the Company’s stock option activity and related information follows:


		Weighted-		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value(1)
	Shares	Price		(in Years)	(in thousands)
Outstanding at December 31, 2017	3,304,166	$	22.45	8.04	$	174,985
Granted	1,083,950		81.36
Exercised	(243,721)		14.57
Canceled	(24,525)		39.32
Outstanding at March 31, 2018	4,119,870	$	38.32	8.39	$	219,934
Exercisable at March 31, 2018	1,438,649	$	13.22	7.20	$	112,909

(1)

Intrinsic value represents the amount by which the fair market value as of March 31, 2018 of the underlying common stock exceeds the exercise price of the option.

The fair value of stock options is estimated on the grant date using the Black‑Scholes option‑pricing model based on the following weighted average assumptions:


	Three Months Ended

	March 31, 2018		March 31, 2017
Risk-free interest rate	2.69	%	2.11	%
Expected dividend yield	—	%	—	%
Expected term (years)	6.0		6.0
Expected stock price volatility	70.18	%	75.43	%

The weighted‑average grant date fair value of options granted in the three months ended March 31, 2018 and 2017 was $52.20 and $23.98, respectively. The total intrinsic value of options exercised in the three months ended March 31, 2018 and 2017 was $17.5 million and $3.5 million, respectively.

Total stock‑based compensation expense recognized for all stock‑based compensation awards in the condensed consolidated statements of operations and comprehensive loss is as follows (in thousands):


	Three Months Ended
	March 31,
	2018		2017
Research and development	$	3,011	$	1,124
General and administrative		2,538		1,116
Total stock-based compensation expense	$	5,549	$	2,240

At March 31, 2018, the Company had $85.6 million of total unrecognized compensation cost related to non-vested stock awards, which is expected to be recognized over a weighted‑average period of 2.89 years. Due to an operating loss, the Company does not record tax benefits associated with stock‑based compensation or option exercises. Tax benefit will be recorded when realized.

2015 Employee Stock Purchase Plan

In 2015, the Company’s board of directors and stockholders approved the 2015 Employee Stock Purchase Plan (the 2015 ESPP), which became effective upon the closing of the IPO in May 2015. The Company initially reserved a total of 243,347 shares of common stock for issuance under the 2015 ESPP. The 2015 ESPP provides that the number of shares reserved and available for issuance under the 2015 ESPP will be cumulatively increased on January 1 of each calendar year by 1% of the number of shares of common stock issued and outstanding on the immediately preceding December 31 or such lesser amount as specified by the compensation committee of the board of directors. For the calendar year beginning January 1, 2018, the number of shares reserved for issuance under the 2015 ESPP was increased by 435,775 shares. The Company issued no shares under the ESPP during the three months ended March 31, 2018 and 2017, respectively.

Net Loss per Share

3 Months Ended

Mar. 31, 2018

Disclosure Text Block

Net Loss per Share

9. Net Loss per Share

Basic net loss per share applicable to common stockholders is calculated by dividing net loss applicable to common stockholders by the weighted average shares outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share applicable to common stockholders is calculated by adjusting weighted average shares outstanding for the dilutive effect of common stock equivalents outstanding for the period. For purposes of the dilutive net loss per share applicable to common stockholders calculation, stock options, and unvested restricted stock are considered to be common stock equivalents but are excluded from the calculation of diluted net loss per share applicable to common stockholders, as their effect would be anti‑dilutive; therefore, basic and diluted net loss per share applicable to common stockholders were the same for all periods presented as a result of the Company’s net loss. The following common stock equivalents were excluded from the calculation of diluted net loss per share applicable to common stockholders for the periods indicated because including them would have had an anti‑dilutive effect.


	Three Months Ended
	March 31,
	2018	2017
Stock options	4,119,870	3,237,043
Unvested restricted stock	—	254
Total	4,119,870	3,237,297

The weighted average number of common shares used in net loss per share applicable to common stockholders on a basic and diluted basis were 43,700,226 and 33,189,759 for the three months ended March 31, 2018 and 2017, respectively.

