Alnylam Pharmaceuticals (ALNY) 10-K 13 Feb 20 2019 FY Annual report Financial data

Cover

Cover - USD ($)	12 Months Ended
	Dec. 31, 2019	Feb. 07, 2020	Jun. 28, 2019
Cover page.
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
Document Transition Report	false
Entity File Number	001-36407
Entity Registrant Name	ALNYLAM PHARMACEUTICALS, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	77-0602661
Entity Address, Address Line One	675 West Kendall Street
Entity Address, Address Line Two	Henri A. Termeer Square
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02142
City Area Code	617
Local Phone Number	551-8200
Title of 12(b) Security	Common Stock, $0.01 par value per share
Trading Symbol	ALNY
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 7,997,224,708
Entity Common Stock, Shares Outstanding		112,583,964
Amendment Flag	false
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Central Index Key	0001178670
Current Fiscal Year End Date	--12-31

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 547,178,000	$ 420,146,000
Marketable debt securities	975,017,000	662,803,000
Marketable equity securities	13,967,000	1,206,000
Accounts receivable, net	43,011,000	18,760,000
Inventory	56,348,000	24,068,000
Prepaid expenses and other current assets	80,343,000	73,713,000
Total current assets	1,715,864,000	1,200,696,000
Property, plant and equipment, net	425,179,000	320,658,000
Operating lease right-of-use assets	221,197,000
Restricted investments	14,825,000	44,825,000
Other assets	18,069,000	8,623,000
Total assets	2,395,134,000	1,574,802,000
Current liabilities:
Accounts payable	49,884,000	59,708,000
Accrued expenses	197,201,000	112,719,000
Operating lease liability	27,688,000
Deferred rent	0	3,571,000
Deferred revenue	77,821,000	3,496,000
Total current liabilities	352,594,000	179,494,000
Operating lease liability, net of current portion	276,135,000
Deferred rent, net of current portion	0	57,920,000
Deferred revenue, net of current portion	318,383,000	458,000
Long-term debt	0	30,000,000
Other liabilities	9,330,000	4,965,000
Total liabilities	956,442,000	272,837,000
Commitments and contingencies (Note 9)
Stockholders’ equity:
Preferred stock, $0.01 par value per share, 5,000 shares authorized and no shares issued and outstanding as of December 31, 2019 and December 31, 2018	0	0
Common stock, $0.01 par value per share, 250,000 and 125,000 shares authorized as of December 31, 2019 and December 31, 2018, respectively; 112,188 shares issued and outstanding as of December 31, 2019; 101,177 shares issued and outstanding as of December 31, 2018	1,122,000	1,011,000
Additional paid-in capital	5,201,176,000	4,175,139,000
Accumulated other comprehensive loss	(36,518,000)	(33,213,000)
Accumulated deficit	(3,727,088,000)	(2,840,972,000)
Total stockholders’ equity	1,438,692,000	1,301,965,000
Total liabilities and stockholders’ equity	$ 2,395,134,000	$ 1,574,802,000

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	5,000,000	5,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	250,000,000	125,000,000
Common stock, shares issued (in shares)	112,188,000	101,177,000
Common stock, shares outstanding (in shares)	112,188,000	101,177,000

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues:
Revenues	$ 219,750	$ 74,908	$ 89,912
Operating costs and expenses:
Cost of goods sold	25,062	1,802	0
Research and development	655,114	505,420	390,635
Selling, general and administrative	479,005	382,359	199,365
Total operating costs and expenses	1,159,181	889,581	590,000
Loss from operations	(939,431)	(814,673)	(500,088)
Other income (expense):
Interest income	33,448	29,262	12,236
Other income (expense)	11,308	4,173	(3,022)
Change in fair value of liability obligation	9,422	0	0
Gain on litigation settlement	0	20,564	0
Total other income	54,178	53,999	9,214
Loss before income taxes	(885,253)	(760,674)	(490,874)
Provision for income taxes	(863)	(823)	0
Net loss	$ (886,116)	$ (761,497)	$ (490,874)
Net loss per common share - basic and diluted (in dollars per share)	$ (8.11)	$ (7.57)	$ (5.42)
Weighted-average common shares used to compute basic and diluted net loss per common share (in shares)	109,264	100,590	90,554
Statements of Comprehensive Loss
Net loss	$ (886,116)	$ (761,497)	$ (490,874)
Unrealized gain (loss) on marketable securities	558	1,220	(2,886)
Foreign currency translations	(343)	0	0
Defined benefit pension plans	(3,520)	0	0
Reclassification adjustment for realized loss on marketable securities included in net loss	0	0	1,894
Comprehensive loss	(889,421)	(760,277)	(491,866)
Net Revenues from Collaborators
Revenues:
Revenues	53,213	62,373	89,912
Product Revenues, Net
Revenues:
Revenues	$ 166,537	$ 12,535	$ 0

