Moderna (MRNA) 10-K 27 Feb 20 2019 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Feb. 26, 2020	Jun. 30, 2019
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Registrant Name	Moderna, Inc.
Entity Central Index Key	0001682852
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		368,642,548
Entity Public Float			$ 3,380

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 235,876	$ 658,364
Investments	867,124	863,063
Accounts receivable	5,032	11,686
Accounts receivable from related party (Note 14)	337	899
Prepaid expenses and other current assets	19,403	28,399
Restricted cash	1,032	595
Total current assets	1,128,804	1,563,006
Investments, non-current	159,987	172,990
Property and equipment, net	201,495
Property and equipment, net		211,977
Right-of-use assets, operating leases	86,414
Restricted cash, non-current	10,791	11,532
Other non-current assets	1,931	2,644
Total assets	1,589,422	1,962,149
Current liabilities:
Accounts payable	7,090	31,210
Accrued liabilities	67,652	79,073
Deferred revenue	63,310	109,056
Other current liabilities	5,063	3,464
Total current liabilities	143,115	222,803
Deferred revenue, non-current	138,995	165,352
Operating lease liabilities, non-current	93,675
Financing lease liabilities, non-current	38,689
Deferred lease obligation, non-current		10,006
Lease financing obligation		33,489
Other non-current liabilities	138	258
Total liabilities	414,612	431,908
Commitments and contingencies (Note 8)
Stockholders’ equity:
Preferred stock, $0.0001 par value; 162,000,000 shares authorized at December 31, 2019 and 2018; 0 shares issued or outstanding at December 31, 2019 and 2018	0	0
Common stock, par value $0.0001; 1,600,000,000 shares authorized as of December 31, 2019 and 2018; 336,536,985 and 328,798,904 shares issued and outstanding as of December 31, 2019 and 2018, respectively	34	33
Additional paid-in capital	2,669,426	2,538,155
Accumulated other comprehensive gain (loss)	1,804	(1,320)
Accumulated deficit	(1,496,454)	(1,006,627)
Total stockholders’ equity	1,174,810	1,530,241
Total liabilities and stockholders’ equity	$ 1,589,422	$ 1,962,149

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Preferred stock, par value (usd per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized (in shares)	162,000,000	162,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (usd per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	1,600,000,000	1,600,000,000
Common stock, shares issued (in shares)	336,536,985	328,798,904
Common stock, shares outstanding (in shares)	336,536,985	328,798,904

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue:
Total revenue	$ 60,209	$ 135,068	$ 205,825
Operating expenses:
Research and development	496,309	454,082	410,459
General and administrative	109,620	94,252	64,722
Total operating expenses	605,929	548,334	475,181
Loss from operations	(545,720)	(413,266)	(269,356)
Interest income	38,530	27,023	15,235
Other (expense) income, net	(7,526)	1,835	(1,875)
Loss before (benefit from) provision for income taxes	(514,716)	(384,408)	(255,996)
(Benefit from) provision for income taxes	(695)	326	(80)
Net loss	(514,021)	(384,734)	(255,916)
Reconciliation of net loss to net loss attributable to common stockholders:
Premium paid on repurchase of preferred stock	0	(4,127)	0
Cumulative preferred stock dividends	0	(12,996)	(13,925)
Net loss attributable to common stockholders	$ (514,021)	$ (401,857)	$ (269,841)
Net loss per share attributable to common stockholders, basic and diluted (usd per share)	$ (1.55)	$ (4.95)	$ (4.18)
Weighted average common shares used in net loss per share attributable to common stockholders, basic and diluted (in shares)	330,802,136	81,114,183	64,497,544
Collaboration revenue
Revenue:
Total revenue	$ 42,803	$ 76,519	$ 146,953
Collaboration revenue from related party (Note 14)
Revenue:
Total revenue	5,233	45,993	30,021
Grant revenue
Revenue:
Total revenue	$ 12,173	$ 12,556	$ 28,851

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net loss	$ (514,021)	$ (384,734)	$ (255,916)
Other comprehensive income (loss):
Unrealized gain (loss) on available-for-sale debt securities, net of tax of $1,148, $0 and $0, for the years ended December 31, 2019, 2018 and 2017, respectively	3,447	(132)	(342)
Less: amounts recognized for net realized gain included in net loss	(323)	(31)	(412)
Total other comprehensive income (loss)	3,124	(163)	(754)
Comprehensive loss	$ (510,897)	$ (384,897)	$ (256,670)

CONSOLIDATED STATEMENTS OF CO_2

CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Unrealized gain (loss) on available-for-sale debt securities, tax	$ 1,148	$ 0	$ 0

CONSOLIDATED STATEMENTS OF REDE

CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Series G Redeemable Convertible Preferred Stock	Series D Redeemable Convertible Preferred Stock	Series E Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred StockSeries G Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred StockSeries H Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred StockSeries D Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred StockSeries E Redeemable Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Additional Paid-In CapitalSeries G Redeemable Convertible Preferred Stock	Additional Paid-In CapitalSeries D Redeemable Convertible Preferred Stock	Additional Paid-In CapitalSeries E Redeemable Convertible Preferred Stock	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit
Balance at beginning of period (in shares) at Dec. 31, 2016					448,686,791
Balance at beginning of period at Dec. 31, 2016					$ 1,176,661
Balance at end of period (in shares) at Dec. 31, 2017	448,686,791		63,291,156	81,428,340	448,686,791	0	0	63,291,156	81,428,340
Balance at end of period at Dec. 31, 2017	$ 1,176,661		$ 164,059	$ 501,880	$ 1,176,661	$ 0	$ 0	$ 164,059	$ 501,880
Balance at beginning of period (in shares) at Dec. 31, 2016										63,541,688
Balance at beginning of period at Dec. 31, 2016	(334,810)									$ 6	$ 31,305				$ (403)	$ (365,718)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted common stock (in shares)										1,644,769
Exercise of options to purchase common stock (in shares)										20,542
Exercise of options to purchase common stock	212										212
Redeemable convertible preferred stock issuance costs	(152)										(152)
Stock-based compensation	40,055										40,314					(259)
Unrealized gain (loss) on marketable securities	(754)														(754)
Net loss	(255,916)															(255,916)
Balance at end of period (in shares) at Dec. 31, 2017										65,206,999
Balance at end of period at Dec. 31, 2017	(551,365)									$ 6	71,679				(1,157)	(621,893)
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of redeemable convertible preferred stock, net of issuance costs (in shares)						55,666,004	5,000,000
Issuance of redeemable convertible preferred stock, net of issuance costs		$ 51				$ 549,413	$ 111,546					$ 51
Repurchase of redeemable convertible preferred stock (in shares)								(269,180)	(544,100)
Repurchase of redeemable convertible preferred stock			$ (2,009)	$ (2,118)				$ (704)	$ (3,355)				$ (2,009)	$ (2,118)
Conversion of redeemable convertible preferred stock into common stock (in shares)					(508,539,515)	(55,666,004)	(5,000,000)	(63,021,976)	(80,884,240)	(236,012,913)
Conversion of redeemable convertible preferred stock into common stock	$ 1,833,561				$ (1,833,561)	$ (549,413)	$ (111,546)	$ (163,355)	$ (498,525)	$ 24	1,833,537
Balance at end of period (in shares) at Dec. 31, 2018	0				0	0	0	0	0
Balance at end of period at Dec. 31, 2018					$ 0	$ 0	$ 0	$ 0	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted common stock (in shares)										856,135
Exercise of options to purchase common stock (in shares)										446,864
Exercise of options to purchase common stock	$ 1,427										1,427
Conversion of redeemable convertible preferred stock into common stock (in shares)					508,539,515	55,666,004	5,000,000	63,021,976	80,884,240	236,012,913
Conversion of redeemable convertible preferred stock into common stock	1,833,561				$ (1,833,561)	$ (549,413)	$ (111,546)	$ (163,355)	$ (498,525)	$ 24	1,833,537
Proceeds of initial public offering, net of issuance costs of $41,322 (in shares)										26,275,993
Proceeds of initial public offering, net of issuance costs of $41,322	563,026									$ 3	563,023
Stock-based compensation	72,565										72,565
Unrealized gain (loss) on marketable securities	(163)														(163)
Net loss	(384,734)															(384,734)
Balance at end of period (in shares) at Dec. 31, 2018										328,798,904
Balance at end of period at Dec. 31, 2018	$ 1,530,241									$ 33	2,538,155				(1,320)	(1,006,627)
Balance at end of period (in shares) at Dec. 31, 2019	0				0
Balance at end of period at Dec. 31, 2019					$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted common stock (in shares)										621,432
Exercise of options to purchase common stock (in shares)	6,945,306									6,945,306
Exercise of options to purchase common stock	$ 47,259									$ 1	47,258
Purchase of common stock under employee stock purchase plan (in shares)										171,343
Purchase of common stock under employee stock purchase plan	2,891										2,891
Stock-based compensation	81,122										81,122
Unrealized gain (loss) on marketable securities	3,124														3,124
Net loss	(514,021)															(514,021)
Balance at end of period (in shares) at Dec. 31, 2019										336,536,985
Balance at end of period at Dec. 31, 2019	$ 1,174,810									$ 34	$ 2,669,426				$ 1,804	$ (1,496,454)

CONSOLIDATED STATEMENTS OF RE_2

CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ EQUITY (DEFICIT) (Parenthetical) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	May 31, 2018	Feb. 28, 2018	Dec. 31, 2019	Dec. 31, 2018
Issuance costs				$ 41,322
Series G Redeemable Convertible Preferred Stock
Issuance costs		$ 10,500	$ 10,517	10,517
Series H Redeemable Convertible Preferred Stock
Issuance costs	$ 400		$ 474	$ 474

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities
Net loss	$ (514,021)	$ (384,734)	$ (255,916)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	81,122	72,565	40,055
Depreciation and amortization	31,021	24,862	20,537
Amortization/accretion of investments	(3,742)	(1,866)	1,086
Loss on disposal of property and equipment	316	891	0
Changes in assets and liabilities:
Accounts receivable	6,654	195	1,476
Accounts receivable from related party (Note 14)	562	637	(1,370)
Prepaid expenses and other assets	9,751	(5,289)	579
Right-of-use assets, operating leases	(5,664)
Accounts payable	(23,964)	15,017	(12,766)
Accrued liabilities	(3,362)	8,787	34,369
Deferred revenue	(44,119)	(65,260)	(162,321)
Deferred lease obligation	0	2,420	2,932
Operating lease liabilities	12,647
Other liabilities	(6,169)	910	(145)
Net cash used in operating activities	(458,968)	(330,865)	(331,484)
Investing activities
Purchases of marketable securities	(1,145,226)	(1,227,709)	(727,203)
Proceeds from maturities of marketable securities	993,181	783,373	800,438
Proceeds from sales of marketable securities	168,654	177,008	402,530
Purchases of property and equipment	(31,554)	(105,766)	(58,401)
Net cash (used in) provided by investing activities	(14,945)	(373,094)	417,364
Financing activities
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs	0	661,111	0
Proceeds from initial public offering of common stock, net of issuance costs	0	563,026	0
Repurchases of redeemable convertible preferred stock	0	(8,182)	0
Repurchases of redeemable convertible preferred stock	0	0	(1,483)
Proceeds from issuance of common stock under equity plans	47,259	1,427	212
Proceeds from purchase of common stock under employee stock purchase plan	2,891	0	0
Charges to financing lease liabilities	971
Reimbursement of assets under financing lease obligation	0	11,635	2,724
Payments on financing lease obligation	0	(2,175)	(1,285)
Net cash provided by financing activities	51,121	1,226,842	168
Net (decrease) increase in cash, cash equivalents and restricted cash	(422,792)	522,883	86,048
Cash, cash equivalents and restricted cash, beginning of year	670,491	147,608	61,560
Cash, cash equivalents and restricted cash, end of year	247,699	670,491	147,608
Supplemental cash flow information
Income taxes paid	416	294	398
Interest in connection with financing lease obligation	0	2,998	0
Non-cash investing and financing activities
Issuance costs included in accounts payable and accrued liabilities	0	2,638	152
Purchases of property and equipment included in accounts payable and accrued liabilities	4,676	12,892	19,959
Leasehold improvements included in prepaid and other current assets	0	10,089	1,748
Lease financing obligation	$ 0	$ 10,089	$ 1,748

Organization and Description of

Organization and Description of Business	12 Months Ended
	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization and Description of Business	Organization and Description of Business Moderna, Inc. (we, us, or the Company) was incorporated in Delaware on July 22, 2016. Our principal executive office is located at 200 Technology Square, Cambridge, MA. We are a clinical stage biotechnology company creating a new generation of transformative medicines based on messenger RNA (mRNA), to improve the lives of patients. mRNA medicines are designed to direct the body’s cells to produce intracellular, membrane, or secreted proteins that have a therapeutic or preventive benefit with the potential to address a broad spectrum of diseases. Our platform builds on continuous advances in basic and applied mRNA science, delivery technology, and manufacturing, providing us the capability to pursue in parallel a robust pipeline of new development candidates. We are developing therapeutics and vaccines for infectious diseases, immuno-oncology, rare diseases, autoimmune and cardiovascular diseases, independently and with our strategic collaborators. Since inception, we have incurred significant net losses, which were $514.0 million , $384.7 million , and $255.9 million for the years ended December 31, 2019 , 2018 and 2017 , respectively. On December 11, 2018, we completed our initial public offering (IPO), whereby we sold 26,275,993 shares of common stock at a price of $23.00 per share. The aggregate net proceeds received by us from the IPO were $563.0 million . As of December 31, 2019 , we had an accumulated deficit of $1.50 billion . We expect to continue to incur significant expenses and operating losses for the foreseeable future. In addition, we anticipate that our expenses will increase significantly in connection with our ongoing activities to support our platform research, drug discovery and clinical development, infrastructure and Research Engine and Early Development Engine, digital infrastructure, creation of a portfolio of intellectual property, and administrative support. We do not expect to generate significant revenue from sales of potential mRNA medicines unless and until we successfully complete clinical development and obtain regulatory approval for one or more of our investigational medicines. If we seek to obtain regulatory approval for any of our investigational medicines, we expect to incur significant commercialization expenses. As a result, we will need substantial additional funding to support our continued operations and pursue our growth strategy. Until we can generate significant revenue from potential mRNA medicines, if ever, we expect to finance our operations through a combination of public or private equity offerings, structured financings and debt financings, government funding arrangements, strategic alliances and marketing, distribution and licensing arrangements. We may be unable to raise additional funds or enter into such other agreements on favorable terms, or at all. If we fail to raise capital or enter into such agreements as, and when, needed, we may have to significantly delay, scale back or discontinue the development and commercialization of one or more of our programs. We believe that our cash, cash equivalents, and investments as of December 31, 2019 will be sufficient to enable us to fund our projected operations through at least the next 12 months from the issuance of our financial statements. Because of the numerous risks and uncertainties associated with pharmaceutical development, we are unable to predict the timing or amount of increased expenses or when or if we will be able to achieve or maintain profitability. Even if we are able to generate revenues from the sale of our medicines, we may not become profitable. If we fail to become profitable or are unable to sustain profitability on a continuing basis, then we may be unable to continue our operations at planned levels and be forced to reduce our operations.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Basis of Presentation and Principles of Consolidation Our consolidated financial statements are prepared in accordance with U.S. generally accepted accounting principles (GAAP). Any reference in these notes to applicable guidance is meant to refer to the authoritative accounting principles generally accepted in the United States as found in the Accounting Standard Codification (ASC) and Accounting Standards Update (ASU) of the Financial Accounting Standards Board (FASB). The consolidated financial statements include the Company and its subsidiaries. All intercompany transactions and balances have been eliminated in consolidation. Use of Estimates We have made estimates and judgments affecting the amounts reported in our consolidated financial statements and the accompanying notes. On an ongoing basis, we evaluate our estimates, including critical accounting policies or estimates related to revenue recognition, research and development expenses, income tax provisions, stock-based compensation, leases, and useful lives of long-lived assets. We base our estimates on historical experience and various relevant assumptions that we believe to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. The actual results that we experience may differ materially from our estimates. Significant estimates relied upon in preparing these financial statements include, among others, those related to fair value of equity awards, revenue recognition, research and development expenses, leases, fair value of financial instruments, useful lives of property and equipment, income taxes, and our valuation allowance on our deferred tax assets. Segment Information We have determined that our chief executive officer is the chief operating decision maker (CODM). The CODM reviews financial information presented on a consolidated basis. Resource allocation decisions are made by the CODM based on consolidated results. There are no segment managers who are held accountable by the CODM for operations, operating results, and planning for levels or components below the consolidated unit level. As such, we have concluded that we operate as one segment. All our long-lived assets are located in the United States. Revenue Recognition On January 1, 2019, we adopted Accounting Standards Codification (ASC) Topic 606, Revenue from Contracts with Customers (ASC 606), using the modified retrospective transition method applied to those contracts which were not completed as of January 1, 2019. We recognized the cumulative effect of the adoption as an adjustment to the opening balance of accumulated deficit in the current period consolidated balance sheet. Results for reporting periods beginning after January 1, 2019 are presented under ASC 606, while prior period amounts have not been adjusted and continue to be reported in accordance with our historic accounting under ASC Topic 605, Revenue Recognition (ASC 605). ASC 606 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. Our revenue is primarily generated through collaboration arrangements and grants from government-sponsored and private organizations. Our collaboration arrangements typically contain multiple promises, including licenses to our intellectual property, options to obtain development and commercialization rights, research and development services, and obligations to develop and manufacture preclinical and clinical material. Such arrangements provide for various types of payments to us, including upfront payments, funding of research and development activities, funding for the purchase of preclinical and clinical material, development, regulatory and commercial milestone payments, licensing fees, option exercise payments, and royalties based on product sales. We have received grants from various government-sponsored and private organizations for research and related activities that provide for payments for reimbursed costs, which may include overhead and general and administrative costs as well as a related profit margin. We analyze our collaboration arrangements to assess whether they are within the scope of ASC Topic 808, Collaborative Arrangements (ASC 808) to determine whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards that are dependent on the commercial success of such activities. To the extent the arrangement is within the scope of ASC 808, we assess whether aspects of the arrangement between us and our collaboration partner are within the scope of other accounting literature. If we conclude that some or all aspects of the arrangement represent a transaction with a customer, we account for those aspects of the arrangement within the scope of ASC 606 . If we conclude that some or all aspects of the arrangement are within the scope of ASC 808 and do not represent a transaction with a customer, we recognize our allocation of the shared costs incurred with respect to the jointly conducted activities as a component of the related expense in the period incurred. Pursuant to ASC 606, a customer is a party that has contracted with an entity to obtain goods or services that are an output of the entity’s ordinary activities in exchange for consideration. 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. If we conclude a counter-party to a transaction is not a customer or otherwise not within the scope of ASC 606 or ASC 808, we consider the guidance in other accounting literature as applicable or by analogy to account for such transaction. We also consider the guidance in ASC 606 with respect to principal versus agent considerations, in determining the appropriate treatment for the transactions between us and the strategic collaborator and the transactions between us and other third parties. The classification of transactions under our arrangements is determined based on the nature and contractual terms of the arrangement along with the nature of the operations of the participants. Any consideration related to activities in which we are considered the principal, which includes being in control of the good or service before such good or service is transferred to the customer, are accounted for as gross revenue. We receive payments from our customers based on billing schedules established in each contract. Upfront payments and fees are recorded as contract liabilities upon receipt or when due and may require deferral of revenue recognition to a future period when we perform our obligations under the arrangement. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as current liabilities in our consolidated balance sheets. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as non-current liabilities in our consolidated balance sheets. Amounts payable to us are recorded as accounts receivable when our right to consideration is unconditional. We expense incremental costs of obtaining a contract as incurred if the expected amortization period of the asset that we would have recognized is one year or less or the amount is immaterial. As of December 31, 2019 , we had not capitalized any costs to obtain any of our contracts. Collaboration Revenue To determine the appropriate amount of revenue to be recognized for arrangements that we determine are within the scope of ASC 606, we perform the following steps: (i) identify the contract(s) with our 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 each performance obligation is satisfied. We account for a contract with a customer that is within the scope of ASC 606 when all of the following criteria are met: (i) the arrangement has been approved by the parties and the parties are committed to perform their respective obligations; (ii) each party’s rights regarding the goods and/or services to be transferred can be identified; (iii) the payment terms for the goods and/or services to be transferred can be identified; (iv) the arrangement has commercial substance; and (v) collection of substantially all of the consideration to which we will be entitled in exchange for the goods and/or services that will be transferred to the customer is probable. We also determine the term of the contract based on the period in which we and our customer have present and enforceable rights and obligations for purposes of identifying the performance obligations and determining the transaction price. We evaluate contracts that contain multiple promises to determine which promises are distinct. Promises are considered to be distinct and therefore, accounted for as separate performance obligations, provided that: (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer and (ii) the promise to transfer the good or service to the customer is separately identifiable from other promises in the contract. In assessing whether a promise is distinct, we consider factors such as whether: (i) we provide a significant service of integrating goods and/or services with other goods and/or services promised in the contract; (ii) one or more of the goods and/or services significantly modifies or customizes, or are significantly modified or customized by one or more of the other goods and/or services promised in the contract; and (iii) the goods and/or services are highly interdependent or highly interrelated. Individual goods or services (or bundles of goods and/or services) that meet both criteria for being distinct are accounted for as separate performance obligations. Promises that are not distinct at contract inception are combined and accounted for as a single performance obligation. Options to acquire additional goods and/or services are evaluated to determine if such option provides a material right to the customer that it would not have received without entering into the contract. If so, the option is accounted for as a separate performance obligation. If not, the option is considered a marketing offer which would be accounted for as a separate contract upon the customer’s election. The transaction price is generally comprised of an upfront payment due at contract inception and variable consideration in the form of payments for our services and materials and milestone payments due upon the achievement of specified events. Other payments the Company could be entitled to include tiered royalties earned when customers recognize net sales of licensed products. We consider the existence of any significant financing component within our arrangements and have determined that a significant financing component does not exist in our arrangements as substantive business purposes exist to support the payment structure other than to provide a significant benefit of financing. We measure the transaction price based on the amount of consideration to which we expect to be entitled in exchange for transferring the promised goods and/or services to the customer. We utilize either the expected value method or the most likely amount method to estimate the amount of variable consideration, depending on which method is expected to better predict the amount of consideration to which we will be entitled. Amounts of variable consideration are included in the transaction price to the extent that it is probable that a significant reversal in the amount of cumulative revenue recognized will not occur when the uncertainty associated with the variable consideration is subsequently resolved. With respect to arrangements that include payments for a development or regulatory milestone payment, we evaluate whether the associated event is considered probable of achievement and estimate the amount to be included in the transaction price using the most likely amount method. Milestone payments that are not within our control or the licensee, such as those dependent upon receipt of regulatory approval, are not considered to be probable of achievement until the triggering event occurs. At the end of each reporting period, we re-evaluate the probability of achievement of each milestone and any related constraint, and if necessary, adjust our estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenue and net loss in the period of adjustment. For arrangements that include sales-based royalties, including milestone payments based upon the achievement of a certain level of product sales, wherein the license is deemed to be the sole or predominant item to which the payments relate, we recognize revenue upon the later of: (i) when the related sales occur or (ii) when the performance obligation to which some or all of the payment has been allocated has been satisfied (or partially satisfied). Consideration that would be received for optional goods and/or services is excluded from the transaction price at contract inception. We generally allocate the transaction price to each performance obligation based on a relative standalone selling price basis. We develop assumptions that require judgment to determine the standalone selling price for each performance obligation in consideration of applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated research and development costs. However, in certain instances, we allocate variable consideration entirely to one or more performance obligation if the terms of the variable consideration relate to the satisfaction of the respective performance obligation and the amount allocated is consistent with the amount we would expect to receive for the satisfaction of the respective performance obligation. We recognize revenue based on the amount of the transaction price that is allocated to each respective performance obligation when or as the performance obligation is satisfied by transferring a promised good or service to the customer. For performance obligations that are satisfied at a point in time, we recognize revenue when control of the goods and/or services is transferred to the customer. For performance obligations that are satisfied over time, we recognize revenue by measuring the progress toward complete satisfaction of the performance obligation using a single method of measuring progress which depicts the performance in transferring control of the associated goods and/or services to the customer. We generally use input methods to measure the progress toward the complete satisfaction of performance obligations satisfied over time. With respect to arrangements containing a license to our intellectual property that is determined to be distinct from the other performance obligations identified in the arrangement, we recognize revenue from amounts allocated to the license when the license is transferred to the licensee and the licensee is able to use and benefit from the license. For licenses that are bundled with other promises, we utilize 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. Significant management judgment is required in determining the level of effort required under an arrangement and the period over which we are expected to complete our performance obligations under an arrangement. We evaluate the measure of progress each reporting period and, if necessary, adjust the measure of performance and related revenue recognition. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenue and net loss in the period of adjustment. Grant Revenue We have contracts with the U.S. government’s Defense Advanced Research Projects Agency (DARPA), Biomedical Advanced Research (BARDA), and the Bill & Melinda Gates Foundation (Gates Foundation). We recognize revenue from these contracts as we perform services under these arrangements when the funding is committed. Revenues and related expenses are presented gross in the consolidated statements of operations as we have determined we are the primary obligor under the arrangements relative to the research and development services we perform as lead technical expert. Cash and Cash Equivalents We consider all highly liquid investments with an original maturity of 90 days or less from the date of purchase to be cash equivalents. Restricted Cash Restricted cash is composed of amounts held on deposit related to our lease arrangements. The funds are maintained in money market accounts and are recorded at fair value. We classify our restricted cash as either current or non-current based on the terms of the underlying lease arrangement. Investments We invest our excess cash balances in marketable debt securities. We classify our investments in marketable debt securities as available-for-sale. We report available-for-sale investments at fair value at each balance sheet date, and include any unrealized holding gains and losses (the adjustment to fair value) in accumulated other comprehensive gain (loss), a component of stockholders’ equity. Realized gains and losses are determined using the specific-identification method, and are included in other (expense) income, net in our consolidated statements of operations. Should any adjustment to fair value reflect a decline in the value of the investment, we consider all available evidence to evaluate the extent to which the decline is “other than temporary” and, if so, we recognize the associated unrealized loss through a charge to our consolidated statement of operations. We did no t record any impairment charges related to our marketable securities during the years ended December 31, 2019 , 2018 and 2017 . We classify our available-for-sale marketable securities as current or non-current based on each instrument’s underlying effective maturity date and for which we have the intent and ability to hold the investment for a period of greater than 12 months. Marketable securities with maturities of less than 12 months are classified as current and are included in investments in the consolidated balance sheets. Marketable securities with maturities greater than 12 months for which we have the intent and ability to hold the investment for greater than 12 months are classified as non-current and are included in investments, non-current in the consolidated balance sheets. Accounts Receivable and Allowance for Doubtful Accounts Accounts receivable are amounts due from strategic collaborators as a result of manufacturing and research and development services provided under collaboration arrangements, or milestones achieved, but not yet paid. We also have accounts receivable amounts due from our grant agreements. Amounts payable to us are recorded as accounts receivable when our right to consideration is unconditional. To estimate the allowance for doubtful accounts, we make judgments about the creditworthiness of our customers based on ongoing credit evaluation and historical experience. There was no allowance for doubtful accounts at December 31, 2019 , and 2018 . There was no bad debt expense for the years ended December 31, 2019 , 2018 or 2017 . Concentrations of Credit Risk Financial instruments that subject us to significant concentrations of credit risk consist primarily of cash, cash equivalents, restricted cash, marketable securities, and accounts receivable. Our investment portfolio comprises money market funds, marketable debt securities, including U.S. Treasury securities, debt securities of U.S. government agencies and corporate entities and commercial paper. Our cash management and investment policy limits investment instruments to investment-grade securities with the objective to preserve capital and to maintain liquidity until the funds can be used in business operations. Bank accounts in the United States are insured by the Federal Deposit Insurance Corporation (FDIC) up to $250,000. Our primary operating accounts significantly exceed the FDIC limits. Significant Customers Our accounts receivable are generally unsecured and are from customers in different countries. We generated 80% , 91% and 86% of our revenue for the years ended December 31, 2019 , 2018 and 2017 , respectively, from strategic collaborators. The remaining 20% , 9% , 14% of our revenue for the years ended December 31, 2019 , 2018 and 2017 , respectively, were generated from grants made by government-sponsored and private organizations. A significant portion of our revenue to date has been generated from the following entities that accounted for more than 10% of total revenue and accounts receivable for the periods presented: Percentage of Revenue Years Ended December 31, Percentage of Accounts Receivable December 31, 2019 2018 2017 2019 2018 Merck 61 % 49 % 31 % 13 % 30 % BARDA 13 % * 10 % 54 % 13 % Vertex 10 % * * 17 % 22 % AstraZeneca * 34 % 15 % * * Alexion * * 36 % * * DARPA * * * * 16 % Massachusetts Life Sciences Center * * * * 12 % ________ * - Represents an amount of less than 10% Fair Value Measurements Fair value is defined as the price that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. When determining the fair value measurements for assets and liabilities, which are required to be recorded at fair value, we consider the principal or most advantageous market in which we would transact and the market-based risk measurements or assumptions that market participants would use in pricing the asset or liability, such as risks inherent in valuation techniques, transfer restrictions and credit risk. FASB ASC Topic 820, Fair Value Measurement (ASC 820), establishes a fair value hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and our assumptions (unobservable inputs). Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from our independent sources. Unobservable inputs are inputs that reflect our assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances. The following fair value hierarchy is used to classify assets and liabilities based on the observable inputs and unobservable inputs used to value the assets and liabilities: • Level 1: Unadjusted quoted prices in active markets that are accessible at the measurement date for identical, unrestricted assets or liabilities; • Level 2: Quoted prices for similar assets and liabilities in active markets, quoted prices in markets that are not active, or inputs which are observable, either directly or indirectly, for substantially the full term of the asset or liability; or • Level 3: Prices or valuation techniques that require inputs that are both significant to the fair value measurement and unobservable (i.e., supported by little or no market activity). To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. Our cash equivalents and marketable securities are reported at fair value determined using Level 1 and Level 2 inputs (Note 5). We do not have any non-financial assets or liabilities that should be recognized or disclosed at fair value on a recurring basis at December 31, 2019 , 2018 and 2017 . As of December 31, 2019 and 2018 , we maintain letters of credit of $11.8 million and $12.1 million , respectively, related to our lease arrangements, which are secured by money market accounts in accordance with certain of our lease agreements. The amounts are recorded at fair value using Level 1 inputs and included as restricted cash in our consolidated balance sheets. Construction in Progress Construction in progress includes direct costs related to the construction of various property and equipment, including leasehold improvements, and is stated at original cost. Construction in progress includes costs incurred under construction contracts including project management services, engineering services, design services and development, construction services and other construction-related fees and services. Such costs are not depreciated until the asset is completed and placed into service. Once the asset is placed into service, these capitalized costs will be allocated to certain property and equipment categories and will be depreciated over the estimated useful life of the underlying assets. Property and Equipment Property and equipment are stated at cost, net of accumulated depreciation. Depreciation is calculated using the straight-line method over the estimated useful lives of the assets. The estimated useful lives of property and equipment are described below: Estimated Useful Life Laboratory equipment 5 years Leasehold improvements Lesser of estimated useful life of improvement or remaining life of related lease Computer equipment and software 3 years Other assets including automobiles, furniture and fixtures 5 years Right of use asset, financing Lease term Expenditures for maintenance and repairs are charged to expense as incurred. Upon retirement or sale, the cost of the assets disposed of, and the related accumulated depreciation, are removed from the accounts, and any resulting gain or loss is recorded to other (expense) income, net. Impairment of Long-Lived Assets We evaluate our long-lived assets, which consist of property and equipment, to determine if facts and circumstances indicate that the carrying amount of assets may not be recoverable. If such facts and circumstances exist, we assess the recoverability of the long-lived assets by comparing the projected future undiscounted net cash flows associated with the related asset or group of assets over their remaining lives against their respective carrying amounts. If such review indicates that such cash flows are not expected to be sufficient to recover the recorded value of the assets, the assets are written down to their estimated fair values based on the expected discounted future cash flows attributable to the assets or based on appraisals. For the years ended December 31, 2019 , 2018 and 2017 , we did no t record any impairment expenses. Leases Leases are classified at their commencement date, which is defined as the date on which the lessor makes the underlying asset available for use by the lessee, as either operating or finance leases based on the economic substance of the agreement. We recognize lease right-of-use assets and related liabilities in our consolidated balance sheets for both operating and finance leases. Lease liabilities are measured at the lease commencement date as the present value of the future lease payments using the interest rate implicit in the lease. If the rate implicit is not readily determinable, we will utilize our incremental borrowing rate as of the lease commencement date. Lease right-of-use assets are measured as the lease liability plus initial direct costs and prepaid lease payments less lease incentives. The lease term is the non-cancelable period of the lease and includes options to extend or terminate the lease when it is reasonably certain that an option will be exercised. We recognize operating lease cost in operating expense in our consolidated statements of operations, inclusive of rent escalation provisions and rent holidays, on a straight-line basis over the respective lease term. For our finance leases, we recognize depreciation expense associated with the leased asset acquired and recognize interest expense related to the portion of the financing in our consolidated statements of operations. Additionally, we recognize tenant improvement allowances as a reduction to rent expense on a straight-line basis over the respective lease term. Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries and benefits, facilities costs, overhead costs, contract services, and other outside costs. The value of goods and services received from contract research organizations and contract manufacturing organizations in the reporting period are estimated based on the level of services performed, and progress in the period in cases when we have not received an invoice from the supplier. Patent Costs Costs to secure, defend and maintain patents are expensed as incurred, and are classified as general and administrative expenses due to the uncertainty of future benefits. Stock-Based Compensation We issue stock-based awards to employees and non-employees, generally in the form of stock options and restricted stock units (RSUs). We account for our stock-based compensation awards in accordance with ASC 718, Compensation—Stock Compensation . Most of our stock-based awards have been made to employees. We measure compensation cost for all equity awards at their grant-date fair value and recognize compensation expense over the requisite service period, which is generally the vesting period, on a straight-line basis. The grant date fair value of stock options is estimated using the Black-Scholes option pricing model, which requires management to make assumptions with respect to the fair value of our common stock on the grant date, including the expected term of the award, the expected volatility of our stock, calculated based on a period of time generally commensurate with the expected term of the award, risk-free interest rates and expected dividend yields of our stock. Historically, for periods prior to our IPO, the fair value of the shares of common stock and common units underlying our stock-based awards were determined on each grant date by our board of directors based on valuation estimates from management considering our most recently available independent third-party valuation of our common stock. Our board of directors also assessed and considered, with input from management, additional objective and subjective factors that we believed were relevant and which may have changed from the date of the most recent valuation through the grant date. The grant date fair value of RSUs is estimated based on the fair value of our underlying common stock. For performance-based stock awards, we recognize stock-based compensation expense over the requisite service period using the accelerated attribution method when achievement is probable. We classify stock-based compensation expense in our consolidated statement of operations in the same manner in which the award recipient’s salary and related costs are classified or in which the award recipient’s service payments are classified. Income Taxes We use an asset and liability approach to account for income taxes. We recognize deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the financial reporting and tax bases of assets and liabilities. These differences are measured using the enacted statutory tax rates that are expected to be in effect for the years in which differences are expected to reverse. Valuation allowances are provided when the expected realization of deferred tax assets does not meet a “more likely than not” criterion. We make estimates and judgments about our future taxable income that are based on assumptions that are consistent with our plans and estimates. Should the actual amounts