Commitments	3 Months Ended
Commitments	Mar. 31, 2018
Disclosure Text Block
Commitments	10. Commitments On February 12, 2015, the Company entered into a lease for approximately 38,500 rentable square feet of office and laboratory space in Cambridge, Massachusetts, which the Company gained control over on June 15, 2015, and occupancy commenced in October 2015. The lease ends on October 31, 2022. The Company has an option to extend the lease for five additional years. The lease has a total commitment of $17.8 million over the seven-year term. The Company has agreed to pay an initial annual base rent of approximately $2.3 million, which rises periodically until it reaches approximately $2.8 million. The Company is recording rent expense on a straight-line basis through the end of the lease term. The Company has recorded deferred rent on the consolidated balance sheet at December 31, 2017, accordingly. The lease provides the Company with an allowance for leasehold improvements of $4.3 million. The Company accounts for leasehold improvement incentives as a reduction to rent expense ratably over the lease term. The balance from the leasehold improvement incentives is included in lease incentive obligations on the balance sheet. The lease agreement required the Company to pay a security deposit of $1.3 million, which is recorded in restricted cash on the Company’s balance sheet. In the first quarter of 2018, the Company subleased its former corporate headquarters at 38 Sidney Street, Cambridge, Massachusetts through October 31, 2020. Subject to the terms of the sublease agreement and the master lease agreement, including a right of recapture by the Company, the sublessee has the option to extend the sublease through October 31, 2022. The sublease includes a total commitment by the sublessee of $8.2 million over the 32 month term of the sublease agreement. During the 32 month term, the Company will be responsible for total rental payments of $6.9 million and an additional $0.7 million in total payments related to the Company’s profit on the sublease income which are payable by the Company to the landlord. On April 28, 2017, the Company entered into a lease agreement for approximately 99,833 rentable square feet of office and laboratory space located at 45 Sidney Street in Cambridge, Massachusetts. The initial term of the lease agreement commenced on October 1, 2017 and will expire on November 30, 2029. The lease agreement also provides the Company with an option to extend the lease agreement for two consecutive five-year periods at the then fair market annual rent, as defined in the lease agreement, as well as a right of first offer with respect to leasing additional space adjacent to the existing leased premises. During the initial term of the lease agreement, the Company has agreed to pay an initial annual base rent of approximately $7.7 million, which rises periodically until it reaches approximately $10.6 million in the last year of the initial term. The Company is recording rent expense on a straight-line basis through the end of the lease term. The Company has recorded deferred rent on the consolidated balance sheet at December 31, 2017 accordingly. The landlord has also agreed to provide the Company with a tenant improvement allowance of approximately $14.2 million for improvements to be made to the premises. The Company accounts for leasehold improvement incentives as a reduction to rent expense ratably over the lease term. The lease agreements required the Company to pay a security deposit of $3.5 million, which is recorded in restricted cash on the Company’s balance sheet. For the three months ended March 31, 2018 and 2017, rent expense was $2.2 million and $0.5 million, respectively.