CONSOLIDATED STATEMENTS OF STOC

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Sanofi Genzyme	Common Stock	Common StockSanofi Genzyme	Additional Paid-in Capital	Additional Paid-in CapitalSanofi Genzyme	Total Accumulated Other Comprehensive (Loss) Income	Accumulated Deficit
Beginning Balance at Dec. 31, 2016	$ 920,221		$ 859		$ 2,609,614		$ (33,441)	$ (1,656,811)
Beginning Balance (in shares) at Dec. 31, 2016			85,941,000
Exercise of common stock options, net of tax withholdings	80,546		$ 19		80,527
Exercise of common stock options, net of tax withholdings (in shares)			1,842,000
Issuance of common stock under equity plans	3,160		$ 1		3,159
Issuance of common stock under equity plans (in shares)			115,000
Issuance of common stock under benefit plans	2,040		$ 1		2,039
Issuance of common stock under benefit plans (in shares)			31,000
Issuance of common stock, net of offering costs	1,139,625	$ 21,381	$ 114	$ 3	1,139,511	$ 21,378
Issuance of common stock, net of offering costs (in shares)			11,440,000	298,000
Stock-based compensation expense related to equity-classified awards	91,324				91,324
Other comprehensive loss, net of tax	(992)						(992)
Net loss	(490,874)							(490,874)
Ending Balance at Dec. 31, 2017	1,766,431		$ 997		3,947,552		(34,433)	(2,147,685)
Ending Balance (in shares) at Dec. 31, 2017			99,667,000
Exercise of common stock options, net of tax withholdings	60,743		$ 12		60,731
Exercise of common stock options, net of tax withholdings (in shares)			1,268,000
Issuance of common stock under equity plans	5,046		$ 2		5,044
Issuance of common stock under equity plans (in shares)			212,000
Issuance of common stock under benefit plans	3,170				3,170
Issuance of common stock under benefit plans (in shares)			30,000
Stock-based compensation expense related to equity-classified awards	158,642				158,642
Other comprehensive loss, net of tax	1,220						1,220
Net loss	(761,497)							(761,497)
Ending Balance at Dec. 31, 2018	$ 1,301,965		$ 1,011		4,175,139		(33,213)	(2,840,972)
Ending Balance (in shares) at Dec. 31, 2018	101,177,000		101,177,000
Exercise of common stock options, net of tax withholdings	$ 63,499		$ 15		63,484
Exercise of common stock options, net of tax withholdings (in shares)			1,374,000
Issuance of common stock under equity plans	7,909		$ 1		7,908
Issuance of common stock under equity plans (in shares)			132,000
Issuance of common stock under benefit plans	5,033		$ 1		5,032
Issuance of common stock under benefit plans (in shares)			61,000
Issuance of common stock, net of offering costs	772,477		$ 94		772,383
Issuance of common stock, net of offering costs (in shares)			9,444,000
Stock-based compensation expense related to equity-classified awards	177,230				177,230
Other comprehensive loss, net of tax	(3,305)						(3,305)
Net loss	(886,116)							(886,116)
Ending Balance at Dec. 31, 2019	$ 1,438,692		$ 1,122		$ 5,201,176		$ (36,518)	$ (3,727,088)
Ending Balance (in shares) at Dec. 31, 2019	112,188,000		112,188,000