Collaboration Agreements

Collaboration Agreements	12 Months Ended
	Dec. 31, 2019
Research and Development [Abstract]
Collaboration Agreements	Collaboration Agreements The following table summarizes our total consolidated net revenue from our strategic collaborators for the periods presented (in thousands): Years Ended Collaboration Revenue by Strategic Collaborator: December 31, 2019 December 31, 2019 December 31, 2018 Merck $ 36,608 $ 38,977 $ 66,082 AstraZeneca 5,233 37,235 45,993 Vertex 6,195 6,593 10,437 Total collaboration revenue $ 48,036 $ 82,805 $ 122,512 The following table presents changes in the balances of our receivables and contract liabilities related to our strategic collaboration agreements during the year ended December 31, 2019 (in thousands): January 1, 2019 Additions Deductions December 31, 2019 Contract Assets: Accounts receivable $ 4,612 $ 11,890 $ (14,530 ) $ 1,972 Contract Liabilities: Deferred revenue $ 240,924 $ 10,591 $ (51,987 ) $ 199,528 During the year ended December 31, 2019, we recognized the following revenue as a result of the change in the contract liability balances related to our collaboration agreements (in thousands): Revenue recognized in the period from: Year Ended December 31, 2019 Amounts included in contract liabilities at the beginning of the period (1) $ 51,987 (1) We first allocate revenue to the individual contract liability balance outstanding at the beginning of the period until the revenue exceeds that balance. If additional consideration is received on those contracts in subsequent periods, we assume all revenue recognized in the reporting period first applies to the beginning contract liability. As of December 31, 2019 , the aggregated amount of the transaction price allocated to performance obligations under our collaboration agreements that are unsatisfied or partially unsatisfied was $249.5 million . AstraZeneca – Strategic Alliances in Cardiovascular and Oncology 2013 Option Agreement and Services and Collaboration Agreement In March 2013, we entered into an Option Agreement, the AZ Option Agreement, and a related Services and Collaboration Agreement, the AZ Services Agreement, with AstraZeneca, which were amended and restated in June 2018. We refer to these agreements in the forms that existed prior to the 2018 amendment and restatement as the 2013 AZ Agreements. Under the 2013 AZ Agreements, we granted AstraZeneca certain exclusive rights and licenses, and options to obtain exclusive rights to develop and commercialize potential therapeutic mRNA medicines directed at certain targets for the treatment of cardiovascular and cardiometabolic diseases and cancer, and agreed to provide related services to AstraZeneca. Pursuant to the 2013 AZ Agreements, AstraZeneca was responsible for all research, development and commercialization activities, while we provided specified research and manufacturing services during a research and evaluation period, as described below, to further AstraZeneca’s activities pursuant to an agreed upon services plan. Under the 2013 AZ Agreements, AstraZeneca could have requested we provide additional services, at AstraZeneca’s expense. Subject to customary “back-up” supply rights granted to AstraZeneca, we exclusively manufactured (or had manufactured) mRNA for all research, development and commercialization purposes under the 2013 AZ Agreements until, on a product-by-product basis, the expiration of the time period for which we are entitled to receive earn-out payments with respect to such product pursuant to the 2013 AZ Agreements. As of the effective date of the 2013 AZ Agreements, AstraZeneca acquired forty options that it may exercise to obtain exclusive rights to clinically develop and commercialize identified development candidates (and related back-up candidates) directed to specified targets that arise during the research and evaluation period. During the research and evaluation period for research candidates under the 2013 AZ Agreements, AstraZeneca could have elected to designate a limited number of research candidates as development candidates in order to continue preclinical development on such development candidates (and related back-up candidates). From such pool of development candidates designated by AstraZeneca, during a specified option exercise period, AstraZeneca could have then exercised one of its options to obtain exclusive rights to clinically develop and commercialize an identified development candidate (and related back-up candidates). If AstraZeneca did not exercise one of its options to acquire exclusive rights to clinically develop and commercialize a particular development candidate during the defined option exercise period for such development candidate, AstraZeneca’s rights to exercise an option and other rights granted under the 2013 AZ Agreements with respect to such development candidate (and related back-up candidates) would terminate, all rights to exploit such development candidate (and related back-up candidates) would be returned to us and all data and results generated by AstraZeneca with respect to such development candidate (and related back-up candidates) would be either assigned or licensed to us. Upon the earlier of termination of the 2013 AZ Agreements for any reason and a specified anniversary of the effective date of the 2013 AZ Agreements, all unexercised options, and the right to exercise any and all options if not previously exercised by AstraZeneca, would automatically terminate. On a target-by-target basis, we and AstraZeneca agreed to certain defined exclusivity obligations under the 2013 AZ Agreements with respect to the research, development and commercialization of mRNA medicines for such target. As of the effective date of the 2013 AZ Agreements, AstraZeneca made upfront cash payments to us totaling $240.0 million . Under the 2013 AZ Agreements, we were entitled to receive payments that are not related to any specific program of up to $180.0 million in the aggregate for the achievement of three technical milestones relating to toxicity, delivery, and competition criteria. We achieved the toxicity and competition milestones in the year ended December 31, 2015. The delivery milestone has expired. Under the 2013 AZ Agreements, AstraZeneca was obligated to pay us a $10.0 million option exercise fee with respect to each development candidate (and related back-up candidates) for which it exercised an option. In addition, upon AstraZeneca’s exercise of each option, we were eligible to receive certain payments contingent upon the achievement of specified clinical, regulatory, and commercial events. For any product candidate optioned by AstraZeneca, we were eligible to receive, per product candidate, up to $100.0 million in payments for achievement of development milestones, up to $100.0 million payments for achievement of regulatory milestones, and up to $200.0 million payments for achievement of commercial milestones. Additionally, under the 2013 AZ Agreements, we were entitled to receive, on a product-by-product basis, earn-out payments on worldwide net sales of products ranging from a high-single digit percentage to 12% , subject to certain reductions, with an aggregate minimum floor. We received from AstraZeneca under the 2013 AZ Agreements an option exercise payment of $10.0 million (the 2016 VEGF Exercise) in the year ended December 31, 2016, and a clinical milestone payment of $30.0 million with respect to AstraZeneca’s VEGF-A product (AZD8601) during the year ended December 31, 2018, that is currently being developed in a Phase 2 clinical trial. Unless earlier terminated, the 2013 AZ Agreements would have continued until the expiration of AstraZeneca’s earn-out and contingent option exercise payment obligations for optioned product candidates. Either party had the right to terminate the 2013 AZ Agreements upon the other party’s material breach, either in its entirety or in certain circumstances, with respect to relevant candidates, subject to a defined materiality threshold and specified notice and cure provisions. If AstraZeneca had the right to terminate the 2013 AZ Agreements for our material breach, then AstraZeneca could have elected, in lieu of terminating the 2013 AZ Agreements, in their entirety or with respect to such candidates, to have the 2013 AZ Agreements remain in effect, subject to reductions in certain payments we were eligible to receive and certain adjustments to AstraZeneca’s obligations under the 2013 AZ Agreements. AstraZeneca had the right to terminate the 2013 AZ Agreements in full, without cause, upon 90 -days’ prior notice to us. 2016 Strategic Alliance with AstraZeneca – IL-12 In January 2016, we entered into a new Strategic Drug Development Collaboration and License Agreement, which we refer to as the 2016 AZ Agreement, with AstraZeneca to discover, develop and commercialize potential mRNA medicines for the treatment of a range of cancers. Under the terms of the 2016 AZ Agreement, we and AstraZeneca have agreed to work together on an immuno-oncology program focused on the intratumoral delivery of a potential mRNA medicine to make the IL-12 protein. The 2016 AZ Agreement initially included research activities with respect to a second discovery program. During a limited period of time, each party had an opportunity to propose additional discovery programs to be conducted under the 2016 AZ Agreement. We are responsible for conducting and funding all discovery and preclinical development activities under the 2016 AZ Agreement in accordance with an agreed upon discovery program plan for the IL-12 program and any other discovery program the parties agree to conduct under the 2016 AZ Agreement. For the IL-12 program and any other discovery program the parties agree to conduct under the 2016 AZ Agreement, during a defined election period that commenced as of the effective date of the 2016 AZ Agreement (for the IL-12 program) and otherwise will commence on initiation of any such new discovery program, AstraZeneca may elect to participate in the clinical development of a development candidate arising under the 2016 AZ Agreement from such program. If AstraZeneca so elects (as it has for the IL-12 program), AstraZeneca will lead clinical development activities worldwide and we will be responsible for certain activities, including being solely responsible for manufacturing activities, all in accordance with an agreed upon development plan. AstraZeneca will be responsible for funding all Phase 1 clinical development activities (including costs associated with our manufacture of clinical materials in accordance with the development plan), and Phase 2 clinical development activities (including costs associated with our manufacture of clinical materials in accordance with the development plan) up to a defined dollar threshold. We and AstraZeneca will equally share the costs of Phase 2 clinical development activities in excess of such dollar threshold, all Phase 3 clinical development activities and certain other costs of late-stage clinical development activities, unless we elect not to participate in further development and commercialization activities and instead receive tiered royalties, as described below. We and AstraZeneca will co-commercialize products in the U.S. in accordance with an agreed upon commercialization plan and budget, and on a product-by-product basis will equally share the U.S. profits or losses arising from such commercialization. Notwithstanding, on a product-by-product basis, prior to a specified stage of development of a given product, we have the right to elect not to participate in the further development and commercialization activities for such product. If we make such election, instead of participating in the U.S. profits and losses share with respect to such product, we are obligated to discuss future financial terms with AstraZeneca. If we are unable to agree on future financial terms within a short, defined period of time, we are entitled to receive tiered royalties at default rates set forth in the 2016 AZ Agreement, ranging from percentages in the mid-single digits to 20% on worldwide net sales of products, subject to certain reductions with an aggregate minimum floor. AstraZeneca has sole and exclusive responsibility for all ex-U.S. commercialization efforts. Unless we have elected to not to participate in further development (in which case royalties on ex-U.S. net sales will be at the default rates as described above, unless otherwise agreed by the parties), we are entitled to tiered royalties at rates ranging from 10% to 30% on ex-U.S. net sales of the products, subject to certain reductions with an aggregate minimum floor. Subject to customary “back-up” supply rights granted to AstraZeneca, we exclusively manufacture (or have manufactured) products for all development and commercialization purposes. We and AstraZeneca have agreed to certain defined exclusivity obligations with each other under the 2016 AZ Agreement with respect to the development and commercialization of mRNA medicines for IL-12. Unless earlier terminated, our strategic alliance under the 2016 AZ Agreement will continue on a product-by-product basis (i) until both parties cease developing and commercializing such product without the intention to resume, if we have not elected our right not to participate in further development and commercialization of such product or (ii) on a country-by-country basis, until the end of the applicable royalty term for such product in such country, if we have elected our right not to participate in further development and commercialization of such product. Either party may terminate the 2016 AZ Agreement upon the other party’s material breach, subject to specified notice and cure provisions. Each party may also terminate the 2016 AZ Agreement in the event the other party challenges such party’s patent rights, subject to certain defined exceptions. AstraZeneca has the right to terminate the 2016 AZ Agreement in full or with respect to any program for scientific, technical, regulatory or commercial reasons at any time upon 90 days’ prior written notice to us. On a product-by-product basis, we have the right to terminate the 2016 AZ Agreement in certain cases if AstraZeneca has suspended or is no longer proceeding with the development or commercialization of such product for a period of twelve consecutive months, subject to specified exceptions, including tolling for events outside of AstraZeneca’s control. On a product-by-product basis, if the 2016 AZ Agreement is terminated with respect to a given product, AstraZeneca’s rights in such product will terminate and, to the extent we terminated for AstraZeneca’s breach, patent challenge or cessation of development or AstraZeneca terminated in its discretion, AstraZeneca will grant us reversion licenses and take certain other actions so as to enable us to continue developing and commercializing such product in the oncology field. If we continue developing and commercializing a given product following termination of the 2016 AZ Agreement by AstraZeneca in its discretion with respect to such product, AstraZeneca is entitled to receive a mid-single digit royalty on our worldwide net sales of such product and a high-single digit percentage of the amounts received by us from a third party in consideration of a license to such third party to exploit such product, in each case, until AstraZeneca recovers an amount equal to specified development costs incurred by AstraZeneca under the 2016 AZ Agreement with respect to such product prior to such termination. Such percentages increase by a low to mid-single digit amount to the extent such termination occurs after such product achieves a specified stage of development. 2017 Strategic Alliance with AstraZeneca – Relaxin In October 2017, we entered a new Collaboration and License Agreement, which we refer to as the 2017 AZ Agreement, under which AstraZeneca may clinically develop and commercialize a development candidate, now known as AZD7970, which is comprised of an mRNA construct for the relaxin protein designed by us and encapsulated in one of our proprietary lipid nonoparticles (LNP). We discovered and performed preclinical development activities for AZD7970 prior to the initiation of the strategic alliance with AstraZeneca under the 2017 AZ Agreement. Under the terms of the 2017 AZ Agreement, we will fund and be responsible for conducting preclinical development activities for AZD7970 through completion of IND-enabling GLP toxicology studies and AstraZeneca will lead pharmacological studies, each in accordance with an agreed upon discovery program plan. During a defined election period that commences as of the effective date of the 2017 AZ Agreement, AstraZeneca may elect to participate in further development and commercialization of AZD7970. Upon such election, AstraZeneca will lead clinical development activities for AZD7970 worldwide and we will be responsible for manufacturing AZD7970, certain regulatory matters and any other development activities that we agree to perform and that are set forth in an agreed upon development plan. AstraZeneca will be responsible for funding Phase 1 clinical development activities (including costs associated with our manufacture of clinical materials in accordance with the development plan, up to a cap above which such costs are shared), and Phase 2 clinical development activities (including costs associated with our manufacture of clinical materials in accordance with the development plan, up to a cap above which such costs are shared) up to a defined dollar threshold. Thereafter, we and AstraZeneca will equally share the costs of Phase 2 clinical development activities in excess of such defined dollar threshold, all Phase 3 clinical development activities and certain other costs of late-stage clinical development activities, unless we elect not to participate in further development and co-commercialization activities and instead receive tiered royalties as described below. If the development candidate is determined to be IND-ready, and AstraZeneca does not timely elect to participate in the clinical development of AZD7970, AstraZeneca is obligated to reimburse us for certain costs we incurred in the manufacture and development of AZD7970, since execution of the 2017 AZ Agreement. We and AstraZeneca will co-commercialize AZD7970 in the United States in accordance with an agreed upon commercialization plan and budget, and will equally share U.S. profits or losses arising from such commercialization. Notwithstanding, prior to a specified stage of development of AZD7970, we have the right to elect not to participate in the further development and commercialization activities for AZD7970. If we make such election, instead of participating in the U.S. profits and losses share with respect to AZD7970, we are obligated to discuss future financial terms with AstraZeneca. If we are unable to agree on future financial terms within a short, defined period of time, we are entitled to receive tiered royalties at default rates set forth in the 2017 AZ Agreement, ranging from percentages in the mid-single digits to the low 20s on worldwide net sales by AstraZeneca of AZD7970, subject to certain reductions, with an aggregate minimum floor. AstraZeneca has sole and exclusive responsibility for all ex-U.S. commercialization efforts. Unless we have elected not to participate in further development (in which case royalties on ex-U.S. net sales will be at the default rates as described above, unless otherwise agreed by the parties), we are entitled to receive tiered royalties at rates ranging from 10% to 30% on annual ex-U.S. net sales of AZD7970, subject to certain reductions with an aggregate minimum floor. Subject to customary “back-up” supply rights granted to AstraZeneca, we exclusively manufacture (or have manufactured) products for all development and commercialization purposes. Additionally, we and AstraZeneca have agreed to certain defined exclusivity obligations under the 2017 AZ Agreement with respect to the development and commercialization of mRNA medicines for Relaxin. Unless earlier terminated, our strategic alliance under the 2017 AZ Agreement will continue (i) until the expiration of AstraZeneca’s election period, if it does not elect to participate in the clinical development of AZD7970, (ii) until both parties cease developing and commercializing AZD7970 without the intention to resume, if we have not elected our right not to participate in further development and commercialization of AZD7970, (iii) on a country-by-country basis, until the end of the applicable royalty term for AZD7970 in such country, if we have elected our right not to participate in further development and commercialization of AZD7970 or (iv) following completion of IND-enabling studies with respect to AZD7970, if we provide AstraZeneca with written notice that we do not reasonably believe that the product is IND-ready. Either party may terminate the 2017 AZ Agreement upon the other party’s material breach, subject to specified notice and cure provisions. Each party may also terminate the 2017 AZ Agreement in the event the other party challenges the validity or enforceability of such party’s patent rights, subject to certain defined exceptions. AstraZeneca has the right to terminate the 2017 AZ Agreement in full for scientific, technical, regulatory or commercial reasons at any time upon 90 days’ prior written notice to us. We have the right to terminate the 2017 AZ Agreement in certain cases if AstraZeneca has suspended or is no longer proceeding with the development or commercialization of AZD7970 for a period of twelve consecutive months, subject to specified exceptions, including tolling for events outside of AstraZeneca’s control. If AstraZeneca does not timely elect to participate in clinical development of AZD7970, or the Agreement is terminated, AstraZeneca’s rights in AZD7970 will terminate and, to the extent we terminated for AstraZeneca’s breach, patent challenge or cessation of development or AstraZeneca terminated in its discretion, AstraZeneca will grant us reversion licenses and take certain other actions so as to enable us to continue developing and commercializing AZD7970 in the cardiovascular and cardiometabolic fields. If we continue developing and commercializing AZD7970 following a termination of the 2017 AZ Agreement by AstraZeneca in its discretion, AstraZeneca is entitled to receive a mid-single digit royalty on our worldwide net sales of AZD7970 and a high-single digit percentage of the amounts received by us from a third party in consideration for a license to such third party to exploit AZD7970, in each case until AstraZeneca recovers an amount equal to specified development costs incurred by AstraZeneca under the 2017 AZ Agreement with respect to AZD7970 prior to such termination. Such percentages increase by a low to mid-single digit amount to the extent such termination occurs after such product achieves a specified stage of development. 2013 Agreements with AstraZeneca, amended and restated in 2018 In June 2018, we entered into an Amended and Restated Option Agreement and a related Amended and Restated Services and Collaboration Agreement with AstraZeneca, or the 2018 A&R Agreements, which amended and restated the 2013 AZ Agreements. Under the 2018 A&R Agreements, we granted AstraZeneca certain exclusive rights and licenses to research, develop and commercialize potential therapeutic mRNA medicines directed at certain targets for the treatment of cardiovascular and cardiometabolic diseases and cancer, and agreed to provide related services to AstraZeneca. The activities to be performed by the parties under the 2018 A&R Agreements are limited to defined biological targets in the cardiovascular and cardiometabolic fields and one defined target in the cancer field. Pursuant to the 2018 A&R Agreements, AstraZeneca is responsible for all research, development and commercialization activities and associated costs, while we provide specified research and manufacturing services during a research and evaluation period, as described below, to further AstraZeneca’s activities conducted pursuant to an agreed upon services plan. During this research and evaluation period, these research services, and manufacturing services in excess of a specified threshold, are provided at AstraZeneca’s expense, and manufacturing services below the specified threshold are provided at no additional expense to AstraZeneca. AstraZeneca may request we provide additional research and manufacturing services, at AstraZeneca’s expense, following the end of the research and evaluation period. Subject to customary “back-up” supply rights granted to AstraZeneca, we exclusively manufacture (or have manufactured) mRNA for all research, development and commercialization purposes under the 2018 A&R Agreements until, on a product-by-product basis, the expiration of the time period for which we are entitled to receive earn-out payments with respect to such product pursuant to the 2018 A&R Agreements. As of the effective date of the 2013 AZ Agreements, and as further reflected in the 2018 A&R Agreements, AstraZeneca acquired forty options that it may exercise to obtain exclusive rights to clinically develop and commercialize identified development candidates (and related back-up candidates) directed to specified targets that arise during the research and evaluation period. During the research and evaluation period for research candidates, AstraZeneca may elect to designate a limited number of research candidates as development candidates in order to continue preclinical development on such development candidates (and related back-up candidates). From such pool of development candidates designated by AstraZeneca, during a specified option exercise period, AstraZeneca may then exercise one of its options to obtain exclusive rights to clinically develop and commercialize an identified development candidate (and related back-up candidates) in certain fields. If AstraZeneca does not exercise one of its options to acquire exclusive rights to clinically develop and commercialize a particular development candidate during the defined option exercise period for such development candidate, AstraZeneca’s rights to exercise an option and other rights granted under the 2018 A&R Agreements with respect to such development candidate (and related back-up candidates) will terminate, all rights to exploit such development candidate (and related back-up candidates) will be returned to us and all data and results generated by AstraZeneca with respect to such development candidate (and related back-up candidates) will be either assigned or licensed to us. Upon the earlier of termination of the 2018 A&R Agreements for any reason and a specified anniversary of the effective date of the 2013 AZ Agreements, all unexercised options, and the right to exercise any and all options if not previously exercised by AstraZeneca, will automatically terminate. On a target-by-target basis, we and AstraZeneca have agreed to certain defined exclusivity obligations under the 2018 A&R Agreements with respect to the research, development and commercialization of mRNA medicines for such target in certain fields. In addition, we and AstraZeneca have agreed to certain defined exclusivity obligations with respect to the research, development and commercialization of mRNA medicines coding for the same polypeptide as any development candidate being developed under the 2018 A&R Agreements. Unless earlier terminated, the 2018 A&R Agreements will continue until the expiration of AstraZeneca’s earn-out and contingent option exercise payment obligations for optioned product candidates. Either party may terminate the 2018 A&R Agreements upon the other party’s material breach, either in its entirety or in certain circumstances, with respect to relevant candidates, subject to a defined materiality threshold and specified notice and cure provisions. If AstraZeneca has the right to terminate the 2018 A&R Agreements for our material breach, then AstraZeneca may elect, in lieu of terminating the 2018 A&R Agreements, in their entirety or with respect to such candidates, to have the 2018 A&R Agreements remain in effect, subject to reductions in certain payments we are eligible to receive and certain adjustments to AstraZeneca’s obligations under the 2018 A&R Agreements. AstraZeneca may terminate the 2018 A&R Agreements in full, without cause, upon 90 days’ prior notice to us. Accounting Treatment For periods prior to January 1, 2019, we applied the provisions of ASC 605 in accounting for these arrangements, except for the 2017 AZ Agreement which was accounted for under ASC 808. In August 2016, AstraZeneca exercised a product option available pursuant to the 2013 AZ Agreements to obtain exclusive rights to clinically develop and commercialize the VEGF-A product (AZD8601). This option exercise is referred to as the 2016 VEGF Exercise. Consistent with our conclusions under ASC 605 and pursuant to ASC 606, we determined that the 2016 VEGF Exercise and the 2017 AZ Agreement should be accounted for as separate transactions as the agreements are not interrelated or interdependent. Conversely, the 2013 Agreements, as amended by the 2018 A&R Agreements, and the 2016 AZ Agreement, were combined for accounting purposes and treated as a single agreement, as these agreements were negotiated in contemplation of each other. As of the date of our initial application of ASC 606, we applied the practical expedient to include the aggregate effect of all modifications to the arrangement that occurred before January 1, 2019 with respect to (i) identification of the satisfied and unsatisfied performance obligations, (ii) determination of the transaction price and (iii) allocation of the transaction price to the satisfied and unsatisfied performance obligations. Therefore, we aggregated effects of all the modifications prior to January 1, 2019 to the 2013 Agreements, including the effects of the 2018 A&R Agreements, and the 2016 AZ Agreement were combined into one transaction for accounting purposes. We will refer to this combined transaction as the Combined 2018 AZ Agreements. We determined that all aspects of Combined 2018 AZ Agreements and the 2016 VEGF Exercise represent a transaction with a customer and therefore is accounted for in accordance with ASC 606 as of the date of the initial application. Combined 2018 AZ Agreements We identified the following performance obligations in the Combined 2018 AZ Agreements: (i) a combined performance obligation that includes a research license, research and development pool services, and manufacturing obligations related to the 2013 AZ Agreements, as amended by the 2018 A&R Agreements, collectively referred to as the Combined 2018 AZ Agreement Performance Obligation, (ii) preclinical development services for IL-12, (iii) preclinical development services for an oncology development target, (iv) a combined performance obligation for a development and commercialization license and manufacturing obligations for IL-12, and (v) a material right to receive development and commercialization rights and manufacturing services for an oncology development target. We concluded that the research license is not distinct from the research and development pool services or the manufacturing obligations related to the 2018 A&R Agreements, as AstraZeneca cannot fully exploit the value of the research license without receipt of such services and supply. Our services and supply involve specialized expertise, particularly as it relates to mRNA technology that is not available in the marketplace. Any supply requested by AstraZeneca in excess of the minimum quantities specified in the agreement are considered customer options and treated as separate contracts for accounting purposes. Further, we concluded that AstraZeneca cannot exploit the value of the development and commercialization license for IL-12 without receipt of supply as the development and commercialization license does not convey to AstraZeneca the right to manufacture and therefore combined the development and commercialization license and the manufacturing obligations for IL-12 into one performance obligation. As of January 1, 2019, the date of initial adoption of ASC 606, the total transaction price was determined to be $400.0 million comprised of the $240.0 million in upfront payments pertaining to the 2013 AZ Agreements and $160.0 million of variable consideration comprised of $40.0 million of estimated reimbursement for IL-12 manufacturing obligations and $120.0 million of milestone payments ( $60.0 mill

Grants

Grants	12 Months Ended
	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
Grants	Grants Biomedical Advanced Research and Development Authority (BARDA) In September 2016, we received an award of up to $125.8 million under Agreement No. HHSO100201600029C from BARDA, a component of the Office of the Assistant Secretary for Preparedness and Response, or ASPR within the U.S. Department of Health and Human Services, or HHS, to help fund our Zika vaccine program. Under the terms of the agreement with BARDA, an initial base award of $8.2 million supported toxicology studies, a Phase 1 clinical trial, and associated manufacturing activities. Contract options were available, for $117.6 million to support an additional Phase 1 study of an improved Zika vaccine candidate, Phase 2 and Phase 3 clinical studies, as well as large-scale manufacturing for the Zika vaccine. As of December 31, 2019 , three of the four contract options had been exercised resulting in $117.3 million of available funding with an additional $8.5 million available if the final contract option is exercised. For the years ended December 31, 2019 , 2018 and 2017 , we recognized revenue of $7.6 million , $6.8 million and $20.1 million , respectively, relating to the BARDA Agreement. The Bill & Melinda Gates Foundation In January 2016, we entered a global health project framework agreement with the Gates Foundation to advance mRNA-based development projects for various infectious diseases. The Gates Foundation has committed up to $20.0 million in grant funding to support our initial project related to the evaluation of antibody combinations in a preclinical setting as well as the conduct of a first-in-human Phase 1 clinical trial of a potential mRNA medicine to help prevent human immunodeficiency virus, or HIV, infections. Follow-on projects which could bring total potential funding under the framework agreement up to $100.0 million (including the HIV antibody project) to support the development of additional mRNA-based projects for various infectious diseases can be proposed and approved until the sixth anniversary of the framework agreement, subject to the terms of the framework agreement, including our obligation to grant to the Gates Foundation certain non-exclusive licenses. In March 2019, the Gates Foundation provided an additional funding commitment up to $1.1 million to support a follow-on project. As of December 31, 2019 , up to $21.1 million has been committed for funding with up to an additional $80.0 million available, if additional follow-on projects are approved. For the years ended December 31, 2019 , 2018 and 2017 , we recognized $3.4 million , $1.4 million , and $1.1 million , respectively, related to the Gates Foundation agreement. Deferred revenue of $1.5 million and $0.8 million was recorded as of December 31, 2019 , and 2018 , respectively, related to the Gates Foundation agreement. Defense Advanced Research Projects Agency (DARPA) In October 2013, DARPA awarded us up to $24.6 million under Agreement No. W911NF-13-1-0417, which was subsequently adjusted to $19.7 million , to research and develop potential mRNA medicines as a part of DARPA’s Autonomous Diagnostics to Enable Prevention and Therapeutics, or ADEPT, program, which is focused on assisting with the development of technologies to rapidly identify and respond to threats posed by natural and engineered diseases and toxins. The DARPA awards have been deployed primarily in support of our vaccine and antibody programs to protect against chikungunya infection. As of December 31, 2019 and 2018 , $19.7 million has been committed. We recognized $0.5 million , $3.5 million , $7.7 million , for the years ended December 31, 2019 , 2018 and 2017 , respectively, related to the DARPA agreement.

Financial Instruments

Financial Instruments	12 Months Ended
	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Financial Instruments	Financial Instruments Cash and Cash Equivalents and Investments The following tables summarize our cash and available-for-sale securities by significant investment category at December 31, 2019 and 2018 (in thousands): December 31, 2019 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash and Cash Equivalents Current Marketable Securities Non- Current Marketable Securities Cash and cash equivalents $ 225,874 $ — $ — $ 225,874 $ 225,874 $ — $ — Available-for-sale: Level 2: Certificates of deposit 82,028 79 (6 ) 82,101 10,002 69,197 2,902 U.S. treasury securities 117,891 260 (2 ) 118,149 — 110,186 7,963 Debt securities of U.S. government agencies and corporate entities 834,187 2,708 (32 ) 836,863 — 687,741 149,122 $ 1,259,980 $ 3,047 $ (40 ) $ 1,262,987 $ 235,876 $ 867,124 $ 159,987 December 31, 2018 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash and Cash Equivalents Current Marketable Securities Non- Current Marketable Securities Cash and cash equivalents $ 658,365 $ 20 $ (21 ) $ 658,364 $ 658,364 $ — $ — Available-for-sale: Level 2: Certificates of deposit 173,102 42 (36 ) 173,108 — 157,920 15,188 U.S. treasury securities 152,205 18 (48 ) 152,175 — 152,175 — Debt securities of U.S. government agencies and corporate entities 712,065 40 (1,335 ) 710,770 — 552,968 157,802 $ 1,695,737 $ 120 $ (1,440 ) $ 1,694,417 $ 658,364 $ 863,063 $ 172,990 The amortized cost and estimated fair value of marketable securities, by contractual maturity at December 31, 2019 and 2018 are as follows (in thousands): December 31, 2019 Amortized Cost Estimated Fair Value Due in one year or less $ 864,958 $ 867,124 Due after one year through five years 159,148 159,987 Total $ 1,024,106 $ 1,027,111 December 31, 2018 Amortized Cost Estimated Fair Value Due in one year or less $ 864,003 $ 863,063 Due after one year through five years 173,369 172,990 Total $ 1,037,372 $ 1,036,053 At December 31, 2019 , there were no available-for-sale securities in our total investment portfolio that were in a continuous unrealized loss position for more than 12 months. At December 31, 2018 , we held 25 available-for-sale securities, or an estimated fair value of $82.8 million , out of our total investment portfolio that were in a continuous unrealized loss position for more than 12 months with a gross unrealized loss of $0.4 million . We concluded that the net declines in market value of our available-for-sale securities investment portfolio were temporary in nature and did not consider any of our investments to be other-than-temporarily impaired. In accordance with our investment policy, we place investments in investment grade securities with high credit quality issuers, and generally limit the amount of credit exposure to any one issuer. We evaluate securities for other-than-temporary impairment at the end of each reporting period. Impairment is evaluated considering numerous factors, and their relative significance varies depending on the situation. Factors considered include the length of time and extent to which fair value has been less than the cost basis, the financial condition and near-term prospects of the issuer, and our intent and ability to hold the investment to allow for an anticipated recovery in fair value. Furthermore, the aggregate of individual unrealized losses that had been outstanding for 12 months or less was not significant as of December 31, 2019 , 2018 . and 2017 . We neither intend to sell these investments nor conclude that we are more-likely-than-not that we will have to sell them before recovery of their carrying values. We also believe that we will be able to collect both principal and interest amounts due to us at maturity.