Summary of Significant Accounting Policies and Recent Accounting Pronouncements (Policies)	3 Months Ended
	Mar. 31, 2018
Policy Text Blocks
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires the Company’s management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results could differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and in developing the estimates and assumptions that are used in the preparation of the financial statements. Management must apply significant judgment in this process. Management’s estimation process often may yield a range of potentially reasonable estimates and management must select an amount that falls within that range of reasonable estimates. Estimates are used in the following areas, among others: stock‑based compensation expense; revenue recognition; accrued expenses; and income taxes.
Significant Accounting Policies	Significant Accounting Policies The Company’s critical accounting policies are those policies that require the most significant judgments and estimates in the preparation of our financial statements. Management has determined that the Company’s most critical accounting policies are those relating to revenue recognition, accrued research and development expenses, available-for-sale investments and stock-based compensation.
Revenue recognition	Revenue Recognition Effective January 1, 2018, the Company adopted ASC 606, using the modified retrospective transition method. Under this method, results for reporting periods beginning after January 1, 2018 are presented under ASC 606, while prior period amounts are not adjusted and continue to be reported in accordance with ASC Topic 605, Revenue Recognition (ASC 605). The Company only applied the modified retrospective transition method to contracts that were not completed as of January 1, 2018, the effective date of adoption for ASC 606. This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance, collaboration arrangements and financial instruments. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the entity performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. The Company enters into licensing agreements that are within the scope of ASC 606, under which it may exclusively license rights to research, develop, manufacture and commercialize its product candidates to third parties. The terms of these arrangements typically include payment to the Company of one or more of the following: non-refundable, upfront license fees; reimbursement of certain costs; customer option exercise fees; development, regulatory and commercial milestone payments; and royalties on net sales of licensed products. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under its agreements, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. As part of the accounting for these arrangements, the Company must use significant judgment to determine: (a) the number of performance obligations based on the determination under step (ii) above; (b) the transaction price under step (iii) above; and (c) the stand-alone selling price for each performance obligation identified in the contract for the allocation of transaction price in step (iv) above. The Company uses judgment to determine whether milestones or other variable consideration, except for royalties, should be included in the transaction price as described further below. The transaction price is allocated to each performance obligation on a relative stand-alone selling price basis, for which the Company recognizes revenue as or when the performance obligations under the contract are satisfied. Amounts received prior to revenue recognition are recorded as deferred revenue. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as current portion of deferred revenue in the accompanying consolidated balance sheets. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as deferred revenue, net of current portion. Exclusive Licenses. If the license to the Company’s intellectual property is determined to be distinct from the other promises or performance obligations identified in the arrangement, the Company recognizes revenue from non-refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. In assessing whether a promise or performance obligation is distinct from the other promises, the Company considers factors such as the research, development, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can benefit from a promise for its intended purpose without the receipt of the remaining promise, whether the value of the promise is dependent on the unsatisfied promise, whether there are other vendors that could provide the remaining promise, and whether it is separately identifiable from the remaining promise. For licenses that are combined with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. The measure of progress, and thereby periods over which revenue should be recognized, are subject to estimates by management and may change over the course of the research and development and licensing agreement. Such a change could have a material impact on the amount of revenue the Company records in future periods. Research and Development Services. The promises under the Company’s collaboration agreements may include research and development services to be performed by the Company on behalf of the partner. Payments or reimbursements resulting from the Company’s research and development efforts are recognized as the services are performed and presented on a gross basis because the Company is the principal for such efforts. Reimbursements from and payments to the partner that are the result of a collaborative relationship with the partner, instead of a customer relationship, such as co-development activities, are recorded as a reduction to research and development expense. Customer Options. If an arrangement is determined to contain customer options that allow the customer to acquire additional goods or services, the goods and services underlying the customer options that are not determined to be material rights are not considered to be performance obligations at the outset of the arrangement, as they are contingent upon option exercise. The Company evaluates the customer options for material rights, or options to acquire additional goods or services for free or at a discount. If the customer options are determined to represent a material right, the material right is recognized as a separate performance obligation at the outset of the arrangement. The Company allocates the transaction price to material rights based on the relative