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (886,116)	$ (761,497)	$ (490,874)
Non-cash adjustments to reconcile net loss to net cash used in operating activities:
Amortization and interest accretion related to operating leases	37,193
Depreciation and amortization	17,175	15,248	11,898
Stock-based compensation	174,841	157,752	92,819
Gain on litigation settlement	0	(10,000)	0
Fair value adjustments on marketable equity securities	(11,288)	(3,564)	1,894
Change in fair value of liability obligation	(9,422)	0	0
Other	(3,191)	(5,654)	4,379
Changes in operating assets and liabilities:
Proceeds from landlord lease incentive for tenant improvements	30,170	25,350	0
Accounts receivable, net	(24,238)	15,242	(10,668)
Inventory	(32,411)	(22,645)	0
Prepaid expenses and other assets	(22,042)	(35,067)	(20,711)
Accounts payable, accrued expenses and other	92,354	74,835	26,629
Operating lease liability	(33,703)
Deferred revenue	392,251	(12,616)	1,848
Net cash used in operating activities	(278,427)	(562,616)	(382,786)
Cash flows from investing activities:
Purchases of property, plant and equipment	(140,156)	(126,887)	(104,209)
Purchases of restricted investments	0	(14,825)	0
Proceeds from maturity of restricted investments	30,000	0	120,000
Purchases of marketable debt securities	(2,075,925)	(1,104,046)	(903,457)
Sales and maturities of marketable debt securities	1,775,404	1,518,703	597,305
Other	(7,000)	0	0
Net cash (used in) provided by investing activities	(417,677)	272,945	(290,361)
Cash flows from financing activities:
Proceeds from exercise of stock options and other types of equity, net	71,284	65,470	83,885
Offering proceeds, net of costs	381,900	0	1,139,625
Repayment of term loan	(30,000)	0	(120,000)
Net cash provided by financing activities	823,184	65,470	1,124,891
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(83)	0	0
Net increase (decrease) in cash, cash equivalents and restricted cash	126,997	(224,201)	451,744
Cash, cash equivalents and restricted cash, beginning of period	422,631	646,832	195,088
Cash, cash equivalents and restricted cash, end of period	549,628	422,631	646,832
Supplemental disclosure of cash flows:
Cash paid for interest	172	775	2,430
Cash paid for income taxes	2,566	1,066	114
Supplemental disclosure of noncash investing and financing activities:
Capital expenditures included in accounts payable and accrued expenses	14,876	33,274	8,176
Sanofi Genzyme
Cash flows from financing activities:
Proceeds from issuance of common stock	0	0	21,381
Regeneron
Cash flows from financing activities:
Proceeds from issuance of common stock	$ 400,000	$ 0	$ 0

NATURE OF BUSINESS

NATURE OF BUSINESS	12 Months Ended
	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
NATURE OF BUSINESS	NATURE OF BUSINESS Alnylam Pharmaceuticals, Inc. (also referred to as Alnylam, we, our or us) commenced operations on June 14, 2002 as a biopharmaceutical company seeking to develop and commercialize novel therapeutics based on RNA interference, or RNAi. We are committed to the advancement of our company strategy of building a multi-product, global, commercial biopharmaceutical company with a deep and sustainable clinical pipeline of RNAi therapeutics for future growth and a robust, organic research engine for sustainable innovation and great potential for patient impact. Since inception, we have focused on discovering, developing and commercializing RNAi therapeutics by establishing and maintaining a strong intellectual property position in the RNAi field, establishing strategic alliances with leading pharmaceutical and life sciences companies, generating revenues through licensing agreements, and ultimately developing and commercializing RNAi therapeutics globally, either independently or with our strategic partners. We have devoted substantially all of our efforts to business planning, research, development, manufacturing and early commercial efforts, acquiring, filing and expanding intellectual property rights, recruiting management and technical staff, and raising capital. In August 2018, we received approval for ONPATTRO from the United States Food and Drug Administration, or FDA, and began commercializing and generating product revenues in the U.S. As of December 31, 2019, we have launched ONPATTRO in the U.S., Japan, Canada and in several countries in Europe. In November 2019, we received approval for GIVLAARI from the FDA and began commercializing and generating product revenues in the U.S. in December 2019. Regulatory filings in additional markets are pending or planned for 2020 and beyond for both commercial products.