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Balance Sheet Components	Balance Sheet Components Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets, as of December 31, 2019 and 2018 consists of the following (in thousands): December 31, 2019 2018 Prepaid expenses $ 8,475 $ 10,401 Tenant incentives receivables 4,093 10,089 Interest receivable on marketable securities 6,835 7,909 Prepaid expenses and other current assets $ 19,403 $ 28,399 Property and Equipment, Net Property and equipment, net as of December 31, 2019 and 2018 consists of the following (in thousands): December 31, 2019 2018 Building (1) $ — $ 140,442 Laboratory equipment 108,257 96,907 Leasehold improvements (1) 152,426 13,741 Furniture, fixtures and other 3,316 2,122 Computer equipment and software 11,985 11,513 Internally developed software 7,020 7,020 Right-of-use asset, financing 9,853 — Construction in progress 3,222 4,688 296,079 276,433 Less: Accumulated depreciation (94,584 ) (64,456 ) Property and equipment, net $ 201,495 $ 211,977 _________ (1) Leasehold improvements related to our Moderna Technology Center manufacturing facility were previously capitalized as building under ASC 840 and are now included in leasehold improvements under ASC 842 (Note 2). Depreciation and amortization expense for the years ended December 31, 2019 , 2018 and 2017 was $31.0 million , $24.9 million , $20.5 million , respectively. Accrued Liabilities Accrued liabilities, as of December 31, 2019 and 2018 consists of the following (in thousands): December 31, 2019 2018 In-licenses $ — $ 22,000 Property and equipment 4,029 12,089 Compensation-related 27,428 23,406 External goods and services 36,195 21,578 Accrued liabilities $ 67,652 $ 79,073

Leases Leases

Leases Leases	12 Months Ended
	Dec. 31, 2019
Leases [Abstract]
Leases	Leases We have entered into various long-term non-cancelable lease arrangements for our facilities and equipment expiring at various times through 2032. Certain of these arrangements have free rent periods or escalating rent payment provisions, which we recognize lease cost under such arrangements on a straight-line basis over the life of the leases. We have two campuses in Massachusetts, our Cambridge facility and our Moderna Technology Center, or MTC, located in Norwood. Operating Leases Cambridge facility We occupy a multi-building campus in Technology Square in Cambridge, MA with a mix of offices and research laboratory space totaling approximately 200,000 square feet. Our Cambridge facility leases have expiry ranges from 2020 to 2029. In August 2019, we entered into an amendment to our lease agreements to consolidate our Technology Square space in Cambridge, MA. This included entering into a forward-starting lease agreement starting in January 2020 to acquire approximately 50,000 square feet of additional space at 200 Technology Square including space previously occupied under a sublease which expired on December 31, 2019. We will also completely early-exit our leased space of approximately 60,000 square feet at 500 Technology Square by May 2020. In addition, our current 200 Technology Square lease has been extended for two years to 2029. The amendment provides an additional aggregated tenant improvement allowance of $3.5 million for the design and construction of improvements at 200 Technology Square. In May 2016, we entered into a lease agreement for 124,760 square feet of office and laboratory space at 200 Technology Square in Cambridge, Massachusetts. The lease commenced on September 1, 2016, with the base rent subject to increases over an 11 -year term. We have occupied the premises in six phases from September 2016 to January 2020. We have the option to extend the lease term for two extension periods of five years each, at market-based rates. In addition to rent payments, the lease also provides that we pay our proportionate share of operating expenses and taxes during the term of the lease. As the amount of square footage that we lease increases over the term of the lease, we have recognized each phase’s total rent payments on a straight-line basis over the respective lease term. The lease provides us with an initial tenant allowance of $10.00 per square foot against which costs incurred are capitalized as leasehold improvements. We have provided a security deposit of $1.9 million , that is classified as non-current restricted cash in the consolidated balance sheet. As we occupy additional space through the six phases of occupancy, the security deposit will increase to $2.2 million . In December 2018, in relation to the expansion of space from the agreement entered in May 2016, we entered into sublease agreements for 34,268 square feet to expand our office and laboratory space at 200 Technology Square. In August 2015, we entered into a facility lease agreement for 61,618 square feet of office and laboratory space at 500 Technology Square. The lease commenced in April 2016, with rental fees beginning at a rate of $3.9 million per annum and escalating over the six -year term of the lease. The lease provides a $3.1 million tenant improvement allowance against which costs incurred are capitalized as leasehold improvements. The lease also provides that we pay our proportionate share of operating expenses and taxes during the term of the lease. We have provided a security deposit of $1.0 million , that is classified as current restricted cash in the consolidated balance sheet as of December 31, 2019. Moderna Technology Center North (MTC North) In February 2019, we entered into a new lease agreement for office and laboratory space of approximately 200,000 square feet, MTC North, located in Norwood, MA. The lease commenced in the second quarter of 2019 and had an initial expiration date of 2031. We have the option to extend the lease for up to four additional five -year terms. Contemporaneously, we entered into an agreement to sublease approximately 64 percent of the leased space to a third party. We have no rent obligations to the landlord for the space occupied by the third party. All sublease payments from the third party are paid directly to the landlord. The sublease can expire between May 2020 and February 2021 at the third party’s option. In April 2017, we entered into a lease agreement for land adjacent to our MTC manufacturing facility (see MTC South below). We determined, for accounting purposes, this land lease should be accounted for separately from the lease entered in August 2016 relating to the building. The lease commenced in April 2017, with rental fees beginning at a rate of $0.3 million per annum and escalating over the thirty-five -year term of the lease. We record operating lease cost for each of our operating leases on a straight-line basis from lease commencement date through the end of the lease term. Operating lease cost is recorded to operating expenses in our consolidated statements of operations. Finance Leases Moderna Technology Center manufacturing facility (MTC South) In August 2016, we entered into a lease agreement for approximately 200,000 square feet of office, laboratory, and light manufacturing space, MTC South, in Norwood, MA. The lease commencement date for accounting purposes was October 1, 2016. In connection with this lease, the landlord provided a tenant improvement allowance of approximately $24.2 million for costs associated with the design, engineering, and construction of tenant improvements for the building. The lease will expire in September 2032. We have the option to extend the term for two extension periods of ten years each at market-based rents. The base rent is subject to increases over the term of the lease. We have provided a security deposit of $8.9 million that is classified as non-current restricted cash in the consolidated balance sheets as of December 31, 2019. Pursuant to ASC 842, the MTC South lease is bifurcated into a building lease and a land lease using an estimated incremental borrowing rate as of the lease commencement date. The building lease is classified as a financing lease and the land lease is classified as an operating lease. For accounting purposes, the lease term is determined to be 35 years , which is the non-cancelable period of the lease and includes the optional extension periods as we are reasonably certain that we will exercise the options to extend the lease term. Upon the adoption of ASC 842 at January 1, 2019, we derecognized the assets and liabilities recorded as a result of historical build-to-suit accounting under ASC 840 and recorded financing lease liabilities and financing right-of-use asset associated with the building lease. The financing right-of-use asset is amortized on a straight-line basis to depreciation expense over the remaining lease term. We record interest expense related to the financing lease liabilities in the consolidated statements of operations. Prior to the adoption of ASC 842, for accounting purposes, we were deemed to be the owner of the building during the construction period as we were involved in the construction project, including having responsibilities for cost overruns for planned tenant improvements that did not qualify as normal tenant improvements under the lease accounting guidance. During the construction period, we capitalized the fair value of the building as of lease commencement along with a corresponding lease financing obligation. We also capitalized project construction costs incurred by us as an asset. Property and equipment, net included $86.4 million and $75.0 million as of December 31, 2018 and 2017, respectively, related to construction in process costs for the building. We completed construction of the building and started our MTC South operation in July 2018. During the third and fourth quarters of 2018, we transferred $161.6 million of construction in process to property and equipment, including the building of $140.4 million . Certain manufacturing equipment and processes were still in progress which have been completed during 2019. The carrying value of the lease financing obligation related to the building, was $33.5 million and $15.7 million , as of December 31, 2018 and 2017, respectively. We recorded $10.5 million and $14.5 million in accrued liabilities in the consolidated balance sheets related to our MTC South property and equipment as of December 31, 2018 and 2017, respectively. During the construction period, we bifurcated our future lease payments pursuant to the lease into: (i) a portion that is allocated to the building; and (ii) a portion that is allocated to the land on which the building is located, which is recorded as rental expense. The fair value of the building and the land were estimated by us with the assistance of a third-party valuation expert and giving consideration to comparable properties. Although we did not begin making lease payments pursuant to the lease until October 2017, the portion of the lease obligation allocated to the land was treated for accounting purposes as an operating lease commencing on October 1, 2016. Upon completion of the construction of the building, we evaluated the lease and determined that it did not meet the criteria for “sale-leaseback” treatment. Accordingly, we depreciated the building and incurred interest expense related to the lease financing obligation recorded in our balance sheet. The portion of the lease obligation allocated to land was treated as an operating lease . Operating and financing lease right-of-use assets and lease liabilities as of December 31, 2019 and January 1, 2019 (the date of adoption of ASC 842) were as follows (in thousands): December 31, January 1, 2019 2019 Assets: Right-of-use assets, operating, net (1) (2) $ 86,414 $ 63,334 Right-of-use assets, financing, net (3) (4) 9,544 9,853 Total $ 95,958 $ 73,187 Liabilities: Current: Operating lease liabilities (5) $ 3,584 $ 6,455 Non-current: Operating lease liabilities, non-current 93,675 64,250 Financing lease liabilities, non-current 38,689 37,718 Total non-current lease liabilities 132,364 101,968 Total $ 135,948 $ 108,423 _______ (1) At December 31, 2019, these assets are real estate related assets, which include land, office and laboratory spaces. (2) Net of accumulated depreciation. (3) At December 31, 2019, these assets are real estate assets related to the MTC South lease. (4) Included in property and equipment in the consolidated balance sheets, net of accumulated depreciation. (5) Included in other current liabilities in the consolidated balance sheets. The components of the lease costs for the year ended December 31, 2019 were as follows (in thousands): December 31, 2019 Operating lease costs $ 17,015 Financing lease costs: Amortization of right-of-use assets, financing leases 309 Interest expense for financing lease liabilities 6,557 Total financing lease costs $ 6,866 Variable lease costs 4,399 Total rent expense for the years ended December 31, 2018 and 2017 was $19.1 million and $18.6 million , respectively. Supplemental cash flow information relating to our leases for the year ended December 31, 2019 was as follows (in thousands): December 31, 2019 Cash paid for amounts included in measurement of lease liabilities: Operating cash flows used in operating leases $ (16,121 ) Operating cash flows used in financing leases (5,585 ) Financing cash flows provided by financing leases 971 Operating lease non-cash items: Right-of-use assets reduced through lease modifications and reassessments 2,717 Right-of-use assets obtained in exchange for operating lease liabilities 34,014 Weighted average remaining lease terms and discount rates as of December 31, 2019 were as follows: December 31, 2019 Remaining lease term: Operating leases 12 years Finance leases 33 years Discount rate: Operating leases 9.7 % Finance leases 17.2 % Future minimum lease payments under non-cancelable lease agreements as of December 31, 2019, were as follows (in thousands): Fiscal Year Operating Leases (1) Financing Leases (1) 2020 $ 12,563 $ 5,738 2021 13,814 5,894 2022 14,120 6,054 2023 14,365 6,219 2024 14,708 6,387 Thereafter 103,439 264,156 Total minimum lease payments 173,009 294,448 Less amounts representing interest or imputed interest (75,750 ) (255,759 ) (2) Present value of lease liabilities $ 97,259 $ 38,689 ______ (1) Include the optional extensions in the MTC South lease term which represent a total of $10.3 million and $208.5 million un-discounted future lease payments in operating leases and financing leases, respectively. (2) Based on an imputed interest rate of 17.2% . Prior to adoption of ASC 842, future minimum lease payments under non-cancelable operating lease agreements as of December 31, 2018, which were undiscounted and excluded non-lease components, were as follows (in thousands): Fiscal Year Minimum Lease Payments 2019 $ 20,027 2020 20,404 2021 20,937 2022 20,208 2023 17,235 Thereafter 112,958 Total $ 211,769 ______ (1) The amounts in the table above do not include any optional extensions in our lease terms.
Leases	Leases We have entered into various long-term non-cancelable lease arrangements for our facilities and equipment expiring at various times through 2032. Certain of these arrangements have free rent periods or escalating rent payment provisions, which we recognize lease cost under such arrangements on a straight-line basis over the life of the leases. We have two campuses in Massachusetts, our Cambridge facility and our Moderna Technology Center, or MTC, located in Norwood. Operating Leases Cambridge facility We occupy a multi-building campus in Technology Square in Cambridge, MA with a mix of offices and research laboratory space totaling approximately 200,000 square feet. Our Cambridge facility leases have expiry ranges from 2020 to 2029. In August 2019, we entered into an amendment to our lease agreements to consolidate our Technology Square space in Cambridge, MA. This included entering into a forward-starting lease agreement starting in January 2020 to acquire approximately 50,000 square feet of additional space at 200 Technology Square including space previously occupied under a sublease which expired on December 31, 2019. We will also completely early-exit our leased space of approximately 60,000 square feet at 500 Technology Square by May 2020. In addition, our current 200 Technology Square lease has been extended for two years to 2029. The amendment provides an additional aggregated tenant improvement allowance of $3.5 million for the design and construction of improvements at 200 Technology Square. In May 2016, we entered into a lease agreement for 124,760 square feet of office and laboratory space at 200 Technology Square in Cambridge, Massachusetts. The lease commenced on September 1, 2016, with the base rent subject to increases over an 11 -year term. We have occupied the premises in six phases from September 2016 to January 2020. We have the option to extend the lease term for two extension periods of five years each, at market-based rates. In addition to rent payments, the lease also provides that we pay our proportionate share of operating expenses and taxes during the term of the lease. As the amount of square footage that we lease increases over the term of the lease, we have recognized each phase’s total rent payments on a straight-line basis over the respective lease term. The lease provides us with an initial tenant allowance of $10.00 per square foot against which costs incurred are capitalized as leasehold improvements. We have provided a security deposit of $1.9 million , that is classified as non-current restricted cash in the consolidated balance sheet. As we occupy additional space through the six phases of occupancy, the security deposit will increase to $2.2 million . In December 2018, in relation to the expansion of space from the agreement entered in May 2016, we entered into sublease agreements for 34,268 square feet to expand our office and laboratory space at 200 Technology Square. In August 2015, we entered into a facility lease agreement for 61,618 square feet of office and laboratory space at 500 Technology Square. The lease commenced in April 2016, with rental fees beginning at a rate of $3.9 million per annum and escalating over the six -year term of the lease. The lease provides a $3.1 million tenant improvement allowance against which costs incurred are capitalized as leasehold improvements. The lease also provides that we pay our proportionate share of operating expenses and taxes during the term of the lease. We have provided a security deposit of $1.0 million , that is classified as current restricted cash in the consolidated balance sheet as of December 31, 2019. Moderna Technology Center North (MTC North) In February 2019, we entered into a new lease agreement for office and laboratory space of approximately 200,000 square feet, MTC North, located in Norwood, MA. The lease commenced in the second quarter of 2019 and had an initial expiration date of 2031. We have the option to extend the lease for up to four additional five -year terms. Contemporaneously, we entered into an agreement to sublease approximately 64 percent of the leased space to a third party. We have no rent obligations to the landlord for the space occupied by the third party. All sublease payments from the third party are paid directly to the landlord. The sublease can expire between May 2020 and February 2021 at the third party’s option. In April 2017, we entered into a lease agreement for land adjacent to our MTC manufacturing facility (see MTC South below). We determined, for accounting purposes, this land lease should be accounted for separately from the lease entered in August 2016 relating to the building. The lease commenced in April 2017, with rental fees beginning at a rate of $0.3 million per annum and escalating over the thirty-five -year term of the lease. We record operating lease cost for each of our operating leases on a straight-line basis from lease commencement date through the end of the lease term. Operating lease cost is recorded to operating expenses in our consolidated statements of operations. Finance Leases Moderna Technology Center manufacturing facility (MTC South) In August 2016, we entered into a lease agreement for approximately 200,000 square feet of office, laboratory, and light manufacturing space, MTC South, in Norwood, MA. The lease commencement date for accounting purposes was October 1, 2016. In connection with this lease, the landlord provided a tenant improvement allowance of approximately $24.2 million for costs associated with the design, engineering, and construction of tenant improvements for the building. The lease will expire in September 2032. We have the option to extend the term for two extension periods of ten years each at market-based rents. The base rent is subject to increases over the term of the lease. We have provided a security deposit of $8.9 million that is classified as non-current restricted cash in the consolidated balance sheets as of December 31, 2019. Pursuant to ASC 842, the MTC South lease is bifurcated into a building lease and a land lease using an estimated incremental borrowing rate as of the lease commencement date. The building lease is classified as a financing lease and the land lease is classified as an operating lease. For accounting purposes, the lease term is determined to be 35 years , which is the non-cancelable period of the lease and includes the optional extension periods as we are reasonably certain that we will exercise the options to extend the lease term. Upon the adoption of ASC 842 at January 1, 2019, we derecognized the assets and liabilities recorded as a result of historical build-to-suit accounting under ASC 840 and recorded financing lease liabilities and financing right-of-use asset associated with the building lease. The financing right-of-use asset is amortized on a straight-line basis to depreciation expense over the remaining lease term. We record interest expense related to the financing lease liabilities in the consolidated statements of operations. Prior to the adoption of ASC 842, for accounting purposes, we were deemed to be the owner of the building during the construction period as we were involved in the construction project, including having responsibilities for cost overruns for planned tenant improvements that did not qualify as normal tenant improvements under the lease accounting guidance. During the construction period, we capitalized the fair value of the building as of lease commencement along with a corresponding lease financing obligation. We also capitalized project construction costs incurred by us as an asset. Property and equipment, net included $86.4 million and $75.0 million as of December 31, 2018 and 2017, respectively, related to construction in process costs for the building. We completed construction of the building and started our MTC South operation in July 2018. During the third and fourth quarters of 2018, we transferred $161.6 million of construction in process to property and equipment, including the building of $140.4 million . Certain manufacturing equipment and processes were still in progress which have been completed during 2019. The carrying value of the lease financing obligation related to the building, was $33.5 million and $15.7 million , as of December 31, 2018 and 2017, respectively. We recorded $10.5 million and $14.5 million in accrued liabilities in the consolidated balance sheets related to our MTC South property and equipment as of December 31, 2018 and 2017, respectively. During the construction period, we bifurcated our future lease payments pursuant to the lease into: (i) a portion that is allocated to the building; and (ii) a portion that is allocated to the land on which the building is located, which is recorded as rental expense. The fair value of the building and the land were estimated by us with the assistance of a third-party valuation expert and giving consideration to comparable properties. Although we did not begin making lease payments pursuant to the lease until October 2017, the portion of the lease obligation allocated to the land was treated for accounting purposes as an operating lease commencing on October 1, 2016. Upon completion of the construction of the building, we evaluated the lease and determined that it did not meet the criteria for “sale-leaseback” treatment. Accordingly, we depreciated the building and incurred interest expense related to the lease financing obligation recorded in our balance sheet. The portion of the lease obligation allocated to land was treated as an operating lease . Operating and financing lease right-of-use assets and lease liabilities as of December 31, 2019 and January 1, 2019 (the date of adoption of ASC 842) were as follows (in thousands): December 31, January 1, 2019 2019 Assets: Right-of-use assets, operating, net (1) (2) $ 86,414 $ 63,334 Right-of-use assets, financing, net (3) (4) 9,544 9,853 Total $ 95,958 $ 73,187 Liabilities: Current: Operating lease liabilities (5) $ 3,584 $ 6,455 Non-current: Operating lease liabilities, non-current 93,675 64,250 Financing lease liabilities, non-current 38,689 37,718 Total non-current lease liabilities 132,364 101,968 Total $ 135,948 $ 108,423 _______ (1) At December 31, 2019, these assets are real estate related assets, which include land, office and laboratory spaces. (2) Net of accumulated depreciation. (3) At December 31, 2019, these assets are real estate assets related to the MTC South lease. (4) Included in property and equipment in the consolidated balance sheets, net of accumulated depreciation. (5) Included in other current liabilities in the consolidated balance sheets. The components of the lease costs for the year ended December 31, 2019 were as follows (in thousands): December 31, 2019 Operating lease costs $ 17,015 Financing lease costs: Amortization of right-of-use assets, financing leases 309 Interest expense for financing lease liabilities 6,557 Total financing lease costs $ 6,866 Variable lease costs 4,399 Total rent expense for the years ended December 31, 2018 and 2017 was $19.1 million and $18.6 million , respectively. Supplemental cash flow information relating to our leases for the year ended December 31, 2019 was as follows (in thousands): December 31, 2019 Cash paid for amounts included in measurement of lease liabilities: Operating cash flows used in operating leases $ (16,121 ) Operating cash flows used in financing leases (5,585 ) Financing cash flows provided by financing leases 971 Operating lease non-cash items: Right-of-use assets reduced through lease modifications and reassessments 2,717 Right-of-use assets obtained in exchange for operating lease liabilities 34,014 Weighted average remaining lease terms and discount rates as of December 31, 2019 were as follows: December 31, 2019 Remaining lease term: Operating leases 12 years Finance leases 33 years Discount rate: Operating leases 9.7 % Finance leases 17.2 % Future minimum lease payments under non-cancelable lease agreements as of December 31, 2019, were as follows (in thousands): Fiscal Year Operating Leases (1) Financing Leases (1) 2020 $ 12,563 $ 5,738 2021 13,814 5,894 2022 14,120 6,054 2023 14,365 6,219 2024 14,708 6,387 Thereafter 103,439 264,156 Total minimum lease payments 173,009 294,448 Less amounts representing interest or imputed interest (75,750 ) (255,759 ) (2) Present value of lease liabilities $ 97,259 $ 38,689 ______ (1) Include the optional extensions in the MTC South lease term which represent a total of $10.3 million and $208.5 million un-discounted future lease payments in operating leases and financing leases, respectively. (2) Based on an imputed interest rate of 17.2% . Prior to adoption of ASC 842, future minimum lease payments under non-cancelable operating lease agreements as of December 31, 2018, which were undiscounted and excluded non-lease components, were as follows (in thousands): Fiscal Year Minimum Lease Payments 2019 $ 20,027 2020 20,404 2021 20,937 2022 20,208 2023 17,235 Thereafter 112,958 Total $ 211,769 ______ (1) The amounts in the table above do not include any optional extensions in our lease terms.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Strategic Collaborations Under our strategic collaboration agreements, we are committed to perform certain research, development, and manufacturing activities. As part of our PCV Agreement and PCV/SAV Agreement with Merck, we are committed to perform certain research, development and manufacturing activities related to PCV products through an initial Phase 2 clinical trial up to a budgeted amount of $243.0 million as of December 31, 2019 and 2018 (Note 3). Legal Proceedings We are not currently a party to any material legal proceedings. Indemnification Obligations As permitted under Delaware law, we indemnify our officers, directors, and employees for certain events, occurrences while the officer, or director is, or was, serving at our request in such capacity. The term of the indemnification is for the officer’s or director’s lifetime. We have standard indemnification arrangements in its leases for laboratory and office space that require it to indemnify the landlord against any liability for injury, loss, accident, or damage from any claims, actions, proceedings, or costs resulting from certain acts, breaches, violations, or non-performance under our leases. Through December 31, 2019 and 2018 , we had no t experienced any losses related to these indemnification obligations, and no material claims were outstanding. We do not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related reserves were established. Purchase Commitments and Purchase Orders As of December 31, 2019 , we had $4.5 million of non-cancelable purchase commitments for clinical services which are expected to be paid from 2020 to 2023. In addition to purchase commitments, we have agreements with third parties for various services, including services related to clinical operations and support and contract manufacturing, for which we are not contractually able to terminate for convenience and avoid any and all future obligations to the vendors. Certain agreements provide for termination rights subject to termination fees or wind down costs. Under such agreements, we are contractually obligated to make certain payments to vendors, mainly, to reimburse them for their unrecoverable outlays incurred prior to cancellation. At December 31, 2019 , we had cancelable open purchase orders of $105.9 million in total under such agreements for our significant clinical operations and support and contract manufacturing. These amounts represent only our estimate of those items for which we had a contractual commitment to pay at December 31, 2019 , assuming we would not cancel these agreements. The actual amounts we pay in the future to the vendors under such agreements may differ from the purchase order amounts. Licenses to Patented Technology On June 26, 2017, we entered into sublicense agreements with Cellscript, LLC and its affiliate, mRNA RiboTherapeutics, Inc. to sublicense certain patent rights. Pursuant to each agreement, we are required to pay certain license fees, annual maintenance fees, minimum royalties on future net sales and milestone payments contingent on achievement of certain development, regulatory and commercial milestones for specified products, on a product-by-product basis. We concluded the assets acquired in connection with the sublicense agreements should be accounted for as an asset acquisition of in-process research and development. Accordingly, all payments to be made that meet the characteristics of research and development expenses with no alternative future use will be expensed in the period in which they are incurred. As such, the sublicense payments were expensed as they incurred. Additionally, the development and regulatory milestone payments, up to $1.5 million for therapeutic and prophylactic products and up to $0.5 million for diagnostic products will be recognized as a cost of the asset acquired upon resolution of the associated contingency and will be capitalized or expensed depending on the nature of the associated asset as of the date of recognition. Conversely, commercial milestone payments, up to $24.0 million for therapeutic and prophylactic products will be accounted for as additional expense of the related product sales in the period in which the corresponding sales occur. In conjunction with the agreements entered in 2017, we recognized expense of $22.0 million and $53.3 million , and paid consideration of $25.0 million and $28.3 million in 2018 and 2017, respectively. In 2019, we paid consideration of $22.0 million , which was previously accrued as of December 31, 2018. Additionally, we have other in-license agreements with third parties which require us to make future development, regulatory and commercial milestone payments for specified products associated with the agreements. The achievement of these milestones has not occurred and such milestone payments are immaterial.