standalone selling price, which is determined based on the identified discount and the probability that the customer will exercise the option. Amounts allocated to a material right are not recognized as revenue until, at the earliest, the option is exercised. Milestone Payments. At the inception of each arrangement that includes development milestone payments, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial, and other risks that must be overcome to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant revenue reversal would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenues and earnings in the period of adjustment. Royalties. For arrangements that include sales-based royalties, including milestone payments based on a level of sales, which are the result of a customer-vendor relationship and for which the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied or partially satisfied. To date, the Company has not recognized any royalty revenue resulting from any of its licensing arrangements. For a complete discussion of accounting for collaboration revenues, see Note 6, “Collaborations.” There have been no other significant changes to the Company’s critical accounting policies discussed in its Annual Report on Form 10-K for the year ended December 31, 2017 related to accrued research and development expenses, available-for-sale investments and stock-based compensation.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted In May 2014, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2014-09, which amends the guidance for accounting for revenue from contracts with customers. ASU No. 2014-09 superseded the revenue recognition requirements in ASC 605 and created ASC 606 described above. In 2015 and 2016, the FASB issued additional ASUs related to ASC 606 that delayed the effective date of the guidance and clarified various aspects of the new revenue guidance, including principal versus agent considerations, identifying performance obligations, and licensing, and they include other improvements and practical expedients. Effective January 1, 2018, the Company adopted ASC 606 using the modified retrospective transition method. As a result of adopting ASC 606, the Company has recorded a cumulative-effect increase to opening accumulated deficit of $5.3 million as of January 1, 2018 and a corresponding increase to deferred revenue. Total revenue recorded in the three months ended March 31, 2018 under ASC 606 was $0.9 million, as compared to $1.3 million that would have been recorded under ASC 605. Total deferred revenue as of March 31, 2018 was $39.7 million under ASC 606, as compared to $34.0 million that would have been recorded under ASC 605. The most significant change to the Company’s accounting for revenue as a result of the adoption of ASC 606 relates to its revenue recognition pattern under step (v) above for the Company’s collaboration and license agreement with Roche (as amended, Roche Agreement). Under ASC 605, the Company was recognizing the revenue allocated to each unit of accounting on straight‑line basis over the period the Company expected to complete its obligations. Under ASC 606, the Company is recognizing the revenue allocated to each performance obligation measuring progress using an input method over the period the Company expects to complete each performance obligation. ASC 606 also requires disclosure of the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. For further discussion of the adoption of this standard, see Note 6, “Collaborations.” In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash (ASU No. 2016-18). The amendments in ASU No. 2016-18 require an entity to reconcile and explain the period-over-period change in total cash, cash equivalents and restricted cash within its statements of cash flows. ASU No. 2016-18 was effective for the Company on January 1, 2018. The Company adopted ASU No. 2016-18 using a full retrospective approach and it did not have a significant impact on its consolidated financial statements and related disclosures. In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230) (ASU No. 2016-15), which simplifies certain elements of cash flow classification. The new guidance is intended to reduce diversity in practice in how certain transactions are classified in the statement of cash flows. ASU No. 2016-15 was effective for the Company on January 1, 2018, and it did not have a material impact on the Company’s consolidated financial statements. In 2016, the FASB issued ASU No. 2016-01 Financial Instruments (ASU No. 2016-01) related to the recording of financial assets and financial liabilities. Under the amended guidance, equity investments (except those accounted for under the equity method of accounting or those that result in consolidation of the investee) are to be measured at fair value with changes in fair value recognized in net income (loss). However, an entity has the option to either measure equity investments without readily determinable fair values either (i) at fair value or (ii) at cost adjusted for changes in observable prices minus impairment. Changes in measurement under either alternative will be recognized in net income (loss). The amended guidance became effective January 1, 2018. As the Company does not currently hold equity securities, there was no impact on the financial statements at the adoption date. The Company may hold equity securities in the future, at which time the Company will apply the provisions of ASU No. 2016-01 and record changes in the fair value of the equity securities in net income (loss). Not Yet Adopted In February 2016, the FASB issued ASU No. 2016-02, Leases (ASU No. 2016-02), which will change the way the Company recognizes its leased assets. ASU No. 2016-02 will require organizations that lease assets—referred to as “lessees”—to recognize on the balance sheet the assets and liabilities representing the rights and obligations created by those leases. ASU No. 2016-02 will also require disclosures to help investors and other financial statement users better understand the amount, timing, and uncertainty of cash flows arising from leases. The standard is effective for annual reporting periods (including interim reporting periods within those years) beginning after December 15, 2018. Early adoption is permitted. The Company is currently evaluating the methods of adoption allowed by the new standard and the effect that adoption of the standard is expected to have on the Company’s consolidated financial statements and related disclosures.