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 The accompanying consolidated financial statements reflect the operations of Alnylam and our wholly-owned subsidiaries. All intercompany accounts and transactions have been eliminated. Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America, or GAAP, requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. In our consolidated financial statements, there are significant estimates and assumptions related to our inventory valuation and related reserves, income taxes, revenue recognition, research and development expenses, and stock-based compensation. We base our estimates on historical experience and on various other assumptions that we believe to be reasonable. Actual results could differ from those estimates. Reclassification Certain prior period amounts in the consolidated financial statements have been reclassified to conform to the current period presentation. Liquidity Based on our current operating plan, we believe that our cash, cash equivalents and marketable debt and equity securities as of December 31, 2019, together with the cash we expect to generate from product sales and under our current alliances, will be sufficient to enable us to advance our long-term strategic goals for at least the next 12 months from the filing of this annual report on Form 10-K. Concentrations of Credit Risk and Significant Customers Financial instruments that potentially expose us to concentrations of credit risk consist primarily of cash, cash equivalents and marketable debt securities. As of December 31, 2019 and 2018, substantially all of our cash, cash equivalents and marketable debt securities were invested in money market funds, certificates of deposit, commercial paper, corporate notes, U.S. government-sponsored enterprise securities and U.S. treasury securities through highly rated financial institutions. Corporate notes may also include foreign bonds denominated in U.S. dollars. Investments are restricted, in accordance with our investment policy, to a concentration limit per issuer. During the years ended December 31, 2019 and 2018, our revenues were generated primarily from distributors for product sales and collaborations with strategic partners. For the years ended December 31, 2019 and 2018, our gross accounts receivable balance was comprised of payments primarily due from distributors for product sales and our strategic partners. The following table summarizes customers that represent 10% or greater of our consolidated total gross revenues for the periods indicated: Year Ended December 31, 2019 2019 2018 2017 Distributor A 44 % 13 % N/A Sanofi Genzyme * 58 % 61 % Vir Biotechnology * 16 % * The Medicines Company * * 34 % __________________________________________ * Represents less than 10% The following table summarizes customers with amounts due that represent 10% or greater of our consolidated gross accounts receivable balance, at the periods indicated: As of December 31, 2019 2019 2018 Distributor A 28 % 44 % Sanofi Genzyme 14 % 29 % Vir Biotechnology 13 % * Distributor B 10 % * __________________________________________ * Represents less than 10% Fair Value Measurements The fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. In general, fair values determined by Level 1 inputs utilize quoted prices (unadjusted) in active markets for identical assets or liabilities. Fair values determined by Level 2 inputs utilize data points that are observable, such as quoted prices (adjusted), interest rates and yield curves. Fair values determined by Level 3 inputs utilize unobservable data points for the asset or liability, and include situations where there is little, if any, market activity for the asset or liability. The fair value hierarchy level is determined by the lowest level of significant input. Investments in Marketable Securities and Cash Equivalents We invest our excess cash balances in marketable debt securities and classify our investments as either held-to-maturity or available-for-sale based on facts and circumstances present at the time we purchased the securities. At each balance sheet date presented, we classified all of our investments in debt securities as available-for-sale and as current assets as they represent the investment of funds available for current operations. We report available-for-sale debt securities 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 (loss) income, a component of stockholders’ equity. Realized gains and losses are determined using the specific identification method and are included in other income (expense). If any adjustment to fair value reflects a decline in the value of the marketable debt securities, we consider all available evidence to evaluate the extent to which the decline is “other than temporary,” including our intention to sell and, if so, mark the investment to market through a charge to our consolidated statements of operations and comprehensive loss. We did not record any impairment charges related to our marketable debt securities during the years ended December 31, 2019, 2018 or 2017. Our marketable debt securities are classified as cash equivalents if the original maturity, from the date of purchase, is 90 days or less, and as marketable debt securities if the original maturity, from the date of purchase, is in excess of 90 days. Our cash equivalents are generally composed of commercial paper, corporate notes, U.S. government-sponsored enterprise securities, U.S. treasury securities and money market funds. We measure marketable equity investments (except those accounted for under the equity method of accounting or those that result in consolidation of an investee), which have readily available prices, at fair value with changes in fair value recognized in other income (expense) on our consolidated statements of operations and comprehensive loss. We obtain fair value measurement data for our marketable debt securities from independent pricing services. We perform validation procedures to ensure the reasonableness of this data. This includes meeting with the independent pricing services to understand the methods and data sources used. For our marketable debt securities, we perform our own review of prices received from the independent pricing services by comparing these prices to other sources and for our marketable equity securities, we confirm those securities are trading in active markets. Prior to January 1, 2018, we recognized