Redeemable Convertible Preferre

Redeemable Convertible Preferred Stock and Common Stock	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
Redeemable Convertible Preferred Stock and Common Stock	Redeemable Convertible Preferred Stock and Common Stock On February 28, 2018 and May 7, 2018, the Board of Directors approved an amendment to our Certificate of Incorporation resulting in a total of 775,000,000 shares of common stock and a total of 509,352,795 shares of redeemable convertible preferred stock being authorized, respectively. Upon completion of our IPO, our authorized capital stock consists of 1,600,000,000 shares of common stock, par value $0.0001 per share, and 162,000,000 shares of preferred stock, par value $0.0001 per share, all of which shares of preferred stock are undesignated. On December 11, 2018, we completed our IPO, whereby we sold 26,275,993 shares of common stock at a price of $23.00 per share. The aggregate net proceeds received by us from the IPO were $563.0 million , net of underwriting discounts and commissions of $33.2 million and offering expenses of $8.1 million payable by us. Upon the closing of the IPO, all of the outstanding shares of our redeemable convertible preferred stock were converted into 236,012,913 shares of the common stock. As of December 31, 2019 and 2018, we did not have any convertible preferred stock issued or outstanding. Each share of Common Stock is entitled to one vote. The holders of Common Stock are also entitled to receive dividends whenever funds are legally available and when declared by the board of directors, subject to all the rights, powers and preferences of the undesignated preferred stock. Our redeemable convertible preferred stock on the day immediately prior to the closing of our IPO, or December 11, 2018, and December 31, 2017 consisted of the following (in thousands, except share amounts): Redeemable Convertible Preferred Shares Authorized Redeemable Carrying Value Liquidation Preference As of December 11, 2018 Series A redeemable convertible preferred stock 42,000,000 42,000,000 $ 182 $ 2,859 Series B redeemable convertible preferred stock 122,296,280 122,296,280 770 12,493 Series C redeemable convertible preferred stock 85,669,774 85,669,774 36,238 41,750 Series D redeemable convertible preferred stock 63,291,156 63,021,976 163,355 188,840 Series E redeemable convertible preferred stock 81,428,340 80,884,240 498,525 498,814 Series F redeemable convertible preferred stock 54,001,241 54,001,241 473,532 474,131 Series G redeemable convertible preferred stock 55,666,004 55,666,004 549,413 560,000 Series H redeemable convertible preferred stock 5,000,000 5,000,000 111,546 125,000 Balance at December 11, 2018 509,352,795 508,539,515 $ 1,833,561 $ 1,903,887 Redeemable Convertible Preferred Shares Authorized Redeemable Liquidation Preference As of December 31, 2017 Carrying Value Series A redeemable convertible preferred stock 42,000,000 42,000,000 $ 182 $ 2,701 Series B redeemable convertible preferred stock 122,296,280 122,296,280 770 11,801 Series C redeemable convertible preferred stock 85,669,774 85,669,774 36,238 39,676 Series D redeemable convertible preferred stock 63,291,156 63,291,156 164,059 179,462 Series E redeemable convertible preferred stock 81,428,340 81,428,340 501,880 502,169 Series F redeemable convertible preferred stock 54,001,241 54,001,241 473,532 474,131 Balance at December 31, 2017 448,686,791 448,686,791 $ 1,176,661 $ 1,209,940 Redeemable Convertible Preferred Stock We determined the exchange of common units, Series A, B, C, D and E redeemable convertible preferred units, incentive units, restricted stock units, and unit options upon the 2016 Reorganization was a modification of such units. Accordingly, the Series A, B, C, D and E redeemable convertible preferred stock were recorded at their historical carrying values, including previously accrued cumulative dividends, on the effective date of the 2016 Reorganization. Contemporaneous with the 2016 Reorganization, we entered into a preferred stock purchase agreement which authorized the sale and issuance of up to 68,337,129 shares of our Series F redeemable convertible preferred stock at a purchase price of $8.78 per share. We completed a financing in August 2016 which resulted in the issuance of an aggregate of 54,001,241 shares of Series F redeemable convertible preferred stock at an issuance price of $8.78 per share for gross proceeds of $474.1 million , less issuance costs of approximately $0.6 million . In February 2018 and May 2018, we completed additional preferred stock financings which resulted in the issuance of 55,666,004 shares of Series G redeemable convertible preferred stock and 5,000,000 shares of Series H redeemable convertible preferred stock, respectively. Series G redeemable convertible preferred stock was issued at a purchase price of $10.06 per share for gross proceeds of $560.0 million , less issuance costs of $10.5 million . Series H redeemable convertible preferred stock was issued at a purchase price of $25.00 per share for gross proceeds of $112.0 million , less issuance costs of $0.4 million . The Series H preferred stock is not convertible at the option of the holder until after February 7, 2020, after which, it will be convertible into common stock on a one-for- 1.14 basis because the applicable original issuance price for such series is $25.00 and the initial applicable conversion price is $21.93 (adjusted for the reverse stock split). The initial carrying amount of the Series H redeemable convertible preferred stock was recorded at its fair value of $22.39 per share, which we determined based in part on an independent third-party valuation contemporaneously performed. The difference between the purchase price and the fair value of Series H redeemable convertible preferred stock was determined to be the premium associated with the Merck PCV/SAV Agreement entered in conjunction with the Series H issuance and recorded to deferred revenue of $13.0 million (Note 3). In September 2018, we repurchased 269,180 shares of our Series D redeemable convertible preferred stock and 544,100 shares of our Series E redeemable convertible preferred stock for an aggregate purchase price of $8.2 million . The repurchase was recorded as a decrease of carrying value of preferred stock and the excess of the purchase price paid to an existing shareholder over the carrying amount of redeemable convertible preferred stock surrendered was recorded to additional paid-in capital. The following table summarizes the activity for each series of our outstanding redeemable convertible preferred stock for the year ended December 31, 2018 (in thousands, except share data): Series A Redeemable Convertible Preferred Stock Series B Redeemable Convertible Preferred Stock Series C Redeemable Convertible Preferred Stock Series D Redeemable Convertible Preferred Stock Series E Redeemable Convertible Preferred Stock Series F Redeemable Convertible Preferred Stock Series G Redeemable Convertible Preferred Stock Series H Redeemable Convertible Preferred Stock Total Redeemable Convertible Preferred Stock Shares Amount Shares Amount Shares Amount Shares Amount Shares Amount Shares Amount Shares Amount Shares Amount Shares Amount Balance at December 31, 2017 42,000,000 $ 182 122,296,280 $ 770 85,669,774 $ 36,238 63,291,156 $ 164,059 81,428,340 $ 501,880 54,001,241 $ 473,532 — $ — — $ — 448,686,791 $ 1,176,661 Issuance of Series G redeemable convertible preferred stock, net of issuance costs of $10,517 — — — — — — — — — — — — 55,666,004 549,413 — — 55,666,004 549,413 Issuance of Series H redeemable convertible preferred stock, net of issuance costs of $474 — — — — — — — — — — — — — — 5,000,000 111,546 5,000,000 111,546 Repurchase of Series D redeemable convertible preferred stock — — — — — — (269,180 ) (704 ) — — — — — — — — (269,180 ) (704 ) Repurchase of Series E redeemable convertible preferred stock — — — — — — — — (544,100 ) (3,355 ) — — — — — — (544,100 ) (3,355 ) Conversion of redeemable convertible preferred stock (42,000,000 ) (182 ) (122,296,280 ) (770 ) (85,669,774 ) (36,238 ) (63,021,976 ) (163,355 ) (80,884,240 ) (498,525 ) (54,001,241 ) (473,532 ) (55,666,004 ) (549,413 ) (5,000,000 ) (111,546 ) (508,539,515 ) (1,833,561 ) Balance at December 31, 2018 — $ — — $ — — $ — — $ — — $ — — $ — — $ — — $ — — $ — Prior to the conversion of the redeemable convertible preferred stock into the common stock upon the closing of the IPO in December 2018, the holders of the redeemable convertible preferred stock had the following rights: Voting Rights The holders of redeemable convertible preferred stock were entitled to vote on all matters and had the number of votes equal to the number of shares of common stock into which the shares of redeemable convertible preferred stock were convertible. Certain directors comprising the Board of Directors were elected by majority vote of holders of redeemable convertible preferred stock. A majority vote of the holders of redeemable convertible preferred stock was required to liquidate or dissolve the Company, amend the Certificate of Incorporation or Bylaws, reclassify common stock or establish another class of capital stock, create shares that would rank senior to or authorize additional shares of redeemable convertible preferred stock, declare a dividend or make a distribution, change the authorized number of directors constituting the Board of Directors, or establish a new employee stock option plan. Dividends Dividends were cumulative and accrued annually, whether or not declared, and whether or not there were net profits available to pay dividends. The holders of Series A redeemable convertible preferred stock, Series B redeemable convertible preferred stock, Series C redeemable convertible preferred stock and Series D redeemable convertible preferred stock were entitled to dividends, at a rate per share, per annum, of $0.004 , $0.006 , $0.02568 , and $0.171 , respectively. The holders of the Series E redeemable convertible preferred stock, Series F redeemable convertible preferred stock, Series G redeemable convertible preferred stock and Series H redeemable convertible preferred stock were not entitled to dividends with respect to such shares. All accrued dividends on the redeemable convertible preferred stock were forfeited, as a result of the conversion to common stock, in connection with the closing of our IPO on December 11, 2018. Liquidation Preference The holders of the redeemable convertible preferred stock had preferences in the event of any voluntary or involuntary liquidation, dissolution or winding-up of the Company, as defined in the Third Amended and Restated Certificate of Incorporation. The preferences are set forth below: i. first, to the holders of Series H redeemable convertible preferred stock and Series G redeemable convertible preferred stock, on a pari passu basis, an amount equal to the greater of (a) each respective original issue price plus dividends declared but unpaid or (b) such amount that would be payable had all respective shares been converted to common stock; ii. next, to the holders of the Series F redeemable convertible preferred stock an amount equal to the greater of (a) the original issue price plus dividends declared but unpaid or (b) such amount that would be payable had all respective shares been converted to common stock; iii. next, to the holders of the Series E redeemable convertible preferred stock an amount equal to the greater of (a) the original issue price plus dividends declared but unpaid or (b) such amount that would be payable had all respective shares been converted to common stock; iv. next, to the holders of the Series C redeemable convertible preferred stock an amount equal to the greater of (a) the original issue price plus the Series C redeemable convertible preferred stock dividends accrued but unpaid or (b) such amount that would be payable had all respective shares been converted to common stock; v. next, pari passu , in relation to the holders of the Series A redeemable convertible preferred stock an amount equal to the greater of (a) the original issue price plus the Series A redeemable convertible preferred stock dividends accrued but unpaid or (b) such amount that would be payable had all respective shares been converted to common stock and in relation to the holders of the Series B redeemable convertible preferred stock an amount equal to the greater of (a) the original issue price plus the Series B redeemable convertible preferred stock dividends accrued but unpaid or (b) such amount that would be payable had all respective shares been converted to common stock; vi. next, to the holders of the Series D redeemable convertible preferred stock an amount equal to the Series D redeemable convertible preferred stock dividends accrued but unpaid; and vii. finally, to all holders of common stock, pro rata based on the number of shares held by each such holder. Redemption Pursuant to the Third Amended and Restated Certificate of Incorporation as of May 7, 2018 the redeemable convertible preferred stock did not have any redemption rights that were at the election of the holder. However, the redeemable convertible preferred stock was entitled to payment upon the occurrence of certain contingent events. As it relates to the payment upon the occurrence of a contingent event, we evaluated the redeemable convertible preferred stock in accordance with the guidance in FASB ASC Topic 480, Distinguishing Liabilities from Equity (ASC 480), and determined that the payment of liquidation amounts due upon the occurrence of a contingent event was not solely within our control and accordingly the redeemable convertible preferred stock was classified in temporary equity in the consolidated balance sheet. As it relates to the accretion to redemption value, the redeemable convertible preferred stock was not then redeemable, nor was it probable that the instrument would become redeemable, as it was only redeemable upon the occurrence of a contingent event. Accordingly, no accretion was recognized for the redeemable convertible preferred stock and it would not have been accreted until it was probable that the shares would become redeemable. At December 31, 2017, the occurrence of the contingent events was not considered probable. Conversion Each share of Series A redeemable convertible preferred stock, Series B redeemable convertible preferred stock, Series C redeemable convertible preferred stock, Series D redeemable convertible preferred stock, Series E redeemable convertible preferred stock, Series F redeemable convertible preferred stock, and Series G redeemable convertible preferred stock was convertible at the option of the holder, at any time, into the number of shares of fully paid and non-assessable shares of common stock determined by dividing the applicable original issue price for such series of redeemable convertible preferred stock by the applicable conversion price then in effect for such series. The applicable conversion price was initially $0.11 , $0.17 , $0.70 , $4.65 , $13.45 , $19.14 , and $21.93 per share (as adjusted for the reverse stock split), for the Series A redeemable convertible preferred stock, Series B redeemable convertible preferred stock, Series C redeemable convertible preferred stock, Series D redeemable convertible preferred stock, Series E redeemable convertible preferred stock, Series F redeemable convertible preferred stock, and Series G redeemable convertible preferred stock, respectively. In the case of Series H redeemable convertible preferred stock, shares were not convertible at the option of the holder until the date that was twenty-one months following the date of filing of the Third Amended and Restated Certificate of Incorporation, February 7, 2020, after which, shares were convertible at the option of the holder with an applicable conversion price of $21.93 per share, consistent with the mechanics of conversion for the other series of redeemable convertible preferred stock (as adjusted for the reverse stock split). Each applicable conversion ratio would have been adjusted, if applicable, at the time of conversion of a share of redeemable convertible preferred stock into common stock. The adjustment contemplated cash distributions made to holders of the redeemable convertible preferred stock through the date of conversion by decreasing the number of shares of common stock into which the redeemable convertible preferred stock converted by an amount equal to the distributions divided by the fair value of the common stock at the time of conversion. All outstanding shares of redeemable convertible preferred stock were automatically converted into fully paid and non-assessable shares of common stock at the applicable conversion ratio then in effect upon: (i) the date and time, or the occurrence of an event, specified by vote or written consent of the requisite vote; (ii) the closing of a public offering pursuant to an effective registration statement under the Securities Act of 1933, as amended, as approved by the Board of Directors; or (iii) the date and time, or occurrence of an event, specified by vote or written consent of the holders of a majority or two-thirds (as applicable) of the then outstanding shares of the associated series of redeemable convertible preferred stock (applicable on a series-by-series basis). In the case of Series H redeemable convertible preferred stock, in the event of an automatic conversion prior to the twenty-one-month anniversary, shares converted at (a) in the case of an IPO, a 10% discount to the price per share of common stock sold pursuant to this Annual Report on Form 10-K; provided that such discounted price was no lower than $21.93 or (b) in the case of a liquidation, dissolution, winding up or deemed liquidation event, a 10% discount to the price per share payable to the holders of common stock; provided that such discounted price was no lower than $21.93 . Common stock Holders of common stock are entitled to one vote per share and are entitled to receive dividends, if and when declared by our Board of Directors. In the event of liquidation, holders of common stock share ratably in our assets legally available for distribution to our shareholders. Holders of common stock have no preemptive, subscription, redemption or conversion rights.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2019
Share-based Payment Arrangement [Abstract]
Stock-Based Compensation	Stock-Based Compensation Equity Plans In October 2013, we adopted the 2013 Equity Incentive Plan (the 2013 Incentive Plan) and the 2013 Unit Option and Grant Plan (the 2013 Option Plan), which provided for the grant of incentive units, non-qualified unit options, and restricted and unrestricted unit awards to our employees, officers, directors, advisors, and outside consultants. Historically, we also granted restricted stock to founders, officers, directors, and advisors outside any of the Plans. In August 2016, we adopted the 2016 Stock Option and Grant Plan (the 2016 Equity Plan), which replaced the 2013 Option Plan and the 2013 Incentive Plan. The 2016 Equity Plan and provided for the grant of incentive stock options, non-qualified stock options, restricted stock, unrestricted stock, and restricted stock units to our employees, officers, directors, consultants, and other key persons. In connection with the IPO, we adopted the 2018 Stock Option and Incentive Plan (the 2018 Equity Plan) in November 2018. The 2018 Equity Plan became effective on the date immediately prior to the effective date of the IPO and replaced our 2016 Plan. The 2018 Equity Plan provides flexibility to our compensation committee to use various equity-based incentive awards as compensation tools to motivate our workforce. We have initially reserved 13,000,000 shares of our common stock for the issuance of awards under the 2018 Equity Plan. The 2018 Equity Plan provides that the number of shares reserved and available for issuance under the plan will automatically increase each January 1, beginning on January 1, 2019, by 4% of the outstanding number of shares of our common stock on the immediately preceding December 31, or such lesser number of shares as determined by our compensation committee. The shares of common stock underlying any awards that are forfeited, canceled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, reacquired by us prior to vesting, satisfied without any issuance of stock, expire or are otherwise terminated (other than by exercise) under the 2018 Equity Plan and the 2016 Plan will be added back to the shares of common stock available for issuance under the 2018 Equity Plan. The terms and conditions of stock-based awards are defined at the sole discretion of our Board of Directors. We issue service-based awards, vesting over a defined period of service, and performance-based awards, vesting upon achievement of defined conditions. Service based awards generally vest over a four -year period, with the first 25% of such awards vesting following twelve months of continued employment or service. The remaining awards vests in twelve quarterly installments over the following twelve quarters. Stock options granted under the 2016 Equity Plan expire ten years from the date of grant and the exercise price must be at least equal to the fair market value of common stock on the grant date. As of December 31, 2019 , we had a total of 64.9 million shares reserved for future issuance under our Equity Plans, of which 46.8 million shares were reserved for equity awards previously granted, and 18.1 million shares were available for future grants under the 2018 Equity Plan. No additional awards will be granted under the 2016 Equity Plan as it was replaced by the 2018 Equity Plan. Options We have granted options generally through the 2018 Equity Plan and 2016 Equity Plan. The following table summarizes our option activity as of December 31, 2019 and 2018: Number of Options Weighted Average Exercise Price per Share Weighted Average Grant Date Fair Value per Share Weighted- Average Remaining Contractual Term Aggregate Intrinsic Value (1) (in thousands) Outstanding at December 31, 2018 50,821,132 12.16 6.59 7.1 years 220,434 Granted 7,354,268 19.58 11.35 Exercised (6,945,306 ) 6.86 4.47 Canceled/forfeited (5,693,179 ) 15.17 8.67 Outstanding at December 31, 2019 45,536,915 13.82 7.35 7.2 years 286,310 Exercisable at December 31, 2019 22,872,221 10.13 4.86 5.9 years 216,241 Expected to vest at December 31, 2019 22,664,694 17.54 9.86 8.6 years 70,069 _______ (1) Aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of common stock for those options in the money as of December 31, 2019 . The total intrinsic value of options exercised was $75.6 million , $5.3 million and less than $0.1 million for the years ended December 31, 2019 , 2018 and 2017 , respectively. The total consideration recorded as a result of stock option exercises was approximately $47.2 million for the year ended December 31, 2019 . Restricted Common Stock We have granted restricted stock awards generally through the 2016 Equity Plan. The following table summarizes our restricted stock activity during the year ended December 31, 2019 : Number of Weighted Average Grant Date Price per Share Outstanding, non-vested at December 31, 2018 198,597 $ 12.15 Issued — — Vested (162,717 ) 12.15 Canceled/forfeited (35,880 ) 12.15 Outstanding, non-vested at December 31, 2019 — — The aggregate fair value of restricted stock awards vested during the years ended December 31, 2019 , 2018 and 2017 , were $2.3 million , $11.9 million , $21.1 million , respectively. Restricted Common Stock Units We have granted restricted stock awards generally through the 2016 Equity Plan. The following table summarizes our restricted stock unit activity during the year ended December 31, 2019 : Number of Units Weighted Average per Unit Outstanding, non-vested at December 31, 2018 458,715 $ 11.93 Issued 1,283,662 19.12 Vested (57,344 ) 11.93 Canceled/forfeited (106,413 ) 20.25 Vested in prior year and settled during the period (401,371 ) 11.93 Outstanding, non-vested at December 31, 2019 1,177,249 19.01 2018 Employee Stock Purchase Plan In November 2018, we adopted the 2018 Employee Stock Purchase Plan (the ESPP), which became effective on December 5, 2018. The ESPP initially reserves and authorizes the issuance of up to a total of 810,000 shares of common stock to participating employees. We will make one or more offerings, consisting of one or more purchase periods, each year to our employees to purchase shares under the ESPP. Offerings will usually begin every six months and will continue for six -month periods, referred to as offering periods. The purchase price at which shares are sold under the ESPP will be equal to 85% of the lower of the fair market value of the shares on the first business day of the offering period or the last business day of the purchase period. Employees are generally eligible to participate through payroll deductions of between 1% to 50% of their compensation and may not purchase more than 3,000 shares of common stock during each purchase period or $25,000 worth of shares of common stock in any calendar year. We began our first ESPP offering on June 1, 2019. There were 171,343 shares of common stock sold at an average price of $16.87 per share under the ESPP during the year ended December 31, 2019. As of December 31, 2019, 0.6 million shares were available for future issuance under the ESPP. Valuation and Stock-Based Compensation Expense Stock-based compensation for options granted under our Equity Plans and share purchases under our ESPP is determined using the Black-Scholes option pricing model. The weighted-average assumptions used to estimate the fair value of options granted and ESPP for the years ended December 31, 2019 , 2018 and 2017 are as follows: Weighted Average Years Ended December 31, 2019 2018 2017 Options: Risk-free interest rate 2.29 % 2.76 % 2.02 % Expected term 6.07 years 6.27 years 6.21 years Expected volatility 61 % 63 % 63 % Expected dividends — % — % — % Weighted average fair value per share $ 11.35 $ 9.33 $ 7.96 ESPP: Risk-free interest rate 1.95 % * * Expected term 0.50 years * * Expected volatility 53 % * * Expected dividends — % * * Weighted average fair value per share $ 5.98 * * _________ * - Not applicable Stock-Based Compensation Expense The following table presents the components and classification of stock-based compensation expense for the years ended December 31, 2019 , 2018 and 2017 as follows (in thousands): Years Ended 2019 2018 2017 Options $ 74,780 $ 63,288 $ 31,724 Restricted common stock and units 5,125 9,277 8,331 Employee stock purchase plan 1,217 — — Total $ 81,122 $ 72,565 $ 40,055 Research and development $ 48,259 $ 37,659 $ 21,679 General and administrative 32,863 34,906 18,376 Total $ 81,122 $ 72,565 $ 40,055 For the years ended December 31, 2019 , 2018 and 2017 , we recognized stock-based compensation expense of $9.8 million , $10.6 million and $0.8 million , respectively, related to performance-based awards, including awards with vesting or commencement contingent upon the IPO, for which achievement of such performance-based condition was deemed probable. Stock-based compensation expenses related to non-employee awards were immaterial for the years ended December 31, 2019 , 2018 and 2017 . As of December 31, 2019 , there were $194.8 million of total unrecognized compensation cost related to non-vested stock-based compensation with respect to options and restricted stock granted. That cost is expected to be recognized over a weighted-average period of 3.1 years at December 31, 2019 .

Employee Benefit Plan

Employee Benefit Plan	12 Months Ended
	Dec. 31, 2019
Retirement Benefits [Abstract]
Employee Benefit Plan	Employee Benefit Plan We provide a retirement savings option to our eligible U.S. employees through the Moderna, Inc. 401(k) Plan (the 401(k) Plan), subject to certain limitations. As allowed under Section 401(k) of the Internal Revenue Code, the 401(k) Plan allows tax deferred salary deductions for eligible employees. We match 50% up to the first 6% contributed by a participant. All matching contributions are immediately vested. Total matching contributions to the 401(k) Plan were $4.2 million , $2.1 million , $2.1 million for the years ended December 31, 2019 , 2018 and 2017 , respectively.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Loss before (benefit from) provision for income taxes for the years ended December 31, 2019 , 2018 and 2017 consist of the following (in thousands): Years Ended December 31, 2019 2018 2017 United States $ (508,595 ) $ (380,473 ) $ (247,784 ) Foreign (6,121 ) (3,935 ) (8,212 ) Loss before (benefit from) provision for income taxes $ (514,716 ) $ (384,408 ) $ (255,996 ) The (benefit from) provision for income taxes for the years ended December 31, 2019 , 2018 and 2017 consist of the following components (in thousands): Years Ended December 31, 2019 2018 2017 Current: Federal $ — $ (26 ) $ (252 ) State 505 352 172 Total current 505 326 (80 ) Deferred: Federal (1,200 ) — — Total deferred (1,200 ) — — Total (benefit from) provision for income taxes $ (695 ) $ 326 $ (80 ) The reconciliation of the U.S. statutory income tax rate to our effective tax rate for the years ended December 31, 2019 , 2018 and 2017 are as follows: Years Ended December 31, 2019 2018 2017 Tax effected at statutory rate 21.0 % 21.0 % 34.0 % State taxes, net of federal benefit 7.9 % 6.3 % 8.5 % Non-deductible items 1.6 % 0.0 % (1.3 )% Change in valuation allowance (33.0 )% (28.5 )% (20.4 )% Federal research and development credits 2.5 % 1.5 % 4.8 % Foreign tax rate differential (0.2 )% (0.2 )% (1.1 )% Impact of federal rate change on net deferred taxes 0.0 % 0.0 % (25.0 )% Other 0.2 % (0.2 )% 0.5 % Effective tax rate 0.0 % (0.1 )% 0.0 % The significant components of our deferred tax assets and tax liabilities as of December 31, 2019 and 2018 are as follows (in thousands): December 31, 2019 2018 Deferred tax assets: Net operating loss carry-forwards $ 268,173 $ 114,932 Stock-based compensation 43,978 33,138 Capitalized licenses, R&D and start-up costs 19,891 22,143 Tax credit carry-forwards 77,222 53,617 Accrued expenses 7,377 16,443 Deferred revenue 53,475 71,209 Operating lease liabilities 26,571 — Lease financing obligation 10,570 9,149 Other 11 2,870 Total gross deferred tax assets 507,268 323,501 Less: valuation allowance (470,753 ) (308,273 ) Total deferred tax assets, net of valuation allowance 36,515 15,228 Deferred tax liabilities: Financing right-of-use assets (2,612 ) — Operating right-of-use assets (24,944 ) — Fixed assets (8,959 ) (15,228 ) Total deferred tax liabilities (36,515 ) (15,228 ) Net deferred tax assets $ — $ — We have evaluated the positive and negative evidence bearing upon the realization of our deferred tax assets, including our history of losses and in accordance with the applicable accounting standards, has fully reserved the net deferred tax asset. We concluded that realization of our net deferred tax assets is not more-likely-than-not to be realized. The valuation allowance increased by $162.5 million in the year ended December 31, 2019 , primarily due to the increase in net operating loss carry-forwards, research and development tax credits, and stock-based compensation. At December 31, 2019 , we had approximately $981.8 million and $978.8 million of federal and state net operating loss carry-forwards, respectively, of which $380.1 million of federal and $978.8 million of state loss carry-forwards begin to expire in 2030. Additionally, $601.7 million of federal net operating loss carry-forward will carry forward indefinitely. At December 31, 2019 we also had federal and state research and development credit carry-forwards of approximately $45.6 million and $23.9 million , which begin to expire in 2030 and 2029 , respectively. At December 31, 2019 , we also had federal orphan drug and state investment tax credit carry-forwards of approximately $1.5 million and $5.9 million which begin to expire in 2039 and 2020, respectively. Upon adoption of ASC 606 (Note 2), we recorded a cumulative-effect adjustment of $28.0 million to the opening balance of accumulated deficit as of January 1, 2019 with a decrease in deferred revenue of $30.7 million and a decrease in accounts receivable of $2.7 million . As a result of the cumulative decrease in deferred revenue, our corresponding deferred tax asset decreased by $8.4 million , which was offset by a corresponding decrease to our valuation allowance. Utilization of the net operating loss (NOL) and tax credit carry-forwards may be subject to a substantial annual limitation due to ownership change limitations that have occurred previously, or that could occur in the future, as provided by Section 382 of the Internal Revenue Code of 1986, as amended, or Section 382, as well as similar state provisions and other provisions of the Internal Revenue Code. Ownership changes may limit the amount of NOLs and tax credit carry-forwards that can be utilized annually to offset future taxable income and tax, respectively. In general, an ownership change, as defined by Section 382, results from transactions that increase the ownership of 5% shareholders in the stock of a corporation by more than 50% in the aggregate over a three-year period. We may experience ownership changes in the future as a result of subsequent shifts in our stock ownership, some of which may be outside our control. We file income tax returns in the United States and the Commonwealth of Massachusetts. All tax years since the date of our incorporation remain open to examination by the major taxing jurisdictions (state and federal) to which we are subject, as carry-forward attributes generated in years past may still be adjusted upon examination by the Internal Revenue Service (IRS) or other authorities if they have or will be used in a future period. We are not currently under examination by the IRS, or any other jurisdictions, for any tax year. We recognize, in our financial statements, the effect of a tax position when it is more likely than not, based on the technical merits, that the position will be sustained upon examination. A reconciliation of the beginning and ending amounts of unrecognized tax benefits during the years ended December 31, 2019 and 2018 are as follows (in thousands): Balance as of December 31, 2017 $ 940 Decrease due to prior positions (799 ) Increase due to current year tax position — Balance as of December 31, 2018 141 Decrease due to prior positions — Increase due to current year tax positions — Balance as of December 31, 2019 $ 141 Unrecognized tax benefits may change during the next twelve months for items that arise in the ordinary course of business. We do not anticipate a material change to our unrecognized tax benefits over the next twelve months that would have an adverse effect on our consolidated operating results. We recognize interest and penalties, if applicable, related to uncertain tax positions as a component of income tax expense; however, there have been none to date.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Net Loss per Share	Net Loss per Share Net Loss per Share Attributable to Common Stockholders Basic and diluted net loss per share attributable to common stockholders for the years ended December 31, 2019 , 2018 and 2017 are calculated as follows (in thousands, except share and per share data): Years Ended 2019 2018 2017 Numerator: Net loss $ (514,021 ) $ (384,734 ) $ (255,916 ) Premium paid on repurchase of redeemable convertible preferred stock — (4,127 ) — Cumulative dividends on redeemable convertible preferred stock — (12,996 ) (13,925 ) Net loss attributable to common stockholders $ (514,021 ) $ (401,857 ) $ (269,841 ) Denominator: Weighted average common shares used in net loss per share attributable to common stockholders, basic and diluted 330,802,136 81,114,183 64,497,544 Net loss per share attributable to common stockholders, basic and diluted $ (1.55 ) $ (4.95 ) $ (4.18 ) The following common stock equivalents, presented based on amounts outstanding as of December 31, 2019 , 2018 and 2017 were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because their inclusion would have been anti-dilutive: December 31, 2019 2018 2017 Redeemable convertible preferred stock — — 205,151,299 Stock options 45,536,915 50,821,132 33,684,134 Restricted common stock — 198,597 1,081,205 Restricted common stock units 1,177,249 458,715 458,715 46,714,164 51,478,444 240,375,353

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2019
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions Series G Redeemable Convertible Preferred Stock Financing On January 30, 2018 and on February 15, 2018, respectively, we entered into Series G Preferred Stock Purchase Agreements, pursuant to which we issued and sold an aggregate of 55,666,004 shares of our Series G redeemable convertible preferred stock at a price per share of $10.06 , for an aggregate purchase price of $560.0 million . The following table sets forth the number of shares of our Series G redeemable convertible preferred stock that we issued to our related parties in this transaction (in thousands, except share data): Name Shares of Total OCHA LLC (1) 50,000 $ 503 Viking Global Investors LP and affiliated entities (2) 745,526 $ 7,500 ______ (1) OCHA LLC is an entity controlled by an officer. (2) Consists of (1) 279,160 shares of Series G redeemable convertible preferred stock held by VGE III Portfolio Ltd.; (2) 148,974 shares of Series G preferred stock held by Viking Global Equities LP; (3) 8,737 shares of Series G redeemable convertible preferred stock held by Viking Global Equities II LP; (4) 129,537 shares of Series G preferred stock held by Viking Global Opportunities Illiquid Investments Sub-Master LP; and (5) 179,118 shares of Series G redeemable convertible preferred stock held by Viking Long Fund Master Ltd. Other Transactions The following is a description of additional transactions we have engaged in for the years ended December 31, 2019 , 2018 and 2017, with our related parties. One of our former board members served as Senior Counsel at Covington & Burling LLP (Covington). We paid Covington approximately $0.1 million , $0.2 million and $0.2 million for legal services for the year ended December 31, 2019 , 2018 and 2017, respectively. We had no outstanding accounts payable balances to Covington at December 31, 2019 and 2018 . AstraZeneca is considered to be a related party due to its equity ownership in us. We have also entered into strategic alliances with AstraZeneca. For the years ended December 31, 2019 , 2018 and 2017, we received payments of $7.9 million , $34.0 million and $1.1 million , respectively. At December 31, 2019 and 2018, our outstanding accounts receivable balances from AstraZeneca were $0.3 million and $0.9 million , respectively. Refer to Note 3 for a discussion of the strategic alliances and related transaction.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data (Unaudited)	Selected Quarterly Financial Data (Unaudited) The following tables provide the unaudited selected quarterly financial data for the years ended December 31, 2019 and 2018 (in thousands, except per share data). We believe that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results of any future period. 2019 First Quarter (1) Second Quarter (1) Third Quarter (1) Fourth Quarter Total revenue $ 16,025 $ 13,083 $ 17,046 $ 14,055 Total operating expenses 157,665 156,792 146,861 144,611 Loss from operations (141,640 ) (143,709 ) (129,815 ) (130,556 ) Net loss per share attributable to common stockholders - basic and diluted $ (0.40 ) $ (0.41 ) $ (0.37 ) $ (0.37 ) 2018 First Quarter Second Quarter Third Quarter Fourth Quarter Total revenue $ 29,039 $ 28,851 $ 41,757 $ 35,421 Total operating expenses 106,441 125,866 127,575 188,452 Loss from operations (77,402 ) (97,015 ) (85,818 ) (153,031 ) Net loss per share attributable to common stockholders - basic and diluted $ (1.16 ) $ (1.43 ) $ (1.32 ) $ (1.14 ) _______ (1) As a result of our adoption of ASC 842 as of January 1, 2019 (Note 2), quarterly amounts presented in our prior Forms 10-Q were revised. Total operating expenses and Loss from operations were both adjusted by $0.2 million , $0.2 million and $1.0 million , for the first, second and third quarters of 2019, respectively.

Subsequent Event

Subsequent Event	12 Months Ended
	Dec. 31, 2019
Subsequent Events [Abstract]
Subsequent Event	Subsequent Event On February 14, 2020, we sold 26,315,790 shares of common stock at a price of $19.00 per share through a public equity offering. The aggregate net proceeds from the offering were approximately $478.1 million , net of underwriting discounts, commissions and estimated offering expenses. In addition, the underwriters have exercised their option to purchase an additional 3,947,368 shares of common stock at the public offering price less the underwriting discount, resulting in additional net proceeds of $71.8 million .