Cash Equivalents and Investments (Tables)

3 Months Ended

Mar. 31, 2018

Table Text Blocks

Schedule of cash equivalents and investments, available-for-sale

Cash equivalents and investments, available-for-sale, consisted of the following at March 31, 2018 and December 31, 2017 (in thousands):


	Average		Amortized		Unrealized		Unrealized		Fair
March 31, 2018	Maturity		Cost		Gain		Losses		Value
Cash equivalents:
Money market funds			$	134,886	$	—	$	—	$	134,886
Investments, available-for-sale:
U.S. treasury obligations		298 Days		486,829		—		(592)		486,237
Total			$	621,715	$	—	$	(592)	$	621,123


	Average		Amortized		Unrealized		Unrealized		Fair
December 31, 2017	Maturity		Cost		Gain		Losses		Value
Cash equivalents:
Money market funds			$	400,304	$	—	$	—	$	400,304
Investments, available-for-sale:
U.S. treasury obligations		348 Days		273,321		—		(269)		273,052
Total			$	673,625	$	—	$	(269)	$	673,356

Fair Value of Financial Instruments (Tables)

3 Months Ended

Mar. 31, 2018

Table Text Blocks

Schedule of financial instruments measured at fair value

Financial instruments measured at fair value as of March 31, 2018 are classified below based on the fair value hierarchy described above:


			Active		Observable		Unobservable
	March 31,		Markets		Inputs		Inputs
Description	2018		(Level 1)		(Level 2)		(Level 3)
Financial Assets
Cash equivalents:
Money market funds	$	134,886	$	134,886	$	—	$	—
Investments, available-for-sale:
U.S Treasury obligations		486,237		486,237		—		—
Total	$	621,123	$	621,123	$	—	$	—

Financial instruments measured at fair value as of December 31, 2017 are classified below based on the fair value hierarchy described above:


			Active		Observable		Unobservable
	December 31,		Markets		Inputs		Inputs
Description	2017		(Level 1)		(Level 2)		(Level 3)
Financial Assets
Cash equivalents:
Money market funds	$	400,304	$	400,304	$	—	$	—
Investments, available-for-sale:
U.S Treasury obligations		273,052		273,052		—		—
Total	$	673,356	$	673,356	$	—	$	—

- Definition

Tabular disclosure of financial instruments measured at fair value, including those classified in shareholders' equity measured on a recurring or nonrecurring basis. Disclosures include, but are not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. Nonrecurring fair value measurements are those that are required or permitted in the statement of financial position in particular circumstances.

+ References + Details

Stock Awards (Tables)

3 Months Ended

Mar. 31, 2018

Table Text Blocks

Summary of stock option activity


		Weighted-		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value(1)
	Shares	Price		(in Years)	(in thousands)
Outstanding at December 31, 2017	3,304,166	$	22.45	8.04	$	174,985
Granted	1,083,950		81.36
Exercised	(243,721)		14.57
Canceled	(24,525)		39.32
Outstanding at March 31, 2018	4,119,870	$	38.32	8.39	$	219,934
Exercisable at March 31, 2018	1,438,649	$	13.22	7.20	$	112,909

(1)

Intrinsic value represents the amount by which the fair market value as of March 31, 2018 of the underlying common stock exceeds the exercise price of the option.

Assumptions used in estimating fair value of stock options on grant date


	Three Months Ended

	March 31, 2018		March 31, 2017
Risk-free interest rate	2.69	%	2.11	%
Expected dividend yield	—	%	—	%
Expected term (years)	6.0		6.0
Expected stock price volatility	70.18	%	75.43	%

Summary of stock-based compensation expense recognized in statements of operations


	Three Months Ended
	March 31,
	2018		2017
Research and development	$	3,011	$	1,124
General and administrative		2,538		1,116
Total stock-based compensation expense	$	5,549	$	2,240

Net Loss per Share (Tables)

3 Months Ended

Mar. 31, 2018

Table Text Blocks

Schedule of common stock equivalents excluded from calculation of diluted net loss per share applicable to common stockholders


	Three Months Ended
	March 31,
	2018	2017
Stock options	4,119,870	3,237,043
Unvested restricted stock	—	254
Total	4,119,870	3,237,297

Nature of Business - Stock Offerings (Details) - USD ($) $ / shares in Units, $ in Thousands					3 Months Ended
	Dec. 15, 2017	Apr. 04, 2017	Dec. 13, 2016	May 05, 2015	Mar. 31, 2018	Mar. 31, 2017
Nature of Business
Proceeds from issuance of common stock					$ 3,643	$ 902
IPO
Nature of Business
Stock sold (in shares)				9,367,708
Share price (in dollars per share)				$ 18.00
Proceeds from issuance of IPO shares				$ 154,800
Follow-on Offering
Nature of Business
Stock sold (in shares)	4,259,259	5,750,000	5,750,000
Share price (in dollars per share)	$ 81.00	$ 40.00	$ 25.00
Proceeds from issuance of common stock	$ 325,700	$ 215,600	$ 134,500
Underwriters' Option
Nature of Business
Stock sold (in shares)	555,555	750,000	750,000	1,221,874

Nature of Business - Cash, Cash Equivalents and Investments (Details) $ in Millions	Mar. 31, 2018 USD ($)
Cash, cash equivalents and investments
Cash, cash equivalents and investments	$ 621.1

Summary of Significant Accounting Policies and Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Jan. 02, 2018	Dec. 31, 2017
Summary of Significant Accounting Policies and Recent Accounting Pronouncements
Accumulated deficit	$ (417,452)		$ (355,589)
Collaboration revenue	900
Deferred revenue	39,700
Accounting Standards Update 2014-09 \| Difference between Revenue Guidance in Effect before and after Topic 606
Summary of Significant Accounting Policies and Recent Accounting Pronouncements
Accumulated deficit		$ (5,300)
Deferred revenue		$ 5,300
Accounting Standards Update 2014-09 \| Calculated under Revenue Guidance in Effect before Topic 606
Summary of Significant Accounting Policies and Recent Accounting Pronouncements
Collaboration revenue	1,300
Deferred revenue	$ 34,000

Cash Equivalents and Investments - Tabular Disclosure (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Cash Equivalents and Investments
Available-for-sale, Unrealized Losses	$ (592)	$ (269)
Total, Amortized Cost	621,715	673,625
Total, Fair Value	$ 621,123	$ 673,356
US Treasury obligations
Cash Equivalents and Investments
Average Maturity	298 days	348 days
Available-for-sale, Amortized Cost	$ 486,829	$ 273,321
Available-for-sale, Unrealized Losses	(592)	(269)
Investments, available-for-sale	486,237	273,052
Money market funds
Cash Equivalents and Investments
Cash equivalents, Amortized Cost	134,886	400,304
Cash equivalents, Fair Value	$ 134,886	$ 400,304

Cash Equivalents and Investments - Gains or Losses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Realized gain (loss) on sales of investments
Realized gain (loss) on sales of investments	$ 0	$ 0
Adjustment for other than temporary fair value decline
Adjustment for other than temporary fair value decline	$ 0	$ 0

Cash Equivalents and Investments - Unrealized Loss Position (Details) $ in Millions	Mar. 31, 2018 USD ($) security
Unrealized loss position, number of positions
Number of held securities in an unrealized loss position \| security	58
Unrealized loss position, aggregate fair value
Unrealized loss position for less than 12 months	$ 456.3
Unrealized loss position for more than 12 months	$ 29.9

Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Recurring
Fair Value of Financial Instruments
Total	$ 621,123	$ 673,356
Recurring \| Active Markets (Level 1)
Fair Value of Financial Instruments
Total	621,123	673,356
Money market funds
Fair Value of Financial Instruments
Cash equivalents, Fair Value	134,886	400,304
Money market funds \| Recurring
Fair Value of Financial Instruments
Cash equivalents, Fair Value	134,886	400,304
Money market funds \| Recurring \| Active Markets (Level 1)
Fair Value of Financial Instruments
Cash equivalents, Fair Value	134,886	400,304
US Treasury obligations
Fair Value of Financial Instruments
Investments, available-for-sale	486,237	273,052
US Treasury obligations \| Recurring
Fair Value of Financial Instruments
Investments, available-for-sale	486,237	273,052
US Treasury obligations \| Recurring \| Active Markets (Level 1)
Fair Value of Financial Instruments
Investments, available-for-sale	$ 486,237	$ 273,052

Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Restricted Cash
Restricted Cash and Cash Equivalents	$ 4,557		$ 1,267
Restricted cash included in long-term assets	4,557	$ 4,555
Cambridge Massachusetts, 38 Sidney Street and 45 Sidney Street Leases
Restricted Cash
Restricted cash included in long-term assets	4,600	4,600
Restricted by bank
Restricted Cash
Restricted Cash and Cash Equivalents	$ 4,600	$ 4,800