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Our consolidated financial statements are prepared in accordance with U.S. generally accepted accounting principles (GAAP). Any reference in these notes to applicable guidance is meant to refer to the authoritative accounting principles generally accepted in the United States as found in the Accounting Standard Codification (ASC) and Accounting Standards Update (ASU) of the Financial Accounting Standards Board (FASB). The consolidated financial statements include the Company and its subsidiaries.
Principles of Consolidation	All intercompany transactions and balances have been eliminated in consolidation.
Use of Estimates	We have made estimates and judgments affecting the amounts reported in our consolidated financial statements and the accompanying notes. On an ongoing basis, we evaluate our estimates, including critical accounting policies or estimates related to revenue recognition, research and development expenses, income tax provisions, stock-based compensation, leases, and useful lives of long-lived assets. We base our estimates on historical experience and various relevant assumptions that we believe to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. The actual results that we experience may differ materially from our estimates. Significant estimates relied upon in preparing these financial statements include, among others, those related to fair value of equity awards, revenue recognition, research and development expenses, leases, fair value of financial instruments, useful lives of property and equipment, income taxes, and our valuation allowance on our deferred tax assets.
Segment Information	We have determined that our chief executive officer is the chief operating decision maker (CODM). The CODM reviews financial information presented on a consolidated basis. Resource allocation decisions are made by the CODM based on consolidated results. There are no segment managers who are held accountable by the CODM for operations, operating results, and planning for levels or components below the consolidated unit level. As such, we have concluded that we operate as one segment. All our long-lived assets are located in the United States.
Revenue Recognition	On January 1, 2019, we adopted Accounting Standards Codification (ASC) Topic 606, Revenue from Contracts with Customers (ASC 606), using the modified retrospective transition method applied to those contracts which were not completed as of January 1, 2019. We recognized the cumulative effect of the adoption as an adjustment to the opening balance of accumulated deficit in the current period consolidated balance sheet. Results for reporting periods beginning after January 1, 2019 are presented under ASC 606, while prior period amounts have not been adjusted and continue to be reported in accordance with our historic accounting under ASC Topic 605, Revenue Recognition (ASC 605). ASC 606 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. Our revenue is primarily generated through collaboration arrangements and grants from government-sponsored and private organizations. Our collaboration arrangements typically contain multiple promises, including licenses to our intellectual property, options to obtain development and commercialization rights, research and development services, and obligations to develop and manufacture preclinical and clinical material. Such arrangements provide for various types of payments to us, including upfront payments, funding of research and development activities, funding for the purchase of preclinical and clinical material, development, regulatory and commercial milestone payments, licensing fees, option exercise payments, and royalties based on product sales. We have received grants from various government-sponsored and private organizations for research and related activities that provide for payments for reimbursed costs, which may include overhead and general and administrative costs as well as a related profit margin. We analyze our collaboration arrangements to assess whether they are within the scope of ASC Topic 808, Collaborative Arrangements (ASC 808) to determine whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards that are dependent on the commercial success of such activities. To the extent the arrangement is within the scope of ASC 808, we assess whether aspects of the arrangement between us and our collaboration partner are within the scope of other accounting literature. If we conclude that some or all aspects of the arrangement represent a transaction with a customer, we account for those aspects of the arrangement within the scope of ASC 606 . If we conclude that some or all aspects of the arrangement are within the scope of ASC 808 and do not represent a transaction with a customer, we recognize our allocation of the shared costs incurred with respect to the jointly conducted activities as a component of the related expense in the period incurred. Pursuant to ASC 606, a customer is a party that has contracted with an entity to obtain goods or services that are an output of the entity’s ordinary activities in exchange for consideration. 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. If we conclude a counter-party to a transaction is not a customer or otherwise not within the scope of ASC 606 or ASC 808, we consider the guidance in other accounting literature as applicable or by analogy to account for such transaction. We also consider the guidance in ASC 606 with respect to principal versus agent considerations, in determining the appropriate treatment for the transactions between us and the strategic collaborator and the transactions between us and other third parties. The classification of transactions under our arrangements is determined based on the nature and contractual terms of the arrangement along with the nature of the operations of the participants. Any consideration related to activities in which we are considered the principal, which includes being in control of the good or service before such good or service is transferred to the customer, are accounted for as gross revenue. We receive payments from our customers based on billing schedules established in each contract. Upfront payments and fees are recorded as contract liabilities upon receipt or when due and may require deferral of revenue recognition to a future period when we perform our obligations under the arrangement. Amounts expected to be recognized as revenue within the 12 months following the balance sheet date are classified as current liabilities in our consolidated balance sheets. Amounts not expected to be recognized as revenue within the 12 months following the balance sheet date are classified as non-current liabilities in our consolidated balance sheets. Amounts payable to us are recorded as accounts receivable when our right to consideration is unconditional. We expense incremental costs of obtaining a contract as incurred if the expected amortization period of the asset that we would have recognized is one year or less or the amount is immaterial. As of December 31, 2019 , we had not capitalized any costs to obtain any of our contracts. Collaboration Revenue To determine the appropriate amount of revenue to be recognized for arrangements that we determine are within the scope of ASC 606, we perform the following steps: (i) identify the contract(s) with our 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 each performance obligation is satisfied. We account for a contract with a customer that is within the scope of ASC 606 when all of the following criteria are met: (i) the arrangement has been approved by the parties and the parties are committed to perform their respective obligations; (ii) each party’s rights regarding the goods and/or services to be transferred can be identified; (iii) the payment terms for the goods and/or services to be transferred can be identified; (iv) the arrangement has commercial substance; and (v) collection of substantially all of the consideration to which we will be entitled in exchange for the goods and/or services that will be transferred to the customer is probable. We also determine the term of the contract based on the period in which we and our customer have present and enforceable rights and obligations for purposes of identifying the performance obligations and determining the transaction price. We evaluate contracts that contain multiple promises to determine which promises are distinct. Promises are considered to be distinct and therefore, accounted for as separate performance obligations, provided that: (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer and (ii) the promise to transfer the good or service to the customer is separately identifiable from other promises in the contract. In assessing whether a promise is distinct, we consider factors such as whether: (i) we provide a significant service of integrating goods and/or services with other goods and/or services promised in the contract; (ii) one or more of the goods and/or services significantly modifies or customizes, or are significantly modified or customized by one or more of the other goods and/or services promised in the contract; and (iii) the goods and/or services are highly interdependent or highly interrelated. Individual goods or services (or bundles of goods and/or services) that meet both criteria for being distinct are accounted for as separate performance obligations. Promises that are not distinct at contract inception are combined and accounted for as a single performance obligation. Options to acquire additional goods and/or services are evaluated to determine if such option provides a material right to the customer that it would not have received without entering into the contract. If so, the option is accounted for as a separate performance obligation. If not, the option is considered a marketing offer which would be accounted for as a separate contract upon the customer’s election. The transaction price is generally comprised of an upfront payment due at contract inception and variable consideration in the form of payments for our services and materials and milestone payments due upon the achievement of specified events. Other payments the Company could be entitled to include tiered royalties earned when customers recognize net sales of licensed products. We consider the existence of any significant financing component within our arrangements and have determined that a significant financing component does not exist in our arrangements as substantive business purposes exist to support the payment structure other than to provide a significant benefit of financing. We measure the transaction price based on the amount of consideration to which we expect to be entitled in exchange for transferring the promised goods and/or services to the customer. We utilize either the expected value method or the most likely amount method to estimate the amount of variable consideration, depending on which method is expected to better predict the amount of consideration to which we will be entitled. Amounts of variable consideration are included in the transaction price to the extent that it is probable that a significant reversal in the amount of cumulative revenue recognized will not occur when the uncertainty associated with the variable consideration is subsequently resolved. With respect to arrangements that include payments for a development or regulatory milestone payment, we evaluate whether the associated event is considered probable of achievement and estimate the amount to be included in the transaction price using the most likely amount method. Milestone payments that are not within our control or the licensee, such as those dependent upon receipt of regulatory approval, are not considered to be probable of achievement until the triggering event occurs. At the end of each reporting period, we re-evaluate the probability of achievement of each milestone and any related constraint, and if necessary, adjust our estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenue and net loss in the period of adjustment. For arrangements that include sales-based royalties, including milestone payments based upon the achievement of a certain level of product sales, wherein the license is deemed to be the sole or predominant item to which the payments relate, we recognize revenue upon the later of: (i) when the related sales occur or (ii) when the performance obligation to which some or all of the payment has been allocated has been satisfied (or partially satisfied). Consideration that would be received for optional goods and/or services is excluded from the transaction price at contract inception. We generally allocate the transaction price to each performance obligation based on a relative standalone selling price basis. We develop assumptions that require judgment to determine the standalone selling price for each performance obligation in consideration of applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated research and development costs. However, in certain instances, we allocate variable consideration entirely to one or more performance obligation if the terms of the variable consideration relate to the satisfaction of the respective performance obligation and the amount allocated is consistent with the amount we would expect to receive for the satisfaction of the respective performance obligation. We recognize revenue based on the amount of the transaction price that is allocated to each respective performance obligation when or as the performance obligation is satisfied by transferring a promised good or service to the customer. For performance obligations that are satisfied at a point in time, we recognize revenue when control of the goods and/or services is transferred to the customer. For performance obligations that are satisfied over time, we recognize revenue by measuring the progress toward complete satisfaction of the performance obligation using a single method of measuring progress which depicts the performance in transferring control of the associated goods and/or services to the customer. We generally use input methods to measure the progress toward the complete satisfaction of performance obligations satisfied over time. With respect to arrangements containing a license to our intellectual property that is determined to be distinct from the other performance obligations identified in the arrangement, we recognize revenue from amounts allocated to the license when the license is transferred to the licensee and the licensee is able to use and benefit from the license. For licenses that are bundled with other promises, we utilize 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. Significant management judgment is required in determining the level of effort required under an arrangement and the period over which we are expected to complete our performance obligations under an arrangement. We evaluate the measure of progress each reporting period and, if necessary, adjust the measure of performance and related revenue recognition. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenue and net loss in the period of adjustment. Grant Revenue We have contracts with the U.S. government’s Defense Advanced Research Projects Agency (DARPA), Biomedical Advanced Research (BARDA), and the Bill & Melinda Gates Foundation (Gates Foundation). We recognize revenue from these contracts as we perform services under these arrangements when the funding is committed. Revenues and related expenses are presented gross in the consolidated statements of operations as we have determined we are the primary obligor under the arrangements relative to the research and development services we perform as lead technical expert.
Cash and Cash Equivalents	We consider all highly liquid investments with an original maturity of 90 days or less from the date of purchase to be cash equivalents.
Restricted Cash	Restricted cash is composed of amounts held on deposit related to our lease arrangements. The funds are maintained in money market accounts and are recorded at fair value. We classify our restricted cash as either current or non-current based on the terms of the underlying lease arrangement.
Investments	We invest our excess cash balances in marketable debt securities. We classify our investments in marketable debt securities as available-for-sale. We report available-for-sale investments at fair value at each balance sheet date, and include any unrealized holding gains and losses (the adjustment to fair value) in accumulated other comprehensive gain (loss), a component of stockholders’ equity. Realized gains and losses are determined using the specific-identification method, and are included in other (expense) income, net in our consolidated statements of operations. Should any adjustment to fair value reflect a decline in the value of the investment, we consider all available evidence to evaluate the extent to which the decline is “other than temporary” and, if so, we recognize the associated unrealized loss through a charge to our consolidated statement of operations. We did no t record any impairment charges related to our marketable securities during the years ended December 31, 2019 , 2018 and 2017 . We classify our available-for-sale marketable securities as current or non-current based on each instrument’s underlying effective maturity date and for which we have the intent and ability to hold the investment for a period of greater than 12 months. Marketable securities with maturities of less than 12 months are classified as current and are included in investments in the consolidated balance sheets. Marketable securities with maturities greater than 12 months for which we have the intent and ability to hold the investment for greater than 12 months are classified as non-current and are included in investments, non-current in the consolidated balance sheets.
Accounts Receivable and Allowance for Doubtful Accounts	Accounts receivable are amounts due from strategic collaborators as a result of manufacturing and research and development services provided under collaboration arrangements, or milestones achieved, but not yet paid. We also have accounts receivable amounts due from our grant agreements. Amounts payable to us are recorded as accounts receivable when our right to consideration is unconditional. To estimate the allowance for doubtful accounts, we make judgments about the creditworthiness of our customers based on ongoing credit evaluation and historical experience.
Concentrations of Credit Risk	Financial instruments that subject us to significant concentrations of credit risk consist primarily of cash, cash equivalents, restricted cash, marketable securities, and accounts receivable. Our investment portfolio comprises money market funds, marketable debt securities, including U.S. Treasury securities, debt securities of U.S. government agencies and corporate entities and commercial paper. Our cash management and investment policy limits investment instruments to investment-grade securities with the objective to preserve capital and to maintain liquidity until the funds can be used in business operations. Bank accounts in the United States are insured by the Federal Deposit Insurance Corporation (FDIC) up to $250,000. Our primary operating accounts significantly exceed the FDIC limits.
Significant Customers	Our accounts receivable are generally unsecured and are from customers in different countries. We generated 80% , 91% and 86% of our revenue for the years ended December 31, 2019 , 2018 and 2017 , respectively, from strategic collaborators. The remaining 20% , 9% , 14% of our revenue for the years ended December 31, 2019 , 2018 and 2017 , respectively, were generated from grants made by government-sponsored and private organizations.
Fair Value Measurements	Fair value is defined as the price that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. When determining the fair value measurements for assets and liabilities, which are required to be recorded at fair value, we consider the principal or most advantageous market in which we would transact and the market-based risk measurements or assumptions that market participants would use in pricing the asset or liability, such as risks inherent in valuation techniques, transfer restrictions and credit risk. FASB ASC Topic 820, Fair Value Measurement (ASC 820), establishes a fair value hierarchy for instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and our assumptions (unobservable inputs). Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from our independent sources. Unobservable inputs are inputs that reflect our assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances. The following fair value hierarchy is used to classify assets and liabilities based on the observable inputs and unobservable inputs used to value the assets and liabilities: • Level 1: Unadjusted quoted prices in active markets that are accessible at the measurement date for identical, unrestricted assets or liabilities; • Level 2: Quoted prices for similar assets and liabilities in active markets, quoted prices in markets that are not active, or inputs which are observable, either directly or indirectly, for substantially the full term of the asset or liability; or • Level 3: Prices or valuation techniques that require inputs that are both significant to the fair value measurement and unobservable (i.e., supported by little or no market activity). To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. Our cash equivalents and marketable securities are reported at fair value determined using Level 1 and Level 2 inputs (Note 5). We do not have any non-financial assets or liabilities that should be recognized or disclosed at fair value on a recurring basis at December 31, 2019 , 2018 and 2017 . As of December 31, 2019 and 2018 , we maintain letters of credit of $11.8 million and $12.1 million , respectively, related to our lease arrangements, which are secured by money market accounts in accordance with certain of our lease agreements. The amounts are recorded at fair value using Level 1 inputs and included as restricted cash in our consolidated balance sheets.
Construction in Progress and Property and Equipment	Property and equipment are stated at cost, net of accumulated depreciation. Depreciation is calculated using the straight-line method over the estimated useful lives of the assets. The estimated useful lives of property and equipment are described below: Estimated Useful Life Laboratory equipment 5 years Leasehold improvements Lesser of estimated useful life of improvement or remaining life of related lease Computer equipment and software 3 years Other assets including automobiles, furniture and fixtures 5 years Right of use asset, financing Lease term Expenditures for maintenance and repairs are charged to expense as incurred. Upon retirement or sale, the cost of the assets disposed of, and the related accumulated depreciation, are removed from the accounts, and any resulting gain or loss is recorded to other (expense) income, net. Construction in progress includes direct costs related to the construction of various property and equipment, including leasehold improvements, and is stated at original cost. Construction in progress includes costs incurred under construction contracts including project management services, engineering services, design services and development, construction services and other construction-related fees and services. Such costs are not depreciated until the asset is completed and placed into service. Once the asset is placed into service, these capitalized costs will be allocated to certain property and equipment categories and will be depreciated over the estimated useful life of the underlying assets.
Impairment of Long-Lived Assets	We evaluate our long-lived assets, which consist of property and equipment, to determine if facts and circumstances indicate that the carrying amount of assets may not be recoverable. If such facts and circumstances exist, we assess the recoverability of the long-lived assets by comparing the projected future undiscounted net cash flows associated with the related asset or group of assets over their remaining lives against their respective carrying amounts. If such review indicates that such cash flows are not expected to be sufficient to recover the recorded value of the assets, the assets are written down to their estimated fair values based on the expected discounted future cash flows attributable to the assets or based on appraisals.
Leases	Leases are classified at their commencement date, which is defined as the date on which the lessor makes the underlying asset available for use by the lessee, as either operating or finance leases based on the economic substance of the agreement. We recognize lease right-of-use assets and related liabilities in our consolidated balance sheets for both operating and finance leases. Lease liabilities are measured at the lease commencement date as the present value of the future lease payments using the interest rate implicit in the lease. If the rate implicit is not readily determinable, we will utilize our incremental borrowing rate as of the lease commencement date. Lease right-of-use assets are measured as the lease liability plus initial direct costs and prepaid lease payments less lease incentives. The lease term is the non-cancelable period of the lease and includes options to extend or terminate the lease when it is reasonably certain that an option will be exercised. We recognize operating lease cost in operating expense in our consolidated statements of operations, inclusive of rent escalation provisions and rent holidays, on a straight-line basis over the respective lease term. For our finance leases, we recognize depreciation expense associated with the leased asset acquired and recognize interest expense related to the portion of the financing in our consolidated statements of operations. Additionally, we recognize tenant improvement allowances as a reduction to rent expense on a straight-line basis over the respective lease term.
Research and Development Costs	Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries and benefits, facilities costs, overhead costs, contract services, and other outside costs. The value of goods and services received from contract research organizations and contract manufacturing organizations in the reporting period are estimated based on the level of services performed, and progress in the period in cases when we have not received an invoice from the supplier.
Patent Costs	Costs to secure, defend and maintain patents are expensed as incurred, and are classified as general and administrative expenses due to the uncertainty of future benefits.
Stock-Based Compensation	We issue stock-based awards to employees and non-employees, generally in the form of stock options and restricted stock units (RSUs). We account for our stock-based compensation awards in accordance with ASC 718, Compensation—Stock Compensation . Most of our stock-based awards have been made to employees. We measure compensation cost for all equity awards at their grant-date fair value and recognize compensation expense over the requisite service period, which is generally the vesting period, on a straight-line basis. The grant date fair value of stock options is estimated using the Black-Scholes option pricing model, which requires management to make assumptions with respect to the fair value of our common stock on the grant date, including the expected term of the award, the expected volatility of our stock, calculated based on a period of time generally commensurate with the expected term of the award, risk-free interest rates and expected dividend yields of our stock. Historically, for periods prior to our IPO, the fair value of the shares of common stock and common units underlying our stock-based awards were determined on each grant date by our board of directors based on valuation estimates from management considering our most recently available independent third-party valuation of our common stock. Our board of directors also assessed and considered, with input from management, additional objective and subjective factors that we believed were relevant and which may have changed from the date of the most recent valuation through the grant date. The grant date fair value of RSUs is estimated based on the fair value of our underlying common stock. For performance-based stock awards, we recognize stock-based compensation expense over the requisite service period using the accelerated attribution method when achievement is probable. We classify stock-based compensation expense in our consolidated statement of operations in the same manner in which the award recipient’s salary and related costs are classified or in which the award recipient’s service payments are classified.
Income Taxes	We use an asset and liability approach to account for income taxes. We recognize deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the financial reporting and tax bases of assets and liabilities. These differences are measured using the enacted statutory tax rates that are expected to be in effect for the years in which differences are expected to reverse. Valuation allowances are provided when the expected realization of deferred tax assets does not meet a “more likely than not” criterion. We make estimates and judgments about our future taxable income that are based on assumptions that are consistent with our plans and estimates. Should the actual amounts differ from our estimates, the amount of our valuation allowance could be materially impacted. Changes in these estimates may result in significant increases or decreases to our tax provision in a period in which such estimates are changed, which in turn would affect net income or loss. We recognize tax benefits from uncertain tax positions if we believe the position is more likely than not to be sustained on examination by the taxing authorities based on the technical merits of the position. We make adjustments to these reserves when facts and circumstances change, such as the closing of a tax audit or the refinement of an estimate. The provision for income taxes includes the effects of any reserves for tax positions that are not more likely than not to be sustained, as well as the related net interest and penalties.
Redeemable Convertible Preferred Units and Redeemable Convertible Preferred Stock	We record all redeemable convertible preferred stock at their respective transaction prices on the dates of issuance less issuance costs. Our redeemable convertible preferred stock are classified as temporary equity and excluded from stockholders’ (deficit) equity as the potential redemption of such units or stock is outside our control. Upon the 2016 Reorganization, the redemption rights upon the passage of time were removed and the redeemable convertible preferred stock became redeemable only upon the occurrence of certain contingent events.
Net Loss per Share Attributable to Common Stockholders	We apply the two-class method to compute basic and diluted net loss per share attributable to common stockholders when we have issued shares that meet the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income (loss) available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to share in the earnings as if all income (loss) for the period had been distributed. During periods of loss, there is no allocation required under the two-class method since the potentially participating securities do not have a contractual obligation to fund our losses. We calculate basic net loss per share attributable to common stockholders by dividing net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period, without consideration for common stock equivalents. Upon the closing of our IPO, all outstanding shares of our redeemable convertible preferred stock were converted into common stock. We calculate diluted net loss per share attributable to common stockholders by dividing net loss attributable to common stockholders by the weighted average number of common shares outstanding after giving consideration to the dilutive effect of restricted common stock, stock options and redeemable convertible preferred stock that are outstanding during the period. We have generated a net loss in all periods presented, therefore the basic and diluted net loss per share attributable to common stockholders are the same as the inclusion of the potentially dilutive securities would be anti-dilutive.
Comprehensive Loss	Comprehensive loss includes net loss and other comprehensive income (loss) for the period. Other comprehensive income (loss) mainly consists of unrealized gains and losses on our investments. Total comprehensive income (loss) for all periods presented have been disclosed in the consolidated statements of comprehensive loss.
Recently Adopted Accounting Standards/Recently Issued Accounting Standards Not Yet Adopted	ASU No. 2018-07, Compensation - Stock Compensation: Improvements to Nonemployee Share-Based Payment Accounting In June 2018, the FASB issued ASU 2018-07, Compensation-Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting, which is intended to simplify aspects of share-based compensation issued to non-employees by making the guidance generally consistent with the accounting for employee share-based compensation. The new standard will become effective for us on January 1, 2020, with early adoption permitted. We early adopted this standard in the first quarter of 2019. The adoption of this standard did not have a material impact on our consolidated financial statements and disclosure. ASU No. 2018-18, Collaborative Arrangements: Clarifying the Interaction between Topic 808 and Topic 606 In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808) : Clarifying the Interaction between Topic 808 and Topic 606, which clarifies the interaction between Topic 808 and Topic 606, Revenue from Contracts with Customers . Currently, Topic 808 does not provide comprehensive recognition or measurement guidance for collaborative arrangements, and the accounting for those arrangements is often based on an analogy to other accounting literature or an accounting policy election. Similarly, aspects of Topic 606 have resulted in diversity in practice on the effect of the revenue standard on the accounting for collaborative arrangements. The standard will become effective for us beginning on January 1, 2020, with early adoption permitted. We early adopted this standard in connection with the adoption of ASC 606 in the first quarter of 2019. The adoption of this standard did not have a material impact on our consolidated financial statements and disclosure. Recently Issued Accounting Standards Not Yet Adopted From time to time, new accounting pronouncements are issued by the FASB or other standard setting bodies and adopted by us as of the specified effective date. Unless otherwise discussed, we believe that the impact of recently issued standards that are not yet effective will not have a material impact on our consolidated financial statements and disclosures. In June 2016, the FASB issued ASU No. 2016-13, Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments . This standard will change how companies account for credit losses for most financial assets and certain other instruments. For trade receivables, loans and held-to-maturity debt securities, companies will be required to recognize an allowance for credit losses rather than reducing the carrying value of the asset. ASU 2016-13 will be effective for us on January 1, 2020. We will adopt this standard in the first quarter of 2020. Based on the composition of our investment portfolio and investment policy, the adoption of this standard is not expected to have a material impact on our consolidated financial statements and disclosure. In August 2018, the FASB issued ASU 2018-15, Intangibles—Goodwill and Other—Internal-Use Software (Topic 350): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. This standard requires capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). This standard should be applied either retrospectively or prospectively, and will be effective for us on January 2020. We will adopt this standard in the first quarter of 2020 using the prospective adoption method. The adoption of this standard is not expected to have a material impact on our consolidated financial statements and disclosure. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes . This standard removes certain exceptions for investments, intraperiod allocations and interim calculations, and adds guidance to reduce complexity in accounting for income taxes. This standard will be effective for us on January 1, 2021, with early adoption permitted. We are currently evaluating the potential impact this standard may have on our condensed consolidated financial statements and results of operations upon adoption. ASU No. 2014-09, Revenue from Contracts with Customers In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) , which supersedes all existing revenue recognition requirements in ASC 605 and most industry specific guidance. ASC 606 provides a single comprehensive model for use in accounting for revenue arising from contracts with customers. We adopted the new revenue standard on January 1, 2019 using the modified retrospective transition method applied to those contracts which were not completed as of January 1, 2019. We recognized the cumulative effect of the adoption as an adjustment to the opening balance of accumulated deficit in the first quarter of 2019. We have elected to use the practical expedient to aggregate the impact of all contract modifications that occurred prior to the ASC 606 adoption with respect to (i) identification of the satisfied and unsatisfied performance obligations, (ii) determination of the transaction price and (iii) allocation of the transaction price to the satisfied and unsatisfied performance obligations. ASC 606 requires significant judgment and estimates and results in changes to, but not limited to: (i) the determination of the transaction price, including estimates of variable consideration, (ii) the allocation of the transaction price, including the determination of estimated selling price, and (iii) the pattern of recognition, including the application of proportional performance as a measure of progress on service-related promises and application of point-in-time recognition for supply-related promises. We recorded the cumulative-effect adjustment of $28.0 million to the opening balance of accumulated deficit as of January 1, 2019 with a corresponding decrease to deferred revenue. The effect of the adoption of ASC 606 on our consolidated balance sheet is shown below together with the effect of adoption of ASU No. 2016-02, Leases. The cumulative-effect adjustment is primarily due to the application of ASC 606 to our strategic collaboration agreements, particularly our Combined 2018 AZ Agreements, 2016 VEGF Exercise and Merck PCV/SAV Agreement (see Note 3). In addition, as a result of the cumulative decrease in deferred revenue, our corresponding deferred tax asset was decreased by $8.4 million , which was offset by a corresponding decrease to our valuation allowance. A substantial portion of the $28.0 million cumulative-effect adjustment is the result of the application of ASC 606 regarding the allocation of the transaction price and the measurement of progress in satisfying performance obligations. In particular, for the Combined 2018 AZ Agreements, the adoption of ASC 606 resulted in the decrease of previously deferred revenue of $39.9 million . For the Merck PCV/SAV Agreement, the adoption of ASC 606 resulted in a reversal of previously recognized revenue and an increase in deferred revenue of $13.9 million . These adjustments are due to the change in the way we allocate the transaction price to each performance obligation and measure our performance under each agreement from a straight-line method to a proportional performance model. In addition, the adoption of ASC 606 resulted in the decrease of $4.3 million of previously deferred revenue relating to the 2016 VEGF Exercise. Under ASC 605, the product option fee and the clinical milestone payment we received pursuant to the VEGF Exercise were deferred until the consideration pertaining to the clinical supply of mRNA can be reasonably estimated. Under ASC 606, we are required to estimate the total variable consideration to determine the total consideration and recognize the revenue as clinical supply is shipped to the customer based on the proportionate amount of the transaction price. As a result, the balance of remaining deferred revenues at January 1, 2019 was $75.7 million , $37.1 million and $125.2 million related to the Combined 2018 AZ Agreements, 2016 VEGF Exercise and the Merck PCV/SAV Agreement, respectively. ASU No. 2016-18, Statement of Cash Flows: Restricted Cash In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash , which requires the statement of cash flows to explain the change during the period in the total of cash, cash equivalents and restricted cash. When cash, cash equivalents and restricted cash are presented in more than one line item on the balance sheet, the new standard requires a reconciliation of the totals in the statement of cash flows to the related captions in the balance sheet. This reconciliation can be presented either on the face of the statement of cash flows or in the notes to the financial statements. The new standard became effective for us on January 1, 2019. As a result of adopting this new standard using a retrospective transition method for each period presented, we include our restricted cash balance in the cash, cash equivalents and restricted cash reconciliation of operating, investing and financing activities in the consolidated statements of cash flows. ASU No. 2016-02, Leases: Amendments to the FASB Accounting Standards Codification In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), which supersedes all existing lease guidance. To increase transparency and comparability among organizations, this guidance requires that entities that lease assets recognize right-of-use (ROU) assets representing its right to use the underlying asset for the lease term and lease liabilities related to the rights and obligations created by those leases on the balance sheet regardless of whether they are classified as finance or operating leases, with classification affecting the pattern and presentation of expenses and cash flows on our consolidated financial statements. In addition, new disclosures are required to meet the objective of enabling users of the financial statements to better understand the amount, timing, and uncertainty of cash flows arising from leases. Prior to December 31, 2019, as an EGC, we elected to use the extended transition period provided by the JOBS Act for the implementation of new or revised accounting standards, and as a result of this election, we did not have to comply with the public company FASB standard’s effective date for ASC 842 until we ceased to be classified as an EGC. Effective on December 31, 2019, we lost our EGC status which accelerated the requirement of ASC 842 adoption. As a result, we adjusted our previously reported consolidated financial statements effective January 1, 2019 in this Form 10-K for the year ended December 31, 2019, and amendments to previously filed Forms 10-Q were not required. We adopted ASC 842 on January 1, 2019 on a modified retrospective basis under which we recognized and measured leases existing at, or entered into after, the beginning of the period of adoption. We elected the optional transition approach of not adjusting our comparative period financial statements for the impacts of adoption. Therefore, we recognized the effects of applying ASC 842 as a cumulative-effect adjustment to accumulated deficit of $3.8 million , as of January 1, 2019, the effective date of this standard, related to the adjustments to our Moderna Technology Center manufacturing facility described in Note 7. Additionally, upon adoption of ASC 842, we recorded right-of-use assets of $63.3 million and corresponding lease liabilities of $70.7 million related to our operating leases. The difference between these assets and liabilities is primarily attributable to adjustments to the right-of-use asset at transition related to lease incentives and deferred rent. The comparative consolidated balance sheet as of December 31, 2018 has not been restated to reflect the adoption of ASC 842. In addition, the amounts presented as deferred lease obligations on our consolidated balance sheet as of December 31, 2018 are now included in the calculation of the operating lease right-of-use assets. The transition guidance associated with ASC 842 also permitted certain practical expedients. We elected the practical expedient, which allowed us to carryforward certain aspects of our historical lease accounting under ASC 840 for leases that commenced before the effective date, including not to reassess (i) whether any expired or existing contracts are or contain leases, (ii) lease classification for any expired or existing leases, and (iii) initial direct costs for any existing leases. We also elected to adopt an accounting policy to not apply ASC 842 to leases with an initial term of 12 months or less and not include them in the consolidated balance sheets. Instead, these lease payments are recognized in profit or loss on a straight-line basis over the lease term. Lastly, we elected the practical expedient to not separate non-lease and lease components and instead account for them as a single lease component for all classes of underlying assets. We do not include variable payments that are not based on an index or rate in the single lease component, regardless of whether they are related to the lease or non-lease component. The lease right-of-use assets and related lease liabilities are classified as either operating or finance. Lease liabilities are measured at the lease commencement date as the present value of future minimum lease payments. Lease right-of-use assets are measured as the lease liability plus initial direct costs and prepaid lease payments less lease incentives. In measuring the present value of the future minimum lease payments, the discount rate for the lease is the rate implicit in the lease unless that rate cannot be readily determined. In that case, the lessee is required to use its incremental borrowing rate. In computing our lease liabilities, we generally use the incremental borrowing rate based on the information available on the commencement date using a company-specific rate in the U.S. that is fully collateralized and consistent with the lease term for each lease. The lease term is the non-cancelable period of the lease and includes options to extend or terminate the lease when it is reasonably certain that an option will be exercised.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Entities that Accounted for More than 10% of Total Revenue and Accounts Receivable	A significant portion of our revenue to date has been generated from the following entities that accounted for more than 10% of total revenue and accounts receivable for the periods presented: Percentage of Revenue Years Ended December 31, Percentage of Accounts Receivable December 31, 2019 2018 2017 2019 2018 Merck 61 % 49 % 31 % 13 % 30 % BARDA 13 % * 10 % 54 % 13 % Vertex 10 % * * 17 % 22 % AstraZeneca * 34 % 15 % * * Alexion * * 36 % * * DARPA * * * * 16 % Massachusetts Life Sciences Center * * * * 12 % ________ * - Represents an amount of less than 10%
Estimated Useful Lives of Property and Equipment	The estimated useful lives of property and equipment are described below: Estimated Useful Life Laboratory equipment 5 years Leasehold improvements Lesser of estimated useful life of improvement or remaining life of related lease Computer equipment and software 3 years Other assets including automobiles, furniture and fixtures 5 years Right of use asset, financing Lease term Property and equipment, net as of December 31, 2019 and 2018 consists of the following (in thousands): December 31, 2019 2018 Building (1) $ — $ 140,442 Laboratory equipment 108,257 96,907 Leasehold improvements (1) 152,426 13,741 Furniture, fixtures and other 3,316 2,122 Computer equipment and software 11,985 11,513 Internally developed software 7,020 7,020 Right-of-use asset, financing 9,853 — Construction in progress 3,222 4,688 296,079 276,433 Less: Accumulated depreciation (94,584 ) (64,456 ) Property and equipment, net $ 201,495 $ 211,977 _________ (1) Leasehold improvements related to our Moderna Technology Center manufacturing facility were previously capitalized as building under ASC 840 and are now included in leasehold improvements under ASC 842 (Note 2).
Components of Accumulated Other Comprehensive Loss	The components of accumulated other comprehensive gain (loss) for the years ended December 31, 2019 and 2018 are as follows (in thousands): Unrealized Gain (Loss) on Available-for-Sale Debt Securities Accumulated other comprehensive loss, balance at December 31, 2017 $ (1,157 ) Other comprehensive loss (163 ) Accumulated other comprehensive loss, balance at December 31, 2018 (1,320 ) Other comprehensive income 3,124 Accumulated other comprehensive gain, balance at December 31, 2019 $ 1,804
Effect of Adoption of ASC 606 on Condensed Financial Statements	The following tables summarize the effects of adopting ASC 606 on our consolidated financial statements at December 31, 2019, and for the year ended December 31, 2019 (in thousands, except per share data): As of December 31, 2019 Consolidated Balance Sheet As reported under ASC 606 Adjustments Balance without adoption of ASC 606 Deferred revenue, current $ 63,310 $ (8,054 ) $ 55,256 Deferred revenue, non-current 138,995 4,012 143,007 Accumulated deficit (1,496,454 ) 6,785 (1,489,669 ) Year Ended December 31, 2019 Consolidated Statement of Operations As reported under ASC 606 Adjustments Amount without adoption of ASC 606 Revenue: Collaboration revenue $ 42,803 $ 2,767 $ 45,570 Collaboration revenue from related party 5,233 32,002 37,235 Total revenue 60,209 34,769 94,978 Loss from operations (545,720 ) 34,769 (510,951 ) Loss before income taxes (514,716 ) 34,769 (479,947 ) Net loss (514,021 ) 34,769 (479,252 ) Net loss per share - basic and diluted (1.55 ) 0.10 (1.45 ) The cumulative effect of applying both ASC 606 and ASC 842 on our consolidated balance sheets as of January 1, 2019 was as follows (in thousands): Balance as of December 31, 2018 (1) Effect of the Adoption of Balance as of January 1, 2019 ASC 842 ASC 606 Assets: Accounts receivable $ 11,686 $ — $ (2,738 ) $ 8,948 Prepaid expenses and other current assets 28,399 (300 ) — 28,099 Property and equipment, net 211,977 (2,483 ) — 209,494 Right-of-use assets, operating leases — 63,334 — 63,334 Total assets $ 1,962,149 $ 60,551 $ (2,738 ) $ 2,019,962 Liabilities and Shareholders’ Equity: Deferred revenue $ 109,056 $ — $ (27,281 ) $ 81,775 Other current liabilities 3,464 5,867 — 9,331 Deferred revenue, non-current 165,352 — (3,441 ) 161,911 Operating lease liabilities, non-current — 64,250 — 64,250 Finance lease liabilities, non-current — 37,718 — 37,718 Deferred lease obligation, non-current 10,006 (10,006 ) — — Lease financing obligation 33,489 (33,489 ) — — Accumulated deficit (1,006,627 ) (3,789 ) 27,984 (982,432 ) Total liabilities and shareholders’ equity $ 1,962,149 $ 60,551 $ (2,738 ) $ 2,019,962 _______ (1) As reported in our 2018 Annual Report on Form 10-K. The following table summarizes our total consolidated net revenue from our strategic collaborators for the periods presented (in thousands): Years Ended Collaboration Revenue by Strategic Collaborator: December 31, 2019 December 31, 2019 December 31, 2018 Merck $ 36,608 $ 38,977 $ 66,082 AstraZeneca 5,233 37,235 45,993 Vertex 6,195 6,593 10,437 Total collaboration revenue $ 48,036 $ 82,805 $ 122,512
Reconciliation of Cash and Cash Equivalents	The following table provides a reconciliation of cash, cash equivalents and restricted cash in the consolidated balance sheets that sum to the total of the same such amounts shown in the consolidated statements of cash flows (in thousands): As of December 31, 2019 2018 Cash and cash equivalents $ 235,876 $ 658,364 Restricted cash 1,032 595 Restricted cash, non-current 10,791 11,532 Total cash, cash equivalents and restricted cash shown in the consolidated statements of cash flows $ 247,699 $ 670,491