Collaborations (Details) $ in Thousands	1 Months Ended		3 Months Ended
Collaborations (Details) $ in Thousands	Mar. 31, 2016 USD ($) item	Mar. 31, 2015 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2017 USD ($)
Collaborations
Collaboration revenue			$ 900
Deferred revenue			39,700
Current portion of deferred revenue			4,803			$ 5,373
Roche
Collaborations
Achieved milestone payment received which will be recognized over period of performance					$ 1,800
Collaboration \| Roche
Collaborations
Cumulative Non-refundable milestone payments received						$ 2,000
Initial cash payment	$ 45,000
Collaboration revenue			$ 5,300
Roche Agreement
Collaborations
Number of potential collaboration programs \| item	5
Number of collaboration programs with agreed upon targets \| item	2
Non-refundable upfront payment received	$ 45,000
Total eligible contingent option fees and milestone payments	965,000
Eligible option fees and milestones prior to licensing for all potential collaboration programs	$ 215,000
Research and development services, performance period			8 years
Roche Agreement \| Exercise Of License Right Option
Collaborations
Number of collaboration programs with specified commercial rights for each party \| item	2
Roche Agreement \| Roche
Collaborations
Number of option rights to obtain exclusive license \| item	5
Deferred revenue			$ 39,700
Current portion of deferred revenue			4,800
Roche Agreement \| Roche \| Prior To Exercise License Right Option
Collaborations
Written termination notice period	120 days
Roche Agreement \| Roche \| Exercise Of License Right Option
Collaborations
Number of collaboration programs with exclusive commercialization rights for licensed products \| item	3
Roche Agreement \| Roche \| Exercise Of License Option Product Not Commercially Sold
Collaborations
Written termination notice period	120 days
Roche Agreement \| Roche \| Exercise Of License Option Product Commercially Sold
Collaborations
Written termination notice period	180 days
Alexion
Collaborations
Non-refundable upfront and milestone payment received		$ 18,800
Non-refundable upfront payment received		15,000
Non-refundable milestone payment received		$ 3,800
Collaborative partner's funding responsibility of research and development costs incurred under the research plan (as a percent)		100.00%
Number of deliverables \| item	3
Reimbursable research and development costs recognized			$ 2,800
Non-substantive milestone payment previously		$ 1,800
Alexion \| Calculated under Revenue Guidance in Effect before Topic 606
Collaborations
Collaboration revenue				$ 4,400

Term Loan - General Information (Details) - Secured Debt $ in Millions	1 Months Ended
	Nov. 30, 2014 USD ($)	May 31, 2013 USD ($) tranche	Mar. 31, 2018
2013 Term Loan
Term Loan
Face amount		$ 5.0
Number of tranches \| tranche		3
2013 Term Loan \| Prime Rate
Term Loan
Fixed rate based on percentage points added to base rate (as a percent)		2.00%
2014 Term Loan
Term Loan
Face amount	$ 5.0
Proceeds from term loan	$ 5.0
Percentage fee payable on total loan advances at end of loan (as a percent)			4.00%
Percentage fee payable on outstanding principal in event of prepayment of loan advances, low end of range (as a percent)			1.00%
Percentage fee payable on outstanding principal in event of prepayment of loan advances, high end of range (as a percent)			2.00%

Stock Awards - 2015 Stock Option and Incentive Plan (Details) - 2015 Stock Option and Incentive Plan - shares		3 Months Ended
	Jan. 01, 2018	Mar. 31, 2018	Apr. 08, 2015
Stock Awards
Initial shares of common stock authorized for issuance of stock awards			1,460,084
Increase in number of shares available for grant (as a percent)		4.00%
Increase in number of shares available for grant	1,743,101
Number of shares available for grant		2,452,704

Stock Awards - Awards - General Information (Details) - 2015 Stock Option and Incentive Plan	3 Months Ended
	Mar. 31, 2018
Stock Awards
Award vesting period	4 years
Cliff vesting period for new employees	1 year
Award expiration period	10 years

Stock Awards - Unvested Restricted Stock - Activity (Details) - Restricted Stock - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Stock Awards
Unvested stock options	0		0
Total fair value of restricted stock that vested	$ 0.0	$ 0.1

Stock Awards - Stock Options - Activity (Details) - Employee Stock Options - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Shares
Outstanding at beginning of period (in shares)	3,304,166
Granted (in shares)	1,083,950
Exercised (in shares)	(243,721)
Cancelled (in shares)	(24,525)
Outstanding at end of period (in shares)	4,119,870	3,304,166
Weighted-Average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 22.45
Granted (in dollars per share)	81.36
Exercised (in dollars per share)	14.57
Cancelled (in dollars per share)	39.32
Outstanding at end of period (in dollars per share)	$ 38.32	$ 22.45
Additional disclosures
Shares - Exercisable (in shares)	1,438,649
Weighted-Average Exercise Price - Exercisable (in dollars per share)	$ 13.22
Remaining Contractual Life - Outstanding	8 years 4 months 21 days	8 years 15 days
Remaining Contractual Life - Exercisable	7 years 2 months 12 days
Aggregate Intrinsic Value - Outstanding	$ 219,934	$ 174,985
Aggregate Intrinsic Value - Exercisable	$ 112,909

Stock Awards - Weighted Average Assumptions - Tabular Disclosure (Details) - Employee Stock Options	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Assumptions used in estimating the fair value of stock options granted
Risk-free interest rate	2.69%	2.11%
Expected dividend yield	0.00%	0.00%
Expected term (in years)	6 years	6 years
Expected stock price volatility	70.18%	75.43%

Stock Awards - Weighted Average Assumptions - Additional Information (Details) - Employee Stock Options - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Additional disclosures
Weighted-average grant date fair value of options granted	$ 52.20	$ 23.98
Total intrinsic value of options exercised	$ 17.5	$ 3.5

Stock Awards - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Total stock based compensation expense
Total stock-based compensation expense	$ 5,549	$ 2,240
Research and development
Total stock based compensation expense
Total stock-based compensation expense	3,011	1,124
General and administrative
Total stock based compensation expense
Total stock-based compensation expense	$ 2,538	$ 1,116

Stock Awards - Unrecognized Compensation Costs (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Stock Awards
Total unrecognized compensation cost related to non-vested stock awards	$ 85.6
Weighted-average period over which unrecognized compensation cost will be recognized	2 years 10 months 21 days

Stock Awards - Employee Stock Purchase Plan (Details) - Employee Stock - 2015 Employee Stock Purchase Plan - shares	1 Months Ended	3 Months Ended
	May 31, 2015	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018
Stock Awards
Number of common shares reserved for future issuance	243,347
Annual increase for common stock for issuance (as a percent)	1.00%
Increase of common shares reserved for future issuance				435,775
ESPP shares issued during period		0	0

Net Loss per Share - Anti-dilutive Securities (Details) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Antidilutive securities excluded from computation of earnings per share
Antidilutive securities excluded from computation of earnings per share	4,119,870	3,237,297
Stock options
Antidilutive securities excluded from computation of earnings per share
Antidilutive securities excluded from computation of earnings per share	4,119,870	3,237,043
Restricted Stock
Antidilutive securities excluded from computation of earnings per share
Antidilutive securities excluded from computation of earnings per share		254

Net Loss per Share - Weighted Average Number of Common Shares (Details) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net Loss per Share
Weighted-average number of common shares used in net loss per share applicable to common stockholders - basic	43,700,226	33,189,759
Weighted-average number of common shares used in net loss per share applicable to common stockholders - diluted	43,700,226	33,189,759

Commitments - General Information (Details) $ in Millions			3 Months Ended
Commitments - General Information (Details) $ in Millions	Oct. 01, 2017 USD ($) ft² period	Feb. 12, 2015 USD ($) ft²	Mar. 31, 2018 USD ($)
Operating Leases
Total commitment under the operating lease			$ 6.9
Corporate Headquarters Lease
Operating Leases
Area leased (in square feet) \| ft²		38,500
Extension period of lease term		5 years
Total commitment under the operating lease		$ 17.8
Lease term		7 years
Base annual rent, initial		$ 2.3
Base annual rent, maximum		2.8
Allowance for leasehold improvements		4.3
Security deposit included in restricted cash		$ 1.3
Total commitment in sublease			$ 8.2
Sublease term			32 months
Sublease payable			$ 0.7
Office and Laboratory Space in Cambridge Massachusetts, 45 Sidney Street
Operating Leases
Area leased (in square feet) \| ft²	99,833
Number of extensions \| period	2
Extension period of lease term	5 years
Base annual rent, initial	$ 7.7
Base annual rent, maximum	10.6
Security deposit included in restricted cash	3.5
Tenant improvement allowance	$ 14.2

Term Loan - Future Minimum Payments (Details) $ in Millions	Mar. 31, 2018 USD ($)
Future minimum payments
Future minimum payments for remainder of year	$ 1.1