Collaboration Agreements (Table

Collaboration Agreements (Tables)	12 Months Ended
	Dec. 31, 2019
Research and Development [Abstract]
Summary of Total Consolidated Net Revenues from Strategic Collaborators	The following tables summarize the effects of adopting ASC 606 on our consolidated financial statements at December 31, 2019, and for the year ended December 31, 2019 (in thousands, except per share data): As of December 31, 2019 Consolidated Balance Sheet As reported under ASC 606 Adjustments Balance without adoption of ASC 606 Deferred revenue, current $ 63,310 $ (8,054 ) $ 55,256 Deferred revenue, non-current 138,995 4,012 143,007 Accumulated deficit (1,496,454 ) 6,785 (1,489,669 ) Year Ended December 31, 2019 Consolidated Statement of Operations As reported under ASC 606 Adjustments Amount without adoption of ASC 606 Revenue: Collaboration revenue $ 42,803 $ 2,767 $ 45,570 Collaboration revenue from related party 5,233 32,002 37,235 Total revenue 60,209 34,769 94,978 Loss from operations (545,720 ) 34,769 (510,951 ) Loss before income taxes (514,716 ) 34,769 (479,947 ) Net loss (514,021 ) 34,769 (479,252 ) Net loss per share - basic and diluted (1.55 ) 0.10 (1.45 ) The cumulative effect of applying both ASC 606 and ASC 842 on our consolidated balance sheets as of January 1, 2019 was as follows (in thousands): Balance as of December 31, 2018 (1) Effect of the Adoption of Balance as of January 1, 2019 ASC 842 ASC 606 Assets: Accounts receivable $ 11,686 $ — $ (2,738 ) $ 8,948 Prepaid expenses and other current assets 28,399 (300 ) — 28,099 Property and equipment, net 211,977 (2,483 ) — 209,494 Right-of-use assets, operating leases — 63,334 — 63,334 Total assets $ 1,962,149 $ 60,551 $ (2,738 ) $ 2,019,962 Liabilities and Shareholders’ Equity: Deferred revenue $ 109,056 $ — $ (27,281 ) $ 81,775 Other current liabilities 3,464 5,867 — 9,331 Deferred revenue, non-current 165,352 — (3,441 ) 161,911 Operating lease liabilities, non-current — 64,250 — 64,250 Finance lease liabilities, non-current — 37,718 — 37,718 Deferred lease obligation, non-current 10,006 (10,006 ) — — Lease financing obligation 33,489 (33,489 ) — — Accumulated deficit (1,006,627 ) (3,789 ) 27,984 (982,432 ) Total liabilities and shareholders’ equity $ 1,962,149 $ 60,551 $ (2,738 ) $ 2,019,962 _______ (1) As reported in our 2018 Annual Report on Form 10-K. The following table summarizes our total consolidated net revenue from our strategic collaborators for the periods presented (in thousands): Years Ended Collaboration Revenue by Strategic Collaborator: December 31, 2019 December 31, 2019 December 31, 2018 Merck $ 36,608 $ 38,977 $ 66,082 AstraZeneca 5,233 37,235 45,993 Vertex 6,195 6,593 10,437 Total collaboration revenue $ 48,036 $ 82,805 $ 122,512
Changes in Balances of Receivables and Contract Liabilities	The following table presents changes in the balances of our receivables and contract liabilities related to our strategic collaboration agreements during the year ended December 31, 2019 (in thousands): January 1, 2019 Additions Deductions December 31, 2019 Contract Assets: Accounts receivable $ 4,612 $ 11,890 $ (14,530 ) $ 1,972 Contract Liabilities: Deferred revenue $ 240,924 $ 10,591 $ (51,987 ) $ 199,528 During the year ended December 31, 2019, we recognized the following revenue as a result of the change in the contract liability balances related to our collaboration agreements (in thousands): Revenue recognized in the period from: Year Ended December 31, 2019 Amounts included in contract liabilities at the beginning of the period (1) $ 51,987 (1) We first allocate revenue to the individual contract liability balance outstanding at the beginning of the period until the revenue exceeds that balance. If additional consideration is received on those contracts in subsequent periods, we assume all revenue recognized in the reporting period first applies to the beginning contract liability.

Financial Instruments (Tables)

Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Summary of Cash and Available-for-Sale Securities by Significant Investment Category	The following tables summarize our cash and available-for-sale securities by significant investment category at December 31, 2019 and 2018 (in thousands): December 31, 2019 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash and Cash Equivalents Current Marketable Securities Non- Current Marketable Securities Cash and cash equivalents $ 225,874 $ — $ — $ 225,874 $ 225,874 $ — $ — Available-for-sale: Level 2: Certificates of deposit 82,028 79 (6 ) 82,101 10,002 69,197 2,902 U.S. treasury securities 117,891 260 (2 ) 118,149 — 110,186 7,963 Debt securities of U.S. government agencies and corporate entities 834,187 2,708 (32 ) 836,863 — 687,741 149,122 $ 1,259,980 $ 3,047 $ (40 ) $ 1,262,987 $ 235,876 $ 867,124 $ 159,987 December 31, 2018 Amortized Cost Unrealized Gains Unrealized Losses Fair Value Cash and Cash Equivalents Current Marketable Securities Non- Current Marketable Securities Cash and cash equivalents $ 658,365 $ 20 $ (21 ) $ 658,364 $ 658,364 $ — $ — Available-for-sale: Level 2: Certificates of deposit 173,102 42 (36 ) 173,108 — 157,920 15,188 U.S. treasury securities 152,205 18 (48 ) 152,175 — 152,175 — Debt securities of U.S. government agencies and corporate entities 712,065 40 (1,335 ) 710,770 — 552,968 157,802 $ 1,695,737 $ 120 $ (1,440 ) $ 1,694,417 $ 658,364 $ 863,063 $ 172,990
Amortized Cost and Estimated Fair Value of Marketable Securities, by Contractual Maturity	The amortized cost and estimated fair value of marketable securities, by contractual maturity at December 31, 2019 and 2018 are as follows (in thousands): December 31, 2019 Amortized Cost Estimated Fair Value Due in one year or less $ 864,958 $ 867,124 Due after one year through five years 159,148 159,987 Total $ 1,024,106 $ 1,027,111 December 31, 2018 Amortized Cost Estimated Fair Value Due in one year or less $ 864,003 $ 863,063 Due after one year through five years 173,369 172,990 Total $ 1,037,372 $ 1,036,053

Balance Sheet Components (Table

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets, as of December 31, 2019 and 2018 consists of the following (in thousands): December 31, 2019 2018 Prepaid expenses $ 8,475 $ 10,401 Tenant incentives receivables 4,093 10,089 Interest receivable on marketable securities 6,835 7,909 Prepaid expenses and other current assets $ 19,403 $ 28,399
Schedule of Property and Equipment, Net	The estimated useful lives of property and equipment are described below: Estimated Useful Life Laboratory equipment 5 years Leasehold improvements Lesser of estimated useful life of improvement or remaining life of related lease Computer equipment and software 3 years Other assets including automobiles, furniture and fixtures 5 years Right of use asset, financing Lease term Property and equipment, net as of December 31, 2019 and 2018 consists of the following (in thousands): December 31, 2019 2018 Building (1) $ — $ 140,442 Laboratory equipment 108,257 96,907 Leasehold improvements (1) 152,426 13,741 Furniture, fixtures and other 3,316 2,122 Computer equipment and software 11,985 11,513 Internally developed software 7,020 7,020 Right-of-use asset, financing 9,853 — Construction in progress 3,222 4,688 296,079 276,433 Less: Accumulated depreciation (94,584 ) (64,456 ) Property and equipment, net $ 201,495 $ 211,977 _________ (1) Leasehold improvements related to our Moderna Technology Center manufacturing facility were previously capitalized as building under ASC 840 and are now included in leasehold improvements under ASC 842 (Note 2).
Schedule of Accrued Liabilities	Accrued liabilities, as of December 31, 2019 and 2018 consists of the following (in thousands): December 31, 2019 2018 In-licenses $ — $ 22,000 Property and equipment 4,029 12,089 Compensation-related 27,428 23,406 External goods and services 36,195 21,578 Accrued liabilities $ 67,652 $ 79,073

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2019
Leases [Abstract]
Assets And Liabilities, Lessee [Table Text Block]	Operating and financing lease right-of-use assets and lease liabilities as of December 31, 2019 and January 1, 2019 (the date of adoption of ASC 842) were as follows (in thousands): December 31, January 1, 2019 2019 Assets: Right-of-use assets, operating, net (1) (2) $ 86,414 $ 63,334 Right-of-use assets, financing, net (3) (4) 9,544 9,853 Total $ 95,958 $ 73,187 Liabilities: Current: Operating lease liabilities (5) $ 3,584 $ 6,455 Non-current: Operating lease liabilities, non-current 93,675 64,250 Financing lease liabilities, non-current 38,689 37,718 Total non-current lease liabilities 132,364 101,968 Total $ 135,948 $ 108,423 _______ (1) At December 31, 2019, these assets are real estate related assets, which include land, office and laboratory spaces. (2) Net of accumulated depreciation. (3) At December 31, 2019, these assets are real estate assets related to the MTC South lease. (4) Included in property and equipment in the consolidated balance sheets, net of accumulated depreciation. (5) Included in other current liabilities in the consolidated balance sheets.
Lease, Cost	Weighted average remaining lease terms and discount rates as of December 31, 2019 were as follows: December 31, 2019 Remaining lease term: Operating leases 12 years Finance leases 33 years Discount rate: Operating leases 9.7 % Finance leases 17.2 % Supplemental cash flow information relating to our leases for the year ended December 31, 2019 was as follows (in thousands): December 31, 2019 Cash paid for amounts included in measurement of lease liabilities: Operating cash flows used in operating leases $ (16,121 ) Operating cash flows used in financing leases (5,585 ) Financing cash flows provided by financing leases 971 Operating lease non-cash items: Right-of-use assets reduced through lease modifications and reassessments 2,717 Right-of-use assets obtained in exchange for operating lease liabilities 34,014 The components of the lease costs for the year ended December 31, 2019 were as follows (in thousands): December 31, 2019 Operating lease costs $ 17,015 Financing lease costs: Amortization of right-of-use assets, financing leases 309 Interest expense for financing lease liabilities 6,557 Total financing lease costs $ 6,866 Variable lease costs 4,399
Finance Lease Maturity	Future minimum lease payments under non-cancelable lease agreements as of December 31, 2019, were as follows (in thousands): Fiscal Year Operating Leases (1) Financing Leases (1) 2020 $ 12,563 $ 5,738 2021 13,814 5,894 2022 14,120 6,054 2023 14,365 6,219 2024 14,708 6,387 Thereafter 103,439 264,156 Total minimum lease payments 173,009 294,448 Less amounts representing interest or imputed interest (75,750 ) (255,759 ) (2) Present value of lease liabilities $ 97,259 $ 38,689 ______ (1) Include the optional extensions in the MTC South lease term which represent a total of $10.3 million and $208.5 million un-discounted future lease payments in operating leases and financing leases, respectively. (2) Based on an imputed interest rate of 17.2% .
Operating Lease Maturity	Future minimum lease payments under non-cancelable lease agreements as of December 31, 2019, were as follows (in thousands): Fiscal Year Operating Leases (1) Financing Leases (1) 2020 $ 12,563 $ 5,738 2021 13,814 5,894 2022 14,120 6,054 2023 14,365 6,219 2024 14,708 6,387 Thereafter 103,439 264,156 Total minimum lease payments 173,009 294,448 Less amounts representing interest or imputed interest (75,750 ) (255,759 ) (2) Present value of lease liabilities $ 97,259 $ 38,689 ______ (1) Include the optional extensions in the MTC South lease term which represent a total of $10.3 million and $208.5 million un-discounted future lease payments in operating leases and financing leases, respectively. (2) Based on an imputed interest rate of 17.2% .
Future Minimum Lease Payments Under Non-Cancelable Operating Lease Agreements	Prior to adoption of ASC 842, future minimum lease payments under non-cancelable operating lease agreements as of December 31, 2018, which were undiscounted and excluded non-lease components, were as follows (in thousands): Fiscal Year Minimum Lease Payments 2019 $ 20,027 2020 20,404 2021 20,937 2022 20,208 2023 17,235 Thereafter 112,958 Total $ 211,769 ______ (1) The amounts in the table above do not include any optional extensions in our lease terms.

Redeemable Convertible Prefer_2

Redeemable Convertible Preferred Stock and Common Stock (Tables)	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
Redeemable Convertible Preferred Stock Prior to IPO	Our redeemable convertible preferred stock on the day immediately prior to the closing of our IPO, or December 11, 2018, and December 31, 2017 consisted of the following (in thousands, except share amounts): Redeemable Convertible Preferred Shares Authorized Redeemable Carrying Value Liquidation Preference As of December 11, 2018 Series A redeemable convertible preferred stock 42,000,000 42,000,000 $ 182 $ 2,859 Series B redeemable convertible preferred stock 122,296,280 122,296,280 770 12,493 Series C redeemable convertible preferred stock 85,669,774 85,669,774 36,238 41,750 Series D redeemable convertible preferred stock 63,291,156 63,021,976 163,355 188,840 Series E redeemable convertible preferred stock 81,428,340 80,884,240 498,525 498,814 Series F redeemable convertible preferred stock 54,001,241 54,001,241 473,532 474,131 Series G redeemable convertible preferred stock 55,666,004 55,666,004 549,413 560,000 Series H redeemable convertible preferred stock 5,000,000 5,000,000 111,546 125,000 Balance at December 11, 2018 509,352,795 508,539,515 $ 1,833,561 $ 1,903,887 Redeemable Convertible Preferred Shares Authorized Redeemable Liquidation Preference As of December 31, 2017 Carrying Value Series A redeemable convertible preferred stock 42,000,000 42,000,000 $ 182 $ 2,701 Series B redeemable convertible preferred stock 122,296,280 122,296,280 770 11,801 Series C redeemable convertible preferred stock 85,669,774 85,669,774 36,238 39,676 Series D redeemable convertible preferred stock 63,291,156 63,291,156 164,059 179,462 Series E redeemable convertible preferred stock 81,428,340 81,428,340 501,880 502,169 Series F redeemable convertible preferred stock 54,001,241 54,001,241 473,532 474,131 Balance at December 31, 2017 448,686,791 448,686,791 $ 1,176,661 $ 1,209,940 The following table summarizes the activity for each series of our outstanding redeemable convertible preferred stock for the year ended December 31, 2018 (in thousands, except share data): Series A Redeemable Convertible Preferred Stock Series B Redeemable Convertible Preferred Stock Series C Redeemable Convertible Preferred Stock Series D Redeemable Convertible Preferred Stock Series E Redeemable Convertible Preferred Stock Series F Redeemable Convertible Preferred Stock Series G Redeemable Convertible Preferred Stock Series H Redeemable Convertible Preferred Stock Total Redeemable Convertible Preferred Stock Shares Amount Shares Amount Shares Amount Shares Amount Shares Amount Shares Amount Shares Amount Shares Amount Shares Amount Balance at December 31, 2017 42,000,000 $ 182 122,296,280 $ 770 85,669,774 $ 36,238 63,291,156 $ 164,059 81,428,340 $ 501,880 54,001,241 $ 473,532 — $ — — $ — 448,686,791 $ 1,176,661 Issuance of Series G redeemable convertible preferred stock, net of issuance costs of $10,517 — — — — — — — — — — — — 55,666,004 549,413 — — 55,666,004 549,413 Issuance of Series H redeemable convertible preferred stock, net of issuance costs of $474 — — — — — — — — — — — — — — 5,000,000 111,546 5,000,000 111,546 Repurchase of Series D redeemable convertible preferred stock — — — — — — (269,180 ) (704 ) — — — — — — — — (269,180 ) (704 ) Repurchase of Series E redeemable convertible preferred stock — — — — — — — — (544,100 ) (3,355 ) — — — — — — (544,100 ) (3,355 ) Conversion of redeemable convertible preferred stock (42,000,000 ) (182 ) (122,296,280 ) (770 ) (85,669,774 ) (36,238 ) (63,021,976 ) (163,355 ) (80,884,240 ) (498,525 ) (54,001,241 ) (473,532 ) (55,666,004 ) (549,413 ) (5,000,000 ) (111,546 ) (508,539,515 ) (1,833,561 ) Balance at December 31, 2018 — $ — — $ — — $ — — $ — — $ — — $ — — $ — — $ — — $ —

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2019
Share-based Payment Arrangement [Abstract]
Options Activity	The following table summarizes our option activity as of December 31, 2019 and 2018: Number of Options Weighted Average Exercise Price per Share Weighted Average Grant Date Fair Value per Share Weighted- Average Remaining Contractual Term Aggregate Intrinsic Value (1) (in thousands) Outstanding at December 31, 2018 50,821,132 12.16 6.59 7.1 years 220,434 Granted 7,354,268 19.58 11.35 Exercised (6,945,306 ) 6.86 4.47 Canceled/forfeited (5,693,179 ) 15.17 8.67 Outstanding at December 31, 2019 45,536,915 13.82 7.35 7.2 years 286,310 Exercisable at December 31, 2019 22,872,221 10.13 4.86 5.9 years 216,241 Expected to vest at December 31, 2019 22,664,694 17.54 9.86 8.6 years 70,069 _______ (1) Aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of common stock for those options in the money as of December 31, 2019 .
Restricted Common Stock Activity	The following table summarizes our restricted stock activity during the year ended December 31, 2019 : Number of Weighted Average Grant Date Price per Share Outstanding, non-vested at December 31, 2018 198,597 $ 12.15 Issued — — Vested (162,717 ) 12.15 Canceled/forfeited (35,880 ) 12.15 Outstanding, non-vested at December 31, 2019 — —
Restricted Common Stock Units Activity	The following table summarizes our restricted stock unit activity during the year ended December 31, 2019 : Number of Units Weighted Average per Unit Outstanding, non-vested at December 31, 2018 458,715 $ 11.93 Issued 1,283,662 19.12 Vested (57,344 ) 11.93 Canceled/forfeited (106,413 ) 20.25 Vested in prior year and settled during the period (401,371 ) 11.93 Outstanding, non-vested at December 31, 2019 1,177,249 19.01
Weighted-Average Assumptions Used to Estimate the Fair Value of Options Granted	The weighted-average assumptions used to estimate the fair value of options granted and ESPP for the years ended December 31, 2019 , 2018 and 2017 are as follows: Weighted Average Years Ended December 31, 2019 2018 2017 Options: Risk-free interest rate 2.29 % 2.76 % 2.02 % Expected term 6.07 years 6.27 years 6.21 years Expected volatility 61 % 63 % 63 % Expected dividends — % — % — % Weighted average fair value per share $ 11.35 $ 9.33 $ 7.96 ESPP: Risk-free interest rate 1.95 % * * Expected term 0.50 years * * Expected volatility 53 % * * Expected dividends — % * * Weighted average fair value per share $ 5.98 * * _________ * - Not applicable
Stock-Based Compensation Expense	The following table presents the components and classification of stock-based compensation expense for the years ended December 31, 2019 , 2018 and 2017 as follows (in thousands): Years Ended 2019 2018 2017 Options $ 74,780 $ 63,288 $ 31,724 Restricted common stock and units 5,125 9,277 8,331 Employee stock purchase plan 1,217 — — Total $ 81,122 $ 72,565 $ 40,055 Research and development $ 48,259 $ 37,659 $ 21,679 General and administrative 32,863 34,906 18,376 Total $ 81,122 $ 72,565 $ 40,055

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Loss Before Provision For (Benefit From) Income Taxes	Loss before (benefit from) provision for income taxes for the years ended December 31, 2019 , 2018 and 2017 consist of the following (in thousands): Years Ended December 31, 2019 2018 2017 United States $ (508,595 ) $ (380,473 ) $ (247,784 ) Foreign (6,121 ) (3,935 ) (8,212 ) Loss before (benefit from) provision for income taxes $ (514,716 ) $ (384,408 ) $ (255,996 )
Provision For (Benefit From) Income Taxes	The (benefit from) provision for income taxes for the years ended December 31, 2019 , 2018 and 2017 consist of the following components (in thousands): Years Ended December 31, 2019 2018 2017 Current: Federal $ — $ (26 ) $ (252 ) State 505 352 172 Total current 505 326 (80 ) Deferred: Federal (1,200 ) — — Total deferred (1,200 ) — — Total (benefit from) provision for income taxes $ (695 ) $ 326 $ (80 )
Reconciliation of U.S. Statutory Income Tax Rate to Effective Tax Rate	The reconciliation of the U.S. statutory income tax rate to our effective tax rate for the years ended December 31, 2019 , 2018 and 2017 are as follows: Years Ended December 31, 2019 2018 2017 Tax effected at statutory rate 21.0 % 21.0 % 34.0 % State taxes, net of federal benefit 7.9 % 6.3 % 8.5 % Non-deductible items 1.6 % 0.0 % (1.3 )% Change in valuation allowance (33.0 )% (28.5 )% (20.4 )% Federal research and development credits 2.5 % 1.5 % 4.8 % Foreign tax rate differential (0.2 )% (0.2 )% (1.1 )% Impact of federal rate change on net deferred taxes 0.0 % 0.0 % (25.0 )% Other 0.2 % (0.2 )% 0.5 % Effective tax rate 0.0 % (0.1 )% 0.0 %
Significant Components of Deferred Tax Assets and Tax Liabilities	The significant components of our deferred tax assets and tax liabilities as of December 31, 2019 and 2018 are as follows (in thousands): December 31, 2019 2018 Deferred tax assets: Net operating loss carry-forwards $ 268,173 $ 114,932 Stock-based compensation 43,978 33,138 Capitalized licenses, R&D and start-up costs 19,891 22,143 Tax credit carry-forwards 77,222 53,617 Accrued expenses 7,377 16,443 Deferred revenue 53,475 71,209 Operating lease liabilities 26,571 — Lease financing obligation 10,570 9,149 Other 11 2,870 Total gross deferred tax assets 507,268 323,501 Less: valuation allowance (470,753 ) (308,273 ) Total deferred tax assets, net of valuation allowance 36,515 15,228 Deferred tax liabilities: Financing right-of-use assets (2,612 ) — Operating right-of-use assets (24,944 ) — Fixed assets (8,959 ) (15,228 ) Total deferred tax liabilities (36,515 ) (15,228 ) Net deferred tax assets $ — $ —
Reconciliation of the Beginning and Ending Amounts of Unrecognized Tax Benefits Roll Forward	A reconciliation of the beginning and ending amounts of unrecognized tax benefits during the years ended December 31, 2019 and 2018 are as follows (in thousands): Balance as of December 31, 2017 $ 940 Decrease due to prior positions (799 ) Increase due to current year tax position — Balance as of December 31, 2018 141 Decrease due to prior positions — Increase due to current year tax positions — Balance as of December 31, 2019 $ 141

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Basic and Diluted Net Loss per Share Attributable to Common Stockholders	Basic and diluted net loss per share attributable to common stockholders for the years ended December 31, 2019 , 2018 and 2017 are calculated as follows (in thousands, except share and per share data): Years Ended 2019 2018 2017 Numerator: Net loss $ (514,021 ) $ (384,734 ) $ (255,916 ) Premium paid on repurchase of redeemable convertible preferred stock — (4,127 ) — Cumulative dividends on redeemable convertible preferred stock — (12,996 ) (13,925 ) Net loss attributable to common stockholders $ (514,021 ) $ (401,857 ) $ (269,841 ) Denominator: Weighted average common shares used in net loss per share attributable to common stockholders, basic and diluted 330,802,136 81,114,183 64,497,544 Net loss per share attributable to common stockholders, basic and diluted $ (1.55 ) $ (4.95 ) $ (4.18 )
Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share	The following common stock equivalents, presented based on amounts outstanding as of December 31, 2019 , 2018 and 2017 were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because their inclusion would have been anti-dilutive: December 31, 2019 2018 2017 Redeemable convertible preferred stock — — 205,151,299 Stock options 45,536,915 50,821,132 33,684,134 Restricted common stock — 198,597 1,081,205 Restricted common stock units 1,177,249 458,715 458,715 46,714,164 51,478,444 240,375,353

Related Party Transactions (Tab

Related Party Transactions (Tables)	12 Months Ended
	Dec. 31, 2019
Related Party Transactions [Abstract]
Schedule of Redeemable Preferred Stock Issued to Related Parties	The following table sets forth the number of shares of our Series G redeemable convertible preferred stock that we issued to our related parties in this transaction (in thousands, except share data): Name Shares of Total OCHA LLC (1) 50,000 $ 503 Viking Global Investors LP and affiliated entities (2) 745,526 $ 7,500 ______ (1) OCHA LLC is an entity controlled by an officer. (2) Consists of (1) 279,160 shares of Series G redeemable convertible preferred stock held by VGE III Portfolio Ltd.; (2) 148,974 shares of Series G preferred stock held by Viking Global Equities LP; (3) 8,737 shares of Series G redeemable convertible preferred stock held by Viking Global Equities II LP; (4) 129,537 shares of Series G preferred stock held by Viking Global Opportunities Illiquid Investments Sub-Master LP; and (5) 179,118 shares of Series G redeemable convertible preferred stock held by Viking Long Fund Master Ltd.

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Unaudited Selected Quarterly Financial Data	The following tables provide the unaudited selected quarterly financial data for the years ended December 31, 2019 and 2018 (in thousands, except per share data). We believe that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results of any future period. 2019 First Quarter (1) Second Quarter (1) Third Quarter (1) Fourth Quarter Total revenue $ 16,025 $ 13,083 $ 17,046 $ 14,055 Total operating expenses 157,665 156,792 146,861 144,611 Loss from operations (141,640 ) (143,709 ) (129,815 ) (130,556 ) Net loss per share attributable to common stockholders - basic and diluted $ (0.40 ) $ (0.41 ) $ (0.37 ) $ (0.37 ) 2018 First Quarter Second Quarter Third Quarter Fourth Quarter Total revenue $ 29,039 $ 28,851 $ 41,757 $ 35,421 Total operating expenses 106,441 125,866 127,575 188,452 Loss from operations (77,402 ) (97,015 ) (85,818 ) (153,031 ) Net loss per share attributable to common stockholders - basic and diluted $ (1.16 ) $ (1.43 ) $ (1.32 ) $ (1.14 ) _______ (1) As a result of our adoption of ASC 842 as of January 1, 2019 (Note 2), quarterly amounts presented in our prior Forms 10-Q were revised. Total operating expenses and Loss from operations were both adjusted by $0.2 million , $0.2 million and $1.0 million , for the first, second and third quarters of 2019, respectively.

Organization and Description _2

Organization and Description of Business (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 11, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Subsidiary, Sale of Stock [Line Items]
Operating losses		$ 514,021	$ 384,734	$ 255,916
Accumulated deficit		$ 1,496,454	$ 1,006,627		$ 982,432
IPO
Subsidiary, Sale of Stock [Line Items]
Shares of common stock issued (in shares)	26,275,993
Price per share (usd per share)	$ 23
Aggregate net proceeds from the offering	$ 563,000

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Accounting Policies Narrative (Details)	12 Months Ended
	Dec. 31, 2019USD ($)segmentshares	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)shares	Jan. 01, 2019USD ($)	Dec. 11, 2018shares
Accounting Policies [Abstract]
Number of operating segments | segment	1
Impairment charges related to marketable securities	$ 0	$ 0	$ 0
Allowance for doubtful accounts	0	0
Bad debt expense	0	0	0
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of long-lived assets	0	0	$ 0
Decrease in deferred tax asset	(507,268,000)	(323,501,000)
Valuation allowance	470,753,000	308,273,000
Remaining deferred revenues	199,528,000	$ 240,924,000
Right-of-use assets, operating leases	86,414,000			$ 63,334,000
Operating lease, liability	$ 97,259,000
Redeemable convertible preferred stock, shares issued | shares	0	0	448,686,791		508,539,515
Redeemable convertible preferred stock, shares outstanding | shares	0	0	448,686,791		508,539,515
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt instrument term	12 months
Marketable securities maturity term	12 months
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities maturity term	12 months
Accounting Standards Update 2014-09
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (decrease) in cumulative effect of applying new standard				27,984,000
Decrease in deferred tax asset				8,400,000
Valuation allowance				(8,400,000)
Increase (decrease) in deferred revenue				(30,700,000)
Accounting Standards Update 2016-02
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (decrease) in cumulative effect of applying new standard				(3,790,000)
Right-of-use assets, operating leases				63,300,000
Operating lease, liability				70,700,000
Accumulated Deficit | Accounting Standards Update 2014-09
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (decrease) in cumulative effect of applying new standard				27,984,000
Accumulated Deficit | Accounting Standards Update 2016-02
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (decrease) in cumulative effect of applying new standard				(3,790,000)
Combined 2018 AZ Agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining deferred revenues	$ 73,700,000	$ 115,600,000		75,700,000
Combined 2018 AZ Agreements | Accounting Standards Update 2014-09
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (decrease) in deferred revenue				39,900,000
PCVSAV Agreement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining deferred revenues	83,800,000	111,300,000		125,200,000
PCVSAV Agreement | Accounting Standards Update 2014-09
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (decrease) in deferred revenue				13,900,000
VEGF Exercise 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining deferred revenues	41,200,000	41,200,000		37,100,000
VEGF Exercise 2016 | Accounting Standards Update 2014-09
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (decrease) in deferred revenue				$ 4,300,000
IPO | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred stock converted into common stock (in shares) | shares					236,012,913
Fair Value, Inputs, Level 1 | Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Letters of credit	$ 11,800,000	$ 12,100,000

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Significant Customers (Details) - Customer Concentration Risk	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue | Strategic Collaborators
Concentration Risk [Line Items]
Concentration risk, percentage	80.00%	91.00%	86.00%
Revenue | Merck
Concentration Risk [Line Items]
Concentration risk, percentage	61.00%	49.00%	31.00%
Revenue | Vertex
Concentration Risk [Line Items]
Concentration risk, percentage	10.00%
Revenue | AstraZeneca
Concentration Risk [Line Items]
Concentration risk, percentage		34.00%	15.00%
Revenue | Alexion
Concentration Risk [Line Items]
Concentration risk, percentage			36.00%
Revenue | Government-Sponsored and Private Organizations
Concentration Risk [Line Items]
Concentration risk, percentage	20.00%	9.00%	14.00%
Revenue | BARDA
Concentration Risk [Line Items]
Concentration risk, percentage	13.00%		10.00%
Accounts Receivable | Merck
Concentration Risk [Line Items]
Concentration risk, percentage	13.00%	30.00%
Accounts Receivable | Vertex
Concentration Risk [Line Items]
Concentration risk, percentage	17.00%	22.00%
Accounts Receivable | BARDA
Concentration Risk [Line Items]
Concentration risk, percentage	54.00%	13.00%
Accounts Receivable | DARPA
Concentration Risk [Line Items]
Concentration risk, percentage		16.00%
Accounts Receivable | Massachusetts Life Sciences Center
Concentration Risk [Line Items]
Concentration risk, percentage		12.00%

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Property and Equipment and Software Capitalization (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment, net		$ 209,494,000	$ 211,977,000
Laboratory equipment
Property, Plant and Equipment [Line Items]
Estimated Useful Life	5 years
Computer equipment and software
Property, Plant and Equipment [Line Items]
Estimated Useful Life	3 years
Other assets including automobiles, furniture and fixtures
Property, Plant and Equipment [Line Items]
Estimated Useful Life	5 years
Capitalized software
Property, Plant and Equipment [Line Items]
Property and equipment, net			$ 0

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 1,530,241	$ (551,365)	$ (334,810)
Other comprehensive loss	3,124	(163)	(754)
Balance at end of period	1,174,810	1,530,241	(551,365)
Unrealized Gain (Loss) on Available-for-Sale Debt Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1,320)	(1,157)
Other comprehensive loss	3,124	(163)
Balance at end of period	$ 1,804	$ (1,320)	$ (1,157)

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Effect of Adoption of ASC 606 on Condensed Consolidated Financial Statements (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									$ 60,209
Loss from operations	$ (130,556)	$ (129,815)	$ (143,709)	$ (141,640)	$ (153,031)	$ (85,818)	$ (97,015)	$ (77,402)	(545,720)	$ (413,266)	$ (269,356)
Loss before income taxes									(514,716)	(384,408)	(255,996)
Net loss									$ (514,021)	$ (384,734)	$ (255,916)
Net loss per share - basic and diluted (in dollars per share)	$ (0.37)	$ (0.37)	$ (0.41)	$ (0.40)	$ (1.14)	$ (1.32)	$ (1.43)	$ (1.16)	$ (1.55)	$ (4.95)	$ (4.18)
Accounts receivable	$ 5,032				$ 11,686				$ 5,032	$ 11,686		$ 8,948
Prepaid expenses and other current assets	19,403				28,399				19,403	28,399		28,099
Property and equipment, net					211,977					211,977		209,494
Right-of-use assets, operating leases	86,414								86,414			63,334
Assets	1,589,422				1,962,149				1,589,422	1,962,149		2,019,962
Deferred revenue, current	63,310				109,056				63,310	109,056		81,775
Other current liabilities	5,063				3,464				5,063	3,464		9,331
Deferred revenue, non-current	138,995				165,352				138,995	165,352		161,911
Operating lease liabilities, non-current	93,675								93,675			64,250
Financing lease liabilities, non-current	38,689								38,689			37,718
Deferred lease obligation, non-current					10,006					10,006
Lease financing obligation					33,489					33,489
Accumulated deficit	(1,496,454)				(1,006,627)				(1,496,454)	(1,006,627)		(982,432)
Total liabilities and stockholders’ equity	1,589,422				1,962,149				1,589,422	1,962,149		2,019,962
Balance without adoption of ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									94,978
Loss from operations									(510,951)
Loss before income taxes									(479,947)
Net loss									$ (479,252)
Net loss per share - basic and diluted (in dollars per share)									$ (1.45)
Deferred revenue, current	55,256								$ 55,256
Deferred revenue, non-current	143,007								143,007
Accumulated deficit	(1,489,669)								(1,489,669)
Accounting Standards Update 2016-02
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Right-of-use assets, operating leases												63,300
Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable												$ (2,700)
Accounting Standards Update 2014-09 | Adjustments
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									34,769
Loss from operations									34,769
Loss before income taxes									34,769
Net loss									$ 34,769
Net loss per share - basic and diluted (in dollars per share)									$ 0.10
Deferred revenue, current	(8,054)								$ (8,054)
Deferred revenue, non-current	4,012								4,012
Accumulated deficit	$ 6,785								6,785
Collaboration revenue
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									42,803
Collaboration revenue | Balance without adoption of ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									45,570
Collaboration revenue | Accounting Standards Update 2014-09 | Adjustments
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									2,767
Collaboration revenue from related party
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									5,233
Collaboration revenue from related party | Balance without adoption of ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									37,235
Collaboration revenue from related party | Accounting Standards Update 2014-09 | Adjustments
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									$ 32,002
Previously Reported
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable					11,686					11,686
Prepaid expenses and other current assets					28,399					28,399
Property and equipment, net					211,977					211,977
Assets					1,962,149					1,962,149
Deferred revenue, current					109,056					109,056
Other current liabilities					3,464					3,464
Deferred revenue, non-current					165,352					165,352
Deferred lease obligation, non-current					10,006					10,006
Lease financing obligation					33,489					33,489
Accumulated deficit					(1,006,627)					(1,006,627)
Total liabilities and stockholders’ equity					1,962,149					1,962,149
Adjustment | Accounting Standards Update 2016-02
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Prepaid expenses and other current assets					(300)					(300)
Property and equipment, net					(2,483)					(2,483)
Right-of-use assets, operating leases					63,334					63,334
Assets					60,551					60,551
Other current liabilities					5,867					5,867
Operating lease liabilities, non-current					64,250					64,250
Financing lease liabilities, non-current					37,718					37,718
Deferred lease obligation, non-current					(10,006)					(10,006)
Lease financing obligation					(33,489)					(33,489)
Accumulated deficit					(3,789)					(3,789)
Total liabilities and stockholders’ equity					60,551					60,551
Adjustment | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable					(2,738)					(2,738)
Assets					(2,738)					(2,738)
Deferred revenue, current					(27,281)					(27,281)
Deferred revenue, non-current					(3,441)					(3,441)
Accumulated deficit					27,984					27,984
Total liabilities and stockholders’ equity					$ (2,738)					$ (2,738)

Summary of Significant Accoun_9

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 235,876	$ 658,364
Restricted cash	1,032	595
Restricted cash, non-current	10,791	11,532
Total cash, cash equivalents and restricted cash shown in the consolidated statements of cash flows	$ 247,699	$ 670,491	$ 147,608	$ 61,560

Collaboration Agreements - Summ

Collaboration Agreements - Summary of Total Consolidated Net Revenues from Strategic Collaborators (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total collaboration revenue	$ 60,209
Collaboration Revenue by Strategic Collaborator
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total collaboration revenue	48,036
Balance without adoption of ASC 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total collaboration revenue	94,978
Balance without adoption of ASC 606 | Collaboration Revenue by Strategic Collaborator
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total collaboration revenue	82,805	$ 122,512
Merck | Collaboration Revenue by Strategic Collaborator
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total collaboration revenue	36,608
Merck | Balance without adoption of ASC 606 | Collaboration Revenue by Strategic Collaborator
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total collaboration revenue	38,977	66,082
AstraZeneca | Collaboration Revenue by Strategic Collaborator
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total collaboration revenue	5,233
AstraZeneca | Balance without adoption of ASC 606 | Collaboration Revenue by Strategic Collaborator
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total collaboration revenue	37,235	45,993
Vertex | Collaboration Revenue by Strategic Collaborator
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total collaboration revenue	6,195
Vertex | Balance without adoption of ASC 606 | Collaboration Revenue by Strategic Collaborator
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total collaboration revenue	$ 6,593	$ 10,437

Collaboration Agreements - Chan

Collaboration Agreements - Changes in Balances of Receivables and Contract Liabilities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Contract Assets:
Beginning balance	$ 4,612
Additions	11,890
Deductions	(14,530)
Ending balance	1,972
Contract Liabilities:
Beginning balance	240,924
Additions	10,591
Deductions	(51,987)
Ending balance	199,528
Amounts included in contract liabilities at the beginning of the period	$ 51,987

Collaboration Agreements - Astr

Collaboration Agreements - AstraZeneca - Strategic Alliances in Cardiovascular and Oncology (Details)

1 Months Ended

3 Months Ended

12 Months Ended

Jun. 30, 2018option

Oct. 31, 2017

Jan. 31, 2016

Mar. 31, 2013USD ($)milestoneoption

Dec. 31, 2019USD ($)

Sep. 30, 2019USD ($)

Jun. 30, 2019USD ($)

Mar. 31, 2019USD ($)

Dec. 31, 2018USD ($)

Sep. 30, 2018USD ($)

Jun. 30, 2018USD ($)option

Mar. 31, 2018USD ($)

Dec. 31, 2019USD ($)

Dec. 31, 2018USD ($)

Dec. 31, 2017USD ($)

Dec. 31, 2016USD ($)

Jan. 01, 2019USD ($)

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

$ 249,500,000

$ 249,500,000

Deferred revenue

199,528,000

$ 240,924,000

199,528,000

$ 240,924,000

Revenue from contract with customer, excluding assessed tax

60,209,000

Collaboration revenue

14,055,000

$ 17,046,000

$ 13,083,000

$ 16,025,000

35,421,000

$ 41,757,000

$ 28,851,000

$ 29,039,000

60,209,000

135,068,000

$ 205,825,000

Collaboration revenue

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Revenue from contract with customer, excluding assessed tax

42,803,000

Collaboration revenue

42,803,000

76,519,000

146,953,000

Combined 2018 AZ Agreements

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

$ 400,000,000

Increase to the transaction price

1,000,000

1,800,000

Change in estimate of variable consideration

800,000

Deferred revenue

73,700,000

115,600,000

73,700,000

115,600,000

75,700,000

Combined 2018 AZ Agreements | Collaboration revenue

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Revenue from contract with customer, excluding assessed tax

4,300,000

45,400,000

$ 30,000,000

2013 AZ Agreements

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

240,000,000

Number of options to develop candidates | option

40

Upfront cash payments

$ 240,000,000

$ 10,000,000

Estimated arrangement consideration

$ 180,000,000

Number of milestones | milestone

3

Option exercise fee

$ 10,000,000

Termination period

90 days

2013 AZ Agreements | VEGF-A product (AZD8601)

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Upfront cash payments

30,000,000

2013 AZ Agreements | Maximum

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Earn-out payments

12.00%

2013 AZ Agreements | Development Milestones

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Estimated arrangement consideration

$ 100,000,000

2013 AZ Agreements | Regulatory Milestones

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Estimated arrangement consideration

100,000,000

2013 AZ Agreements | Commercial Milestones

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Estimated arrangement consideration

$ 200,000,000

VEGF Exercise 2016

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

55,100,000

Increase to the transaction price

$ 2,300,000

2,800,000

Change in estimate of variable consideration

500,000

Deferred revenue

41,200,000

$ 41,200,000

41,200,000

41,200,000

37,100,000

2016 AZ Agreement

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Termination period

90 days

Suspension period

12 months

2016 AZ Agreement | Minimum

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Tiered royalties

10.00%

2016 AZ Agreement | Maximum

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Tiered royalties at default rate

20.00%

Tiered royalties

30.00%

2017 AZ Agreement

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Termination period

90 days

Suspension period

12 months

Collaboration revenue

0

$ 600,000

2017 AZ Agreement | Minimum

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Tiered royalties

10.00%

2017 AZ Agreement | Maximum

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Tiered royalties

30.00%

2018 A&R Agreements

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Number of options to develop candidates | option

40

40

Termination period

90 days

Variable Consideration | Combined 2018 AZ Agreements

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

160,000,000

Variable Consideration | VEGF Exercise 2016

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

15,100,000

Estimated Reimbursement | Combined 2018 AZ Agreements

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

40,000,000

Estimated Reimbursement | Combined 2018 AZ Agreements | Toxicity Milestone

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

60,000,000

60,000,000

Fixed Payments | VEGF Exercise 2016

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

40,000,000

Milestone Payments | Combined 2018 AZ Agreements

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

120,000,000

Milestone Payments | Combined 2018 AZ Agreements | Toxicity Milestone

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

60,000,000

Milestone Payments | Combined 2018 AZ Agreements | Competition Milestone

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

60,000,000

Milestone Payments | VEGF Exercise 2016

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

30,000,000

Option Exercise Fee | VEGF Exercise 2016

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

$ 10,000,000

Combined 2018 AZ Agreement Performance Obligation | Combined 2018 AZ Agreements

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

293,200,000

293,200,000

IL12 Performance Obligation | Combined 2018 AZ Agreements

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

8,100,000

8,100,000

Oncology Development Target Performance Obligation | Combined 2018 AZ Agreements

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

8,100,000

8,100,000

Development And Commercialization License And Manufacturing Obligations For IL12 | Combined 2018 AZ Agreements

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

90,700,000

90,700,000

Development And Commercialization Rights And Manufacturing Services For An Oncology Development Target | Combined 2018 AZ Agreements

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

$ 1,600,000

$ 1,600,000

Collaboration Agreements - Merc

Collaboration Agreements - Merck – Strategic Alliances in Infectious Diseases and Cancer Vaccines (Details)

1 Months Ended

3 Months Ended

12 Months Ended

34 Months Ended

53 Months Ended

May 31, 2018USD ($)

Apr. 30, 2018USD ($)

Jun. 30, 2016USD ($)

Jan. 31, 2016USD ($)

Jan. 31, 2015USD ($)

Dec. 31, 2019USD ($)

Sep. 30, 2019USD ($)

Jun. 30, 2019USD ($)

Mar. 31, 2019USD ($)

Dec. 31, 2018USD ($)

Sep. 30, 2018USD ($)

Jun. 30, 2018USD ($)

Mar. 31, 2018USD ($)

Dec. 31, 2019USD ($)

Dec. 31, 2018USD ($)

Dec. 31, 2017USD ($)

Dec. 31, 2015

May 31, 2019USD ($)candidate

Jan. 01, 2019USD ($)

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Revenue from contract with customer, excluding assessed tax

$ 60,209,000

Collaboration revenue

$ 14,055,000

$ 17,046,000

$ 13,083,000

$ 16,025,000

$ 35,421,000

$ 41,757,000

$ 28,851,000

$ 29,039,000

60,209,000

$ 135,068,000

$ 205,825,000

Transaction price

249,500,000

249,500,000

Deferred revenue

199,528,000

240,924,000

199,528,000

240,924,000

Series H Redeemable Convertible Preferred Stock

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Aggregate net proceeds from the offering

$ 112,000,000

Premium recorded to deferred revenue

$ 13,000,000

Collaboration revenue

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Revenue from contract with customer, excluding assessed tax

42,803,000

Collaboration revenue

42,803,000

76,519,000

146,953,000

Merck Agreement 2015

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Performance period

3 years

3 years

Number of product candidates | candidate

5

Increase (decrease) in deferred revenue

$ 50,000,000

Upfront cash payments

$ 10,000,000

Estimated arrangement consideration

$ 300,000,000

Revenue from contract with customer, excluding assessed tax

24,600,000

22,900,000

Proceeds from equity investment

$ 50,000,000

Research period

4 years

4 years

Transaction price

$ 65,000,000

Increase to the transaction price

(5,300,000)

Deferred revenue

0

0

Separate Agreements With Merck | Collaboration revenue

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Revenue from contract with customer, excluding assessed tax

(4,800,000)

PCV Agreement

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Upfront cash payments

$ 200,000,000

Estimated arrangement consideration

$ 250,000,000

PCV Agreement | Series H Redeemable Convertible Preferred Stock

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Aggregate net proceeds from the offering

$ 125,000,000

Premium recorded to deferred revenue

13,000,000

PCV Agreement | PCV products

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Estimated arrangement consideration

$ 243,000,000

243,000,000

243,000,000

243,000,000

243,000,000

PCVSAV Agreement

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

213,000,000

Increase to the transaction price

0

Deferred revenue

83,800,000

$ 111,300,000

83,800,000

111,300,000

125,200,000

PCVSAV Agreement | Collaboration revenue

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Revenue from contract with customer, excluding assessed tax

41,400,000

$ 41,500,000

$ 40,000,000

Milestone Payments | Merck Agreement 2015

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Revenue from contract with customer, excluding assessed tax

$ 5,000,000

Transaction price

5,000,000

Upfront Payment | Merck Agreement 2015

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

60,000,000

Upfront Payment | PCVSAV Agreement

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

200,000,000

Upfront Payment, Premium Associated With Contemporaneous Sale Of Preferred Stock | PCVSAV Agreement

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

13,000,000

Upfront Payment, Research And Development | Merck Agreement 2015

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

50,000,000

Upfront Payment, Funding From 2016 Amendment | Merck Agreement 2015

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

$ 10,000,000

PCV Performance Obligation | PCVSAV Agreement

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

206,300,000

206,300,000

KRAS Performance Obligation | PCVSAV Agreement

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

$ 6,700,000

$ 6,700,000

Collaboration Agreements - Vert

Collaboration Agreements - Vertex – 2016 Strategic Alliance in Cystic Fibrosis (Details)

1 Months Ended

3 Months Ended

12 Months Ended

Jul. 31, 2019USD ($)extension_period

Jul. 31, 2016USD ($)

Dec. 31, 2019USD ($)

Sep. 30, 2019USD ($)

Jun. 30, 2019USD ($)

Mar. 31, 2019USD ($)

Dec. 31, 2018USD ($)

Sep. 30, 2018USD ($)

Jun. 30, 2018USD ($)

Mar. 31, 2018USD ($)

Dec. 31, 2019USD ($)

Dec. 31, 2018USD ($)

Dec. 31, 2017USD ($)

Jan. 01, 2019USD ($)

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

$ 249,500,000

$ 249,500,000

Deferred revenue

199,528,000

$ 240,924,000

199,528,000

$ 240,924,000

Collaboration revenue

14,055,000

$ 17,046,000

$ 13,083,000

$ 16,025,000

35,421,000

$ 41,757,000

$ 28,851,000

$ 29,039,000

60,209,000

135,068,000

$ 205,825,000

Revenue from contract with customer, excluding assessed tax

60,209,000

Collaboration revenue

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Collaboration revenue

42,803,000

76,519,000

146,953,000

Revenue from contract with customer, excluding assessed tax

42,803,000

Vertex Agreement

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Upfront cash payments

$ 20,000,000

Research period

3 years

Transaction price

4,100,000

$ 24,400,000

Deferred revenue

$ 800,000

$ 3,300,000

800,000

3,300,000

Research extension period

1 year

Proceeds from equity investment

$ 20,000,000

Termination period

90 days

Termination period upon marketing approval

180 days

Research extension period elected

6 months

Revenue from contract with customer, excluding assessed tax

$ 2,000,000

Research extension period

18 months

Number of extension periods | extension_period

2

Vertex Agreement | Collaboration revenue

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Revenue from contract with customer, excluding assessed tax

6,200,000

$ 10,400,000

$ 9,100,000

Vertex Agreement | Development Milestones

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Estimated arrangement consideration

$ 55,000,000

Vertex Agreement | Regulatory Milestones

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Estimated arrangement consideration

220,000,000

Vertex Agreement | Regulatory Milestones, Subsequent Products

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Estimated arrangement consideration

$ 3,000,000

Upfront Payment | Vertex Agreement

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

2,000,000

20,000,000

Research And Development Funding | Vertex Agreement

Research and Development Arrangement, Contract to Perform for Others [Line Items]

Transaction price

$ 2,100,000

$ 4,400,000

Change in estimation

$ 100,000

Collaboration Agreements - Alex

Collaboration Agreements - Alexion – 2014 Strategic Alliance in Rare Diseases (Details)	1 Months Ended	3 Months Ended								12 Months Ended
	Jan. 31, 2014USD ($)candidate	Dec. 31, 2019USD ($)	Sep. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Deferred revenue recognized										$ 51,987,000
Collaboration revenue		$ 14,055,000	$ 17,046,000	$ 13,083,000	$ 16,025,000	$ 35,421,000	$ 41,757,000	$ 28,851,000	$ 29,039,000	60,209,000	$ 135,068,000	$ 205,825,000
Deferred revenue		199,528,000				240,924,000				199,528,000	240,924,000
Collaboration revenue
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue										42,803,000	76,519,000	146,953,000
2014 Alexion Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Upfront cash payments	$ 100,000,000
Proceeds from equity investment	$ 25,000,000
Number of product candidates | candidate	10
Arrangement term	10 years
Deferred revenue recognized											70,300,000
Deferred revenue		$ 0				$ 0				0	$ 0
2014 Alexion Agreements | Collaboration revenue
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration revenue										$ 0		$ 74,400,000

Collaboration Agreements - Timi

Collaboration Agreements - Timing of Differences (Details) - USD ($) $ in Millions	Dec. 31, 2019	Jun. 30, 2019	Jan. 01, 2019
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price	$ 249.5
PCVSAV Agreement
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price			$ 213
PCVSAV Agreement | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price	83.8
Combined 2018 AZ Agreements
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price			400
Combined 2018 AZ Agreements | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price	100.7
Combined 2018 AZ Agreements | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: (nil)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price	110.4
VEGF Exercise 2016
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price			55.1
VEGF Exercise 2016 | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price	53.7
Vertex Agreement
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price		$ 4.1	$ 24.4
Vertex Agreement | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price	$ 1.6

Grants (Details)

Grants (Details)	1 Months Ended				3 Months Ended								12 Months Ended
	Mar. 31, 2019USD ($)	Sep. 30, 2016USD ($)	Jan. 31, 2016USD ($)	Oct. 31, 2013USD ($)	Dec. 31, 2019USD ($)contract_option	Sep. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2019USD ($)contract_option	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Disaggregation of Revenue [Line Items]
Total revenue					$ 14,055,000	$ 17,046,000	$ 13,083,000	$ 16,025,000	$ 35,421,000	$ 41,757,000	$ 28,851,000	$ 29,039,000	$ 60,209,000	$ 135,068,000	$ 205,825,000
Deferred revenue					199,528,000				240,924,000				199,528,000	240,924,000
Grant revenue
Disaggregation of Revenue [Line Items]
Total revenue													12,173,000	12,556,000	28,851,000
BARDA
Disaggregation of Revenue [Line Items]
Award amount		$ 125,800,000
Amount committed for funding					117,300,000								117,300,000
Available funding					$ 8,500,000								$ 8,500,000
BARDA | Initial base award
Disaggregation of Revenue [Line Items]
Award amount		8,200,000
BARDA | Contract options
Disaggregation of Revenue [Line Items]
Award amount		$ 117,600,000
Number of contract options exercised | contract_option					3								3
Number of contract options | contract_option					4								4
BARDA | Grant revenue
Disaggregation of Revenue [Line Items]
Total revenue													$ 7,600,000	6,800,000	20,100,000
The Bill & Melinda Gates Foundation
Disaggregation of Revenue [Line Items]
Award amount			$ 100,000,000
Available funding					$ 80,000,000								80,000,000
Deferred revenue					1,500,000				800,000				1,500,000	800,000
The Bill & Melinda Gates Foundation | Initial project
Disaggregation of Revenue [Line Items]
Award amount	$ 1,100,000		$ 20,000,000
Amount committed for funding					21,100,000								21,100,000
The Bill & Melinda Gates Foundation | Grant revenue
Disaggregation of Revenue [Line Items]
Total revenue													3,400,000	1,400,000	1,100,000
DARPA
Disaggregation of Revenue [Line Items]
Award amount				$ 24,600,000									19,700,000
Amount committed for funding					$ 19,700,000				$ 19,700,000				19,700,000	19,700,000
DARPA | Grant revenue
Disaggregation of Revenue [Line Items]
Total revenue													$ 500,000	$ 3,500,000	$ 7,700,000

Financial Instruments - Summary

Financial Instruments - Summary of Cash and Available-for-Sale Securities by Significant Investment Category (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,024,106	$ 1,037,372
Level 2
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,259,980	1,695,737
Unrealized Gains	3,047	120
Unrealized Losses	(40)	(1,440)
Fair Value	1,262,987	1,694,417
Level 2 | Cash and Cash Equivalents
Debt Securities, Available-for-sale [Line Items]
Fair Value	235,876	658,364
Level 2 | Current Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	867,124	863,063
Level 2 | Non- Current Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	159,987	172,990
Cash and Cash Equivalents
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	225,874	658,365
Unrealized Gains	0	20
Unrealized Losses	0	(21)
Fair Value	225,874	658,364
Cash and Cash Equivalents | Cash and Cash Equivalents
Debt Securities, Available-for-sale [Line Items]
Fair Value	225,874	658,364
Cash and Cash Equivalents | Current Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	0	0
Cash and Cash Equivalents | Non- Current Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	0	0
Certificates of deposit | Level 2
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	82,028	173,102
Unrealized Gains	79	42
Unrealized Losses	(6)	(36)
Fair Value	82,101	173,108
Certificates of deposit | Level 2 | Cash and Cash Equivalents
Debt Securities, Available-for-sale [Line Items]
Fair Value	10,002	0
Certificates of deposit | Level 2 | Current Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	69,197	157,920
Certificates of deposit | Level 2 | Non- Current Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	2,902	15,188
U.S. treasury securities | Level 2
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	117,891	152,205
Unrealized Gains	260	18
Unrealized Losses	(2)	(48)
Fair Value	118,149	152,175
U.S. treasury securities | Level 2 | Cash and Cash Equivalents
Debt Securities, Available-for-sale [Line Items]
Fair Value	0	0
U.S. treasury securities | Level 2 | Current Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	110,186	152,175
U.S. treasury securities | Level 2 | Non- Current Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	7,963	0
Debt securities of U.S. government agencies and corporate entities | Level 2
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	834,187	712,065
Unrealized Gains	2,708	40
Unrealized Losses	(32)	(1,335)
Fair Value	836,863	710,770
Debt securities of U.S. government agencies and corporate entities | Level 2 | Cash and Cash Equivalents
Debt Securities, Available-for-sale [Line Items]
Fair Value	0	0
Debt securities of U.S. government agencies and corporate entities | Level 2 | Current Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	687,741	552,968
Debt securities of U.S. government agencies and corporate entities | Level 2 | Non- Current Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 149,122	$ 157,802

Financial Instruments - Amortiz

Financial Instruments - Amortized Cost and Estimated Fair Value of Marketable Securities, by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Amortized Cost
Due in one year or less	$ 864,958	$ 864,003
Due after one year through five years	159,148	173,369
Amortized Cost	1,024,106	1,037,372
Estimated Fair Value
Due in one year or less	867,124	863,063
Due after one year through five years	159,987	172,990
Total	$ 1,027,111	$ 1,036,053

Financial Instruments - Narrati

Financial Instruments - Narrative (Details) $ in Millions	Dec. 31, 2019security	Dec. 31, 2018USD ($)security
Investments, Debt and Equity Securities [Abstract]
Number of available-for-sale securitiesin a continuous unrealized loss position for more than 12 months | security	0	25
Estimated fair value of total investment portfolio in a continuous unrealized loss position for more than 12 months		$ 82.8
Gross unrealized loss of total investment portfolio in a continuous unrealized loss position for more than 12 months		$ 0.4

Balance Sheet Components - Prep

Balance Sheet Components - Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Prepaid expenses	$ 8,475		$ 10,401
Tenant incentives receivables	4,093		10,089
Interest receivable on marketable securities	6,835		7,909
Prepaid expenses and other current assets	$ 19,403	$ 28,099	$ 28,399

Balance Sheet Components - Prop

Balance Sheet Components - Property and Equipment, Net (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 296,079,000
Less: Accumulated depreciation	(94,584,000)
Property and equipment, net	201,495,000
Property, plant and equipment, gross		$ 276,433,000
Less: Accumulated depreciation		(64,456,000)
Property and equipment, net		211,977,000		$ 209,494,000
Depreciation and amortization	31,021,000	24,862,000	$ 20,537,000
Building
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	0
Property, plant and equipment, gross		140,442,000
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	108,257,000
Property, plant and equipment, gross		96,907,000
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	152,426,000
Property, plant and equipment, gross		13,741,000
Furniture, fixtures and other
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	3,316,000
Property, plant and equipment, gross		2,122,000
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	11,985,000
Property, plant and equipment, gross		11,513,000
Internally developed software
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	7,020,000
Property, plant and equipment, gross		7,020,000
Property and equipment, net		0
Right-of-use asset, financing
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	9,853,000
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 3,222,000
Property, plant and equipment, gross		$ 4,688,000

Balance Sheet Components - Accr

Balance Sheet Components - Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
In-licenses	$ 0	$ 22,000
Property and equipment	4,029	12,089
Compensation-related	27,428	23,406
External goods and services	36,195	21,578
Accrued liabilities	$ 67,652	$ 79,073

Leases (Details)

Leases (Details) $ in Thousands

1 Months Ended

6 Months Ended

12 Months Ended

Apr. 30, 2017USD ($)

Aug. 31, 2015USD ($)ft²

Dec. 31, 2018USD ($)ft²

Dec. 31, 2019USD ($)ft²campus

Dec. 31, 2018USD ($)ft²

Dec. 31, 2017USD ($)

Dec. 01, 2020USD ($)

Aug. 31, 2019USD ($)ft²

Feb. 28, 2019ft²extension_period

Jan. 01, 2019USD ($)

Aug. 31, 2016USD ($)ft²extension_period

May 31, 2016USD ($)ft²extension_period$ / ft²

Lessee, Lease, Description [Line Items]

Rent expense

$ 19,100

$ 18,600

Number of campuses | campus

2

Property and equipment, net

$ 211,977

211,977

$ 209,494

Carrying value of the construction financing obligation

$ 33,489

$ 33,489

Amounts representing interest or imputed interest, operating lease

$ 75,750

Amounts representing interest or imputed interest, finance lease

$ 255,759

Imputed interest rate

17.20%

Cambridge Leases

Lessee, Lease, Description [Line Items]

Area of office space (in sqft) | ft²

200,000

Norwood Leases

Lessee, Lease, Description [Line Items]

Area of office space (in sqft) | ft²

200,000

Finance lease, term of lease

35 years

Extension term

5 years

10 years

Percentage subleased

64.00%

Number of extension periods | extension_period

4

2

200 Technology Square | Cambridge Leases

Lessee, Lease, Description [Line Items]

Area of office space (in sqft) | ft²

50,000

124,760

Tenant allowance (in usd per sqft) | $ / ft²

10

Security deposit

$ 1,900

Extension term

2 years

5 years

Tenant improvement allowance

$ 3,500

Lease term

11 years

Number of extension periods | extension_period

2

200 Technology Square, Sublease Agreements | Cambridge Leases

Lessee, Lease, Description [Line Items]

Area of office space (in sqft) | ft²

34,268

34,268

500 Technology Square | Cambridge Leases

Lessee, Lease, Description [Line Items]

Area of office space (in sqft) | ft²

61,618

60,000

Rate per annum

$ 3,900

Security deposit

1,000

Tenant improvement allowance

$ 3,100

Lease term

6 years

Land adjacent to Norwood | Norwood Leases

Lessee, Lease, Description [Line Items]

Rate per annum

$ 300

Lease term

35 years

Office, laboratory, and light manufacturing space | Norwood Leases

Lessee, Lease, Description [Line Items]

Area of office space (in sqft) | ft²

200,000

Security deposit

$ 8,900

Tenant improvement allowance

$ 24,200

Carrying value of the construction financing obligation

$ 33,500

$ 33,500

15,700

Financing obligation in accrued liabilities

10,500

10,500

14,500

Amounts representing interest or imputed interest, operating lease

10,300

Amounts representing interest or imputed interest, finance lease

$ 208,500

Scenario, Forecast | 200 Technology Square | Cambridge Leases

Lessee, Lease, Description [Line Items]

Security deposit

$ 2,200

Construction in progress | Office, laboratory, and light manufacturing space | Norwood Leases

Lessee, Lease, Description [Line Items]

Property and equipment, net

86,400

$ 86,400

$ 75,000

Transfers of property and equipment

161,600

Building | Office, laboratory, and light manufacturing space | Norwood Leases

Lessee, Lease, Description [Line Items]

Transfers of property and equipment

$ 140,400

Leases - Balance Sheet Informat

Leases - Balance Sheet Information (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019
Leases [Abstract]
Right-of-use assets, operating, net	$ 86,414	$ 63,334
Right-of-use assets, financing, net	9,544	9,853
Total	95,958	73,187
Operating lease liabilities, current	3,584	6,455
Operating lease liabilities, non-current	93,675	64,250
Financing lease liabilities, non-current	38,689	37,718
Total non-current lease liabilities	132,364	101,968
Total	$ 135,948	$ 108,423

Leases - Lease Costs (Details)

Leases - Lease Costs (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Leases [Abstract]
Operating lease costs	$ 17,015
Amortization of right-of-use assets, financing leases	309
Interest expense for financing lease liabilities	6,557
Total financing lease costs	6,866
Variable lease costs	4,399
Operating cash flows used in operating leases	(16,121)
Operating cash flows used in financing leases	(5,585)
Financing cash flows provided by financing leases	971
Right-of-use assets reduced through lease modifications and reassessments	2,717
Right-of-use assets obtained in exchange for operating lease liabilities	$ 34,014
Operating leases, remaining lease term	12 years
Finance leases, remaining lease term	33 years
Operating leases, discount rate	9.70%
Finance leases, discount rate	17.20%

Leases - Minimum Lease Payments

Leases - Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2019USD ($)
Operating Leases
2020	$ 12,563
2021	13,814
2022	14,120
2023	14,365
2024	14,708
Thereafter	103,439
Total minimum lease payments	173,009
Less amounts representing interest or imputed interest	(75,750)
Present value of lease liabilities	97,259
Financing Leases
2020	5,738
2021	5,894
2022	6,054
2023	6,219
2024	6,387
Thereafter	264,156
Total minimum lease payments	294,448
Less amounts representing interest or imputed interest	(255,759)
Present value of lease liabilities	$ 38,689

Leases - Topic 840 (Details)

Leases - Topic 840 (Details) $ in Thousands	Dec. 31, 2018USD ($)
Leases [Abstract]
2019	$ 20,027
2020	20,404
2021	20,937
2022	20,208
2023	17,235
Thereafter	112,958
Total	$ 211,769

Commitments and Contingencies -

Commitments and Contingencies - Strategic Collaborations (Details) - PCV Agreement - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Apr. 30, 2018	Jun. 30, 2016
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Budgeted amount				$ 250
PCV products
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Budgeted amount	$ 243	$ 243	$ 243

Commitments and Contingencies_2

Commitments and Contingencies - Indemnification Obligations (Details) - Indemnifications	12 Months Ended
	Dec. 31, 2019USD ($)claim	Dec. 31, 2018USD ($)claim
Loss Contingencies [Line Items]
Losses related to indemnification obligations	$ 0	$ 0
Number of claims outstanding | claim	0	0
Reserves established	$ 0	$ 0

Commitments and Contingencies_3

Commitments and Contingencies - Purchase Commitments and Purchase Orders (Details) $ in Millions	Dec. 31, 2019USD ($)
Purchase commitments
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Contractual commitment to pay	$ 4.5
Clinical operations and support commitments
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Contractual commitment to pay	$ 105.9

Commitments and Contingencies_4

Commitments and Contingencies - Licenses to Patented Technology (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jun. 26, 2017
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Accrued liabilities	$ 67,652,000	$ 79,073,000
Sublicense agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Expenses associated with license agreements		22,000,000	$ 53,300,000
Consideration paid	$ 22,000,000	$ 25,000,000	$ 28,300,000
Therapeutic and prophylactic products | Sublicense agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and regulatory milestone payments				$ 1,500,000
Commercial milestone payments				24,000,000
Diagnostic products | Sublicense agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and regulatory milestone payments				$ 500,000

Redeemable Convertible Prefer_3

Redeemable Convertible Preferred Stock and Common Stock - Narrative (Details)	Dec. 11, 2018USD ($)$ / sharesshares	Aug. 10, 2016USD ($)$ / sharesshares	Sep. 30, 2018USD ($)shares	May 31, 2018USD ($)$ / sharesshares	Feb. 28, 2018USD ($)$ / sharesshares	Feb. 15, 2018USD ($)$ / sharesshares	Dec. 31, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)shares	May 07, 2018shares
Subsidiary, Sale of Stock [Line Items]
Common stock, shares authorized (in shares)	1,600,000,000				775,000,000		1,600,000,000	1,600,000,000
Redeemable convertible preferred stock, shares authorized	509,352,795								448,686,791	509,352,795
Common stock, par value (usd per share) | $ / shares	$ 0.0001						$ 0.0001	$ 0.0001
Preferred stock, shares authorized (in shares)	162,000,000						162,000,000	162,000,000
Preferred stock, par value (usd per share) | $ / shares	$ 0.0001						$ 0.0001	$ 0.0001
Proceeds from initial public offering of common stock, net of issuance costs | $							$ 0	$ 563,026,000	$ 0
Offering expenses | $								$ 41,322,000
Redeemable convertible preferred stock, shares issued	508,539,515						0	0	448,686,791
Redeemable convertible preferred stock, shares outstanding	508,539,515						0	0	448,686,791
Accretion of redeemable convertible preferred units | $								$ 0
Series F Redeemable Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Redeemable convertible preferred stock, shares authorized	54,001,241								54,001,241
Shares of common stock issued (in shares)		54,001,241
Price per share (usd per share) | $ / shares		$ 8.78
Aggregate net proceeds from the offering | $		$ 474,100,000
Offering expenses | $		$ 600,000
Redeemable convertible preferred stock, shares issued	54,001,241								54,001,241
Redeemable convertible preferred stock, shares outstanding	54,001,241								54,001,241
Authorized number of shares (in shares)		68,337,129
Conversion price (usd per share) | $ / shares								$ 19.14
Series G Redeemable Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Redeemable convertible preferred stock, shares authorized	55,666,004
Shares of common stock issued (in shares)					55,666,004	55,666,004
Price per share (usd per share) | $ / shares					$ 10.06	$ 10.06
Aggregate net proceeds from the offering | $					$ 560,000,000	$ 560,000,000
Offering expenses | $					$ 10,500,000		$ 10,517,000	$ 10,517,000
Redeemable convertible preferred stock, shares issued	55,666,004
Redeemable convertible preferred stock, shares outstanding	55,666,004
Conversion price (usd per share) | $ / shares								$ 21.93
Series H Redeemable Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Redeemable convertible preferred stock, shares authorized	5,000,000
Shares of common stock issued (in shares)				5,000,000
Price per share (usd per share) | $ / shares				$ 25
Aggregate net proceeds from the offering | $				$ 112,000,000
Offering expenses | $				$ 400,000			$ 474,000	$ 474,000
Redeemable convertible preferred stock, shares issued	5,000,000
Redeemable convertible preferred stock, shares outstanding	5,000,000
Conversion ratio				0.8771930
Conversion price (usd per share) | $ / shares				$ 21.93				$ 21.93
Fair value per share (usd per share) | $ / shares				$ 22.39
Premium recorded to deferred revenue | $				$ 13,000,000
Convertible holding period								21 months
Percentage discount on shares								10.00%
Series D Redeemable Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Redeemable convertible preferred stock, shares authorized	63,291,156								63,291,156
Redeemable convertible preferred stock, shares issued	63,021,976								63,291,156
Redeemable convertible preferred stock, shares outstanding	63,021,976								63,291,156
Conversion price (usd per share) | $ / shares								$ 4.65
Repurchase of redeemable convertible preferred stock (in shares)			269,180
Repurchase of redeemable convertible preferred stock | $								$ 2,009,000
Dividend rate per annum (usd per share | $ / shares								$ 0.171
Series E Redeemable Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Redeemable convertible preferred stock, shares authorized	81,428,340								81,428,340
Redeemable convertible preferred stock, shares issued	80,884,240								81,428,340
Redeemable convertible preferred stock, shares outstanding	80,884,240								81,428,340
Conversion price (usd per share) | $ / shares								$ 13.45
Repurchase of redeemable convertible preferred stock (in shares)			544,100
Repurchase of redeemable convertible preferred stock | $			$ 8,200,000					$ 2,118,000
Series A Redeemable Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Redeemable convertible preferred stock, shares authorized	42,000,000								42,000,000
Redeemable convertible preferred stock, shares issued	42,000,000								42,000,000
Redeemable convertible preferred stock, shares outstanding	42,000,000								42,000,000
Conversion price (usd per share) | $ / shares								$ 0.11
Dividend rate per annum (usd per share | $ / shares								0.004
Series B Redeemable Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Redeemable convertible preferred stock, shares authorized	122,296,280								122,296,280
Redeemable convertible preferred stock, shares issued	122,296,280								122,296,280
Redeemable convertible preferred stock, shares outstanding	122,296,280								122,296,280
Conversion price (usd per share) | $ / shares								0.17
Dividend rate per annum (usd per share | $ / shares								0.006
Series C Redeemable Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Redeemable convertible preferred stock, shares authorized	85,669,774								85,669,774
Redeemable convertible preferred stock, shares issued	85,669,774								85,669,774
Redeemable convertible preferred stock, shares outstanding	85,669,774								85,669,774
Conversion price (usd per share) | $ / shares								0.70
Dividend rate per annum (usd per share | $ / shares								$ 0.02568
IPO
Subsidiary, Sale of Stock [Line Items]
Shares of common stock issued (in shares)	26,275,993
Price per share (usd per share) | $ / shares	$ 23
Aggregate net proceeds from the offering | $	$ 563,000,000
Underwriting discounts | $	33,200,000
Offering expenses | $	$ 8,100,000
IPO | Common Stock
Subsidiary, Sale of Stock [Line Items]
Preferred stock converted into common stock (in shares)	236,012,913

Redeemable Convertible Prefer_4

Redeemable Convertible Preferred Stock and Common Stock - Schedule of Redeemable Convertible Preferred Stock (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 11, 2018	May 07, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Redeemable Convertible Preferred Shares Authorized			509,352,795	509,352,795	448,686,791
Redeemable Convertible Preferred Shares Issued	0	0	508,539,515		448,686,791
Redeemable Convertible Preferred Shares Outstanding	0	0	508,539,515		448,686,791
Carrying Value			$ 1,833,561		$ 1,176,661
Liquidation Preference			$ 1,903,887		$ 1,209,940
Series A redeemable convertible preferred stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Shares Authorized			42,000,000		42,000,000
Redeemable Convertible Preferred Shares Issued			42,000,000		42,000,000
Redeemable Convertible Preferred Shares Outstanding			42,000,000		42,000,000
Carrying Value			$ 182		$ 182
Liquidation Preference			$ 2,859		$ 2,701
Series B redeemable convertible preferred stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Shares Authorized			122,296,280		122,296,280
Redeemable Convertible Preferred Shares Issued			122,296,280		122,296,280
Redeemable Convertible Preferred Shares Outstanding			122,296,280		122,296,280
Carrying Value			$ 770		$ 770
Liquidation Preference			$ 12,493		$ 11,801
Series C redeemable convertible preferred stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Shares Authorized			85,669,774		85,669,774
Redeemable Convertible Preferred Shares Issued			85,669,774		85,669,774
Redeemable Convertible Preferred Shares Outstanding			85,669,774		85,669,774
Carrying Value			$ 36,238		$ 36,238
Liquidation Preference			$ 41,750		$ 39,676
Series D redeemable convertible preferred stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Shares Authorized			63,291,156		63,291,156
Redeemable Convertible Preferred Shares Issued			63,021,976		63,291,156
Redeemable Convertible Preferred Shares Outstanding			63,021,976		63,291,156
Carrying Value			$ 163,355		$ 164,059
Liquidation Preference			$ 188,840		$ 179,462
Series E redeemable convertible preferred stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Shares Authorized			81,428,340		81,428,340
Redeemable Convertible Preferred Shares Issued			80,884,240		81,428,340
Redeemable Convertible Preferred Shares Outstanding			80,884,240		81,428,340
Carrying Value			$ 498,525		$ 501,880
Liquidation Preference			$ 498,814		$ 502,169
Series F redeemable convertible preferred stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Shares Authorized			54,001,241		54,001,241
Redeemable Convertible Preferred Shares Issued			54,001,241		54,001,241
Redeemable Convertible Preferred Shares Outstanding			54,001,241		54,001,241
Carrying Value			$ 473,532		$ 473,532
Liquidation Preference			$ 474,131		$ 474,131
Series G redeemable convertible preferred stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Shares Authorized			55,666,004
Redeemable Convertible Preferred Shares Issued			55,666,004
Redeemable Convertible Preferred Shares Outstanding			55,666,004
Carrying Value			$ 549,413
Liquidation Preference			$ 560,000
Series H redeemable convertible preferred stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Shares Authorized			5,000,000
Redeemable Convertible Preferred Shares Issued			5,000,000
Redeemable Convertible Preferred Shares Outstanding			5,000,000
Carrying Value			$ 111,546
Liquidation Preference			$ 125,000

Redeemable Convertible Prefer_5

Redeemable Convertible Preferred Stock and Common Stock - Summary of Activity for Each Series of Outstanding Redeemable Convertible Preferred Stock (Details) - USD ($) $ in Thousands	Dec. 11, 2018	Aug. 10, 2016	Sep. 30, 2018	May 31, 2018	Feb. 28, 2018	Dec. 31, 2019	Dec. 31, 2018
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	448,686,791
Balance at beginning of period							$ 1,176,661
Conversion of redeemable convertible preferred stock into common stock							$ 1,833,561
Balance at end of period (in shares)	508,539,515					0	0
Balance at end of period	$ 1,833,561
Issuance costs							$ 41,322
Series A Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)							42,000,000
Balance at beginning of period							$ 182
Balance at end of period (in shares)	42,000,000
Balance at end of period	$ 182
Series B Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)							122,296,280
Balance at beginning of period							$ 770
Balance at end of period (in shares)	122,296,280
Balance at end of period	$ 770
Series C Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)							85,669,774
Balance at beginning of period							$ 36,238
Balance at end of period (in shares)	85,669,774
Balance at end of period	$ 36,238
Series D Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)							63,291,156
Balance at beginning of period							$ 164,059
Repurchase of redeemable convertible preferred stock (in shares)			(269,180)
Repurchase of redeemable convertible preferred stock							$ (2,009)
Balance at end of period (in shares)	63,021,976
Balance at end of period	$ 163,355
Series E Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)							81,428,340
Balance at beginning of period							$ 501,880
Repurchase of redeemable convertible preferred stock (in shares)			(544,100)
Repurchase of redeemable convertible preferred stock			$ (8,200)				$ (2,118)
Balance at end of period (in shares)	80,884,240
Balance at end of period	$ 498,525
Series F Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)							54,001,241
Balance at beginning of period							$ 473,532
Balance at end of period (in shares)	54,001,241
Balance at end of period	$ 473,532
Issuance costs		$ 600
Series G Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of redeemable convertible preferred stock, net of issuance costs							51
Balance at end of period (in shares)	55,666,004
Balance at end of period	$ 549,413
Issuance costs					$ 10,500	$ 10,517	10,517
Series H Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at end of period (in shares)	5,000,000
Balance at end of period	$ 111,546
Issuance costs				$ 400		$ 474	$ 474
Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	448,686,791
Balance at beginning of period						$ 0	$ 1,176,661
Conversion of redeemable convertible preferred stock into common stock (in shares)							(508,539,515)
Conversion of redeemable convertible preferred stock into common stock							$ (1,833,561)
Balance at end of period (in shares)						0	0
Balance at end of period						$ 0	$ 0
Redeemable Convertible Preferred Stock | Series A Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	42,000,000
Balance at beginning of period						$ 0	$ 182
Conversion of redeemable convertible preferred stock into common stock (in shares)							(42,000,000)
Conversion of redeemable convertible preferred stock into common stock							$ (182)
Balance at end of period (in shares)							0
Balance at end of period							$ 0
Redeemable Convertible Preferred Stock | Series B Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	122,296,280
Balance at beginning of period						$ 0	$ 770
Conversion of redeemable convertible preferred stock into common stock (in shares)							(122,296,280)
Conversion of redeemable convertible preferred stock into common stock							$ (770)
Balance at end of period (in shares)							0
Balance at end of period							$ 0
Redeemable Convertible Preferred Stock | Series C Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	85,669,774
Balance at beginning of period						$ 0	$ 36,238
Conversion of redeemable convertible preferred stock into common stock (in shares)							(85,669,774)
Conversion of redeemable convertible preferred stock into common stock							$ (36,238)
Balance at end of period (in shares)							0
Balance at end of period							$ 0
Redeemable Convertible Preferred Stock | Series D Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	63,291,156
Balance at beginning of period						$ 0	$ 164,059
Repurchase of redeemable convertible preferred stock (in shares)							(269,180)
Repurchase of redeemable convertible preferred stock							$ (704)
Conversion of redeemable convertible preferred stock into common stock (in shares)							(63,021,976)
Conversion of redeemable convertible preferred stock into common stock							$ (163,355)
Balance at end of period (in shares)							0
Balance at end of period							$ 0
Redeemable Convertible Preferred Stock | Series E Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	81,428,340
Balance at beginning of period						$ 0	$ 501,880
Repurchase of redeemable convertible preferred stock (in shares)							(544,100)
Repurchase of redeemable convertible preferred stock							$ (3,355)
Conversion of redeemable convertible preferred stock into common stock (in shares)							(80,884,240)
Conversion of redeemable convertible preferred stock into common stock							$ (498,525)
Balance at end of period (in shares)							0
Balance at end of period							$ 0
Redeemable Convertible Preferred Stock | Series F Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	54,001,241
Balance at beginning of period						$ 0	$ 473,532
Conversion of redeemable convertible preferred stock into common stock (in shares)							(54,001,241)
Conversion of redeemable convertible preferred stock into common stock							$ (473,532)
Balance at end of period (in shares)							0
Balance at end of period							$ 0
Redeemable Convertible Preferred Stock | Series G Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	0
Balance at beginning of period						$ 0	$ 0
Issuance of redeemable convertible preferred stock, net of issuance costs (in shares)							55,666,004
Issuance of redeemable convertible preferred stock, net of issuance costs							$ 549,413
Conversion of redeemable convertible preferred stock into common stock (in shares)							(55,666,004)
Conversion of redeemable convertible preferred stock into common stock							$ (549,413)
Balance at end of period (in shares)							0
Balance at end of period							$ 0
Redeemable Convertible Preferred Stock | Series H Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	0
Balance at beginning of period						$ 0	$ 0
Issuance of redeemable convertible preferred stock, net of issuance costs (in shares)							5,000,000
Issuance of redeemable convertible preferred stock, net of issuance costs							$ 111,546
Conversion of redeemable convertible preferred stock into common stock (in shares)							(5,000,000)
Conversion of redeemable convertible preferred stock into common stock							$ (111,546)
Balance at end of period (in shares)							0
Balance at end of period							$ 0

Stock-Based Compensation - Narr

Stock-Based Compensation - Narrative (Details) $ / shares in Units, $ in Thousands	Dec. 05, 2018USD ($)shares	Nov. 30, 2018shares	Dec. 31, 2019USD ($)installment$ / sharesshares	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Aug. 31, 2016shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate number of shares authorized for issuance (in shares)			64,900,000
Total intrinsic value of options exercised (less than for the year ended December 31, 2017) | $			$ 75,600	$ 5,300	$ 100
Consideration for options exercised | $			$ 47,200
Options issued (in shares)			7,354,268
Strike price (usd per share) | $ / shares			$ 19.58
Weighted average grant date fair value of such options (usd per share) | $ / shares			$ 11.35
Compensation costs | $			$ 81,122	72,565	40,055
Total unrecognized compensation cost related to non-vested stock-based compensation | $			$ 194,800
Weighted-average period of cost expected to be recognized			3 years 1 month
Service-based awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			4 years
Service-based awards | Twelve months of continued employment or service
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			12 months
Vesting percentage			25.00%
Service-based awards | Following twelve quarters
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			36 months
Number of installments | installment			12
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate number of shares authorized for issuance (in shares)			46,800,000
Compensation costs | $			$ 74,780	63,288	31,724
Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation costs | $			1,217	0	0
Restricted common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate fair value of restricted stock awards vested | $			$ 2,300	11,900	21,100
Stock issued (in shares)			0
Weighted average grant date fair value of such restricted stock (usd per share) | $ / shares			$ 0
Performance-based awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation costs | $			$ 9,800	$ 10,600	$ 800
2018 Equity Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate number of shares authorized for issuance (in shares)		13,000,000
Annual percentage increase of shares reserved and available for issuance		4.00%
Shares available for future grant (in shares)			18,100,000
2016 Equity Plan | Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period			10 years
2013 Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant (in shares)						0
ESPP | Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate number of shares authorized for issuance (in shares)	810,000
Shares available for future grant (in shares)			600,000
Offering period	6 months
Purchase period	6 months
Purchase price at which shares are sold, percent	85.00%
Minimum percentage of compensation through payroll deductions	1.00%
Maximum percentage of compensation through payroll deductions	50.00%
Maximum shares to be purchased during purchase period (in shares)	3,000
Maximum value of shares to be purchased during purchase period | $	$ 25,000
Purchase of common stock under employee stock purchase plan (in shares)			171,343
Weighted average fair value per share (usd per share) | $ / shares			$ 16.87

Stock-Based Compensation - Opti

Stock-Based Compensation - Options Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Number of Options
Outstanding at beginning of period (in shares)	50,821,132
Granted (in shares)	7,354,268
Exercised (in shares)	(6,945,306)
Cancelled/forfeited (in shares)	(5,693,179)
Outstanding at end of period (in shares)	45,536,915	50,821,132
Weighted Average Exercise Price per Share
Outstanding at beginning of period (usd per share)	$ 12.16
Granted (usd per share)	19.58
Exercised (usd per share)	6.86
Cancelled/forfeited (usd per share)	15.17
Outstanding at end of period (usd per share)	13.82	$ 12.16
Weighted Average Grant Date Fair Value per Share
Outstanding at beginning of period (usd per share)	6.59
Granted (usd per share)	11.35
Exercised (usd per share)	4.47
Cancelled/forfeited (usd per share)	8.67
Outstanding at end of period (usd per share)	$ 7.35	$ 6.59
Outstanding and Exercisable
Number of Options, Exercisable (in shares)	22,872,221
Weighted- Average Exercise Price per Share, Exercisable (usd per share)	$ 10.13
Weighted- Average Grant Date Fair Value per Share, Exercisable (usd per share)	$ 4.86
Weighted- Average Remaining Contractual Term, Outstanding	7 years 2 months	7 years 1 month
Weighted- Average Remaining Contractual Term, Exercisable	5 years 11 months
Aggregate Intrinsic Value, Outstanding	$ 286,310	$ 220,434
Aggregate Intrinsic Value, Exercisable	$ 216,241
Vested and expected to vest
Number of Options (in shares)	22,664,694
Weighted- Average Exercise Price per Share (usd per share)	$ 17.54
Weighted- Average Grant Date Fair Value per Share	$ 9.86
Weighted- Average Remaining Contractual Term	8 years 7 months
Aggregate Intrinsic Value	$ 70,069

Stock-Based Compensation - Rest

Stock-Based Compensation - Restricted Common Stock and Common Stock Units Activity (Details)	12 Months Ended
	Dec. 31, 2019$ / sharesshares
Restricted Common Stock
Number of Shares/Units
Outstanding, non-vested at beginning of period (in shares) | shares	198,597
Issued (in shares) | shares	0
Vested (in shares) | shares	(162,717)
Cancelled/forfeited (in shares) | shares	(35,880)
Outstanding, non-vested at end of period (in shares) | shares	0
Weighted-Average Grant Date per Share/Unit
Outstanding, non-vested at beginning of period (usd per share) | $ / shares	$ 12.15
Issued (usd per share) | $ / shares	0
Vested (usd per share) | $ / shares	12.15
Cancelled/forfeited (usd per share) | $ / shares	12.15
Outstanding, non-vested at end of period (usd per share) | $ / shares	$ 0
Restricted Common Stock Units
Number of Shares/Units
Outstanding, non-vested at beginning of period (in shares) | shares	458,715
Issued (in shares) | shares	1,283,662
Vested (in shares) | shares	(57,344)
Cancelled/forfeited (in shares) | shares	(106,413)
Vested in prior year and settled during the period (in shares) | shares	(401,371)
Outstanding, non-vested at end of period (in shares) | shares	1,177,249
Weighted-Average Grant Date per Share/Unit
Outstanding, non-vested at beginning of period (usd per share) | $ / shares	$ 11.93
Issued (usd per share) | $ / shares	19.12
Vested (usd per share) | $ / shares	11.93
Cancelled/forfeited (usd per share) | $ / shares	20.25
Pending settlement (usd per share) | $ / shares	11.93
Outstanding, non-vested at end of period (usd per share) | $ / shares	$ 19.01

Stock-Based Compensation - Weig

Stock-Based Compensation - Weighted-Average Assumptions Used to Estimate the Fair Value of Options Granted (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.29%	2.76%	2.02%
Expected term	6 years 25 days	6 years 3 months 7 days	6 years 2 months 14 days
Expected volatility	61.00%	63.00%	63.00%
Expected dividends	0.00%	0.00%	0.00%
Options | Weighted Average
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value per share (usd per share)	$ 11.35	$ 9.33	$ 7.96
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.95%
Expected term	6 months	0 years	0 years
Expected volatility	53.00%
Expected dividends	0.00%
ESPP | Weighted Average
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair value per share (usd per share)	$ 5.98

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 81,122	$ 72,565	$ 40,055
Research and development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	48,259	37,659	21,679
General and administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	32,863	34,906	18,376
Options
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	74,780	63,288	31,724
Restricted common stock and units
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	5,125	9,277	8,331
Employee stock purchase plan
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 1,217	$ 0	$ 0

Employee Benefit Plan (Details)

Employee Benefit Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Retirement Benefits [Abstract]
Percent of match	50.00%
Percent contributed by participant	6.00%
Total matching contributions	$ 4.2	$ 2.1	$ 2.1

Income Taxes - Loss Before Prov

Income Taxes - Loss Before Provision For (Benefit From) Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
United States	$ (508,595)	$ (380,473)	$ (247,784)
Foreign	(6,121)	(3,935)	(8,212)
Loss before (benefit from) provision for income taxes	$ (514,716)	$ (384,408)	$ (255,996)

Income Taxes - Provision For (B

Income Taxes - Provision For (Benefit From) Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current:
Federal	$ 0	$ (26)	$ (252)
State	505	352	172
Total current	505	326	(80)
Deferred:
Federal	(1,200)	0	0
Total deferred	(1,200)	0	0
Total (benefit from) provision for income taxes	$ (695)	$ 326	$ (80)

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of U.S. Statutory Income Tax Rate to Effective Tax Rate (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Tax effected at statutory rate	21.00%	21.00%	34.00%
State taxes, net of federal benefit	7.90%	6.30%	8.50%
Non-deductible items	1.60%	0.00%	(1.30%)
Change in valuation allowance	(33.00%)	(28.50%)	(20.40%)
Federal research and development credits	2.50%	1.50%	4.80%
Foreign tax rate differential	(0.20%)	(0.20%)	(1.10%)
Impact of federal rate change on net deferred taxes	0.00%	0.00%	(25.00%)
Other	0.20%	(0.20%)	0.50%
Effective tax rate	0.00%	(0.10%)	0.00%

Income Taxes - Significant Comp

Income Taxes - Significant Components of Deferred Tax Assets and Tax Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carry-forwards	$ 268,173	$ 114,932
Stock-based compensation	43,978	33,138
Capitalized licenses, R&D and start-up costs	19,891	22,143
Tax credit carry-forwards	77,222	53,617
Accrued expenses	7,377	16,443
Deferred revenue	53,475	71,209
Operating lease liabilities	26,571
Lease financing obligation	10,570
Lease financing obligation		9,149
Other	11	2,870
Total gross deferred tax assets	507,268	323,501
Less: valuation allowance	(470,753)	(308,273)
Total deferred tax assets, net of valuation allowance	36,515	15,228
Deferred tax liabilities:
Financing right-of-use assets	(2,612)
Operating right-of-use assets	(24,944)
Fixed assets	(8,959)	(15,228)
Total deferred tax liabilities	(36,515)	(15,228)
Net deferred tax assets	$ 0	$ 0

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Increase in valuation allowance	$ 162,500
Operating Loss Carryforwards [Line Items]
Decrease in accounts receivable	(5,032)	$ (8,948)	$ (11,686)
Decrease in deferred tax asset	(507,268)		$ (323,501)
Federal
Operating Loss Carryforwards [Line Items]
Net operating losses	981,800
Net operating losses that begin to expire in 2030	380,100
Net operating losses that will carry-forward indefinitely	601,700
Federal | Research and development
Operating Loss Carryforwards [Line Items]
Tax credit carry-forwards	45,600
State
Operating Loss Carryforwards [Line Items]
Net operating losses	978,800
Net operating losses that begin to expire in 2030	978,800
State | Research and development
Operating Loss Carryforwards [Line Items]
Tax credit carry-forwards	23,900
State | Federal Orphan Drug
Operating Loss Carryforwards [Line Items]
Tax credit carry-forwards	1,500
State | Investment tax credit
Operating Loss Carryforwards [Line Items]
Tax credit carry-forwards	$ 5,900
Accounting Standards Update 2014-09
Operating Loss Carryforwards [Line Items]
Cumulative-effect adjustment		27,984
Decrease in deferred revenue		30,700
Decrease in accounts receivable		2,700
Decrease in deferred tax asset		8,400
Accounting Standards Update 2014-09 | Accumulated Deficit
Operating Loss Carryforwards [Line Items]
Cumulative-effect adjustment		$ 27,984

Income Taxes - Reconciliation_2

Income Taxes - Reconciliation of the Beginning and Ending Amounts of Unrecognized Tax Benefits Roll Forward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance as of beginning of period	$ 141	$ 940
Decrease due to prior positions	0	(799)
Increase due to current year tax positions	0	0
Balance as of end of period	$ 141	$ 141

Net Loss per Share - Basic and

Net Loss per Share - Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss									$ (514,021)	$ (384,734)	$ (255,916)
Premium paid on repurchase of preferred stock									0	(4,127)	0
Cumulative dividends on redeemable convertible preferred stock									0	(12,996)	(13,925)
Net loss attributable to common stockholders									$ (514,021)	$ (401,857)	$ (269,841)
Denominator:
Weighted average common shares used in net loss per share attributable to common stockholders, basic and diluted (in shares)									330,802,136	81,114,183	64,497,544
Net loss per share attributable to common stockholders, basic and diluted (usd per share)	$ (0.37)	$ (0.37)	$ (0.41)	$ (0.40)	$ (1.14)	$ (1.32)	$ (1.43)	$ (1.16)	$ (1.55)	$ (4.95)	$ (4.18)

Net Loss per Share - Common Sto

Net Loss per Share - Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (in shares)	46,714,164	51,478,444	240,375,353
Redeemable convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (in shares)	0	0	205,151,299
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (in shares)	45,536,915	50,821,132	33,684,134
Restricted common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (in shares)	0	198,597	1,081,205
Restricted common stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities (in shares)	1,177,249	458,715	458,715

Related Party Transactions - Na

Related Party Transactions - Narrative (Details) - USD ($)	1 Months Ended		12 Months Ended
	Feb. 28, 2018	Feb. 15, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Accounts receivable			$ 337,000	$ 899,000
Affiliated Entity | Covington
Related Party Transaction [Line Items]
Legal services			100,000	200,000	$ 200,000
Outstanding accounts payable balances			0	0
Equity Method Investee | AstraZeneca
Related Party Transaction [Line Items]
Payments received			7,900,000	34,000,000	$ 1,100,000
Accounts receivable			$ 300,000	$ 900,000
Series G Redeemable Convertible Preferred Stock
Related Party Transaction [Line Items]
Shares of stock issued (in shares)	55,666,004	55,666,004
Price per share (usd per share)	$ 10.06	$ 10.06
Aggregate purchase price	$ 560,000,000	$ 560,000,000

Related Party Transactions - Sc

Related Party Transactions - Schedule of Redeemable Preferred Stock Issued to Related Parties (Details) - Series G Redeemable Convertible Preferred Stock - USD ($) $ in Thousands	1 Months Ended
	Feb. 28, 2018	Feb. 15, 2018	Feb. 12, 2018
Related Party Transaction [Line Items]
Shares of stock issued (in shares)	55,666,004	55,666,004
Aggregate purchase price	$ 560,000	$ 560,000
Affiliated Entity | OCHA LLC
Related Party Transaction [Line Items]
Shares of stock issued (in shares)		50,000
Aggregate purchase price		$ 503
Affiliated Entity | VGE III Portfolio Ltd
Related Party Transaction [Line Items]
Shares of stock issued (in shares)		279,160
Affiliated Entity | Viking Global Equities LP
Related Party Transaction [Line Items]
Shares of stock issued (in shares)		148,974
Affiliated Entity | Viking Global Equities II LP
Related Party Transaction [Line Items]
Shares of stock issued (in shares)		8,737
Affiliated Entity | Viking Global Opportunities Illiquid Investments Sub-Master LP
Related Party Transaction [Line Items]
Shares of stock issued (in shares)		129,537
Affiliated Entity | Viking Long Fund Master Ltd
Related Party Transaction [Line Items]
Shares of stock issued (in shares)			179,118
Investee | Viking Global Investors LP and affiiated entities
Related Party Transaction [Line Items]
Shares of stock issued (in shares)		745,526
Aggregate purchase price		$ 7,500

Selected Quarterly Financial _3

Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 14,055	$ 17,046	$ 13,083	$ 16,025	$ 35,421	$ 41,757	$ 28,851	$ 29,039	$ 60,209	$ 135,068	$ 205,825
Total operating expenses	144,611	146,861	156,792	157,665	188,452	127,575	125,866	106,441	605,929	548,334	475,181
Loss from operations	$ (130,556)	$ (129,815)	$ (143,709)	$ (141,640)	$ (153,031)	$ (85,818)	$ (97,015)	$ (77,402)	$ (545,720)	$ (413,266)	$ (269,356)
Net loss per share attributable to common stockholders, basic and diluted (usd per share)	$ (0.37)	$ (0.37)	$ (0.41)	$ (0.40)	$ (1.14)	$ (1.32)	$ (1.43)	$ (1.16)	$ (1.55)	$ (4.95)	$ (4.18)
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New accounting pronouncement or change in accounting principle, effect of change on operating results		$ 1,000	$ 200	$ 200

Subsequent Events (Details)

Subsequent Events (Details) - Subsequent Event $ / shares in Units, $ in Millions	Feb. 14, 2020USD ($)$ / sharesshares
Public Equity Offering
Subsequent Event [Line Items]
Shares of stock issued (in shares) | shares	26,315,790
Price per share (usd per share) | $ / shares	$ 19
Proceeds from public offering | $	$ 478.1
Underwriting Option
Subsequent Event [Line Items]
Shares of stock issued (in shares) | shares	3,947,368
Proceeds from public offering | $	$ 71.8