unrealized gains and losses on our marketable equity securities through accumulated other comprehensive income (loss) on our consolidated balance sheets. Accounts Receivable We record accounts receivable net of customer allowances for distribution services, prompt payment discounts and chargebacks based on contractual terms. As of December 31, 2019 and 2018, we determined an allowance for doubtful accounts was not required based upon our review of contractual payment terms and individual customer circumstances. We have standard payment terms that generally require payment within approximately 30 to 90 days. Accounts receivable, net on our consolidated balance sheets also includes billed and unbilled collaboration receivables. Inventory Inventory is measured at the lower of cost or estimated net realizable value. We use a standard cost basis, which approximates cost determined on a first-in, first-out basis. Inventory costs include all raw materials, direct conversion costs and overhead. Raw and intermediate materials that may be used for either research and development or commercial purposes are classified as inventory until the material is consumed or otherwise allocated for research and development. If the material is used for research and development, it is expensed as research and development once that determination is made. We capitalize inventory costs that are expected to be sold commercially once we determine it is probable that the inventory costs will be recovered through commercial sale based on the review of several factors, including (i) the likelihood that all required regulatory approvals will be received, considering any special filing status, (ii) the expected timing of validation (if not yet completed) of manufacturing processes in the associated facility, (iii) the expected expiration of the inventory, (iv) logistical or commercial constraints that may impede the timely distribution and sale of the product, including transport requirements and reimbursement status, (v) current market factors, including competitive landscape and pricing, (vi) threatened or anticipated litigation challenges, (vii) history of approvals of similar products or formulations and (viii) FDA (or other appropriate regulatory agencies) correspondence regarding the safety and efficacy of the product. Prior to the capitalization of inventory costs, we record such costs as research and development expenses on our consolidated statements of operations and comprehensive loss. We reduce our inventory to net realizable value for potentially excess, dated or obsolete inventory based on our quarterly assessment of the recoverability of our capitalized inventory. We periodically review inventory levels to identify what may expire prior to expected sale or has a cost basis in excess of its estimated realizable value and write-down such inventories as appropriate. Property, Plant and Equipment Property, plant and equipment are stated at cost, net of accumulated depreciation. Depreciation expense is recorded on a straight-line basis over the estimated useful life of the asset. Leasehold improvements are amortized over the shorter of the asset’s estimated useful life or the lease term. Construction in progress reflects amounts incurred for construction or improvements of property, plant or equipment that have not been placed in service. Costs of construction of certain long-lived assets include capitalized interest, which is amortized over the estimated useful life of the related asset. The cost and accumulated depreciation of assets retired or sold are removed from the respective asset category, and any gain or loss is recognized in our consolidated statements of operations and comprehensive loss. During the years ended December 31, 2019, 2018 and 2017, we recorded $16.6 million, $12.8 million and $11.9 million, respectively, of depreciation expense related to our property, plant and equipment. The estimated useful lives of property, plant and equipment are as follows: Asset Category Useful Life Laboratory equipment 5 Computer equipment and software 3-10 years Furniture and fixtures 5 Leasehold improvements Shorter of asset life or lease term Manufacturing Equipment 7-15 years Buildings 40 years Land — Construction in progress — Research and Development Accruals We record accrued liabilities related to products we have received or services that we have incurred, specifically related to ongoing pre-clinical studies and clinical trials, for which service providers have not yet billed us, or when billing terms under these contracts do not coincide with the timing of when the work is performed, as of our period-end. These costs primarily relate to third-party clinical management costs, laboratory and analysis costs, toxicology studies and investigator fees. The assessment of these costs is a subjective process, requiring judgment based on our knowledge of the research and development programs, services performed for the period, experience with related activities and the expected duration of the third-party service contract, where applicable. Upon settlement, these costs may differ materially from the amounts accrued in our consolidated financial statements. Our historical accrual estimates have not been materially different from our actual costs. Revenue Recognition We recognize revenue when control of promised goods or services is transferred to a customer at an amount that reflects the consideration to which we expect to be entitled in exchange for those goods or services. To determine revenue recognition, we perform the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price, including variable consideration, if any; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) we satisfy a performance obligation. We only apply the five-step model to contracts when collectability of the consideration to which we are entitled in exchange for the goods or services we transfer to the customer is determined to be probable. At contract inception, once the contract is determined to be within the scope of Accounting Standards Codification Topic 606, Revenue from Contracts with Customers, or ASC 606, we assess whether the goods or services promised within each contract are distinct and, therefore, represent a separate performance obligation. Goods and services that are determined not to be distinct are combined with other promised goods and services until a distinct bundle is identified. We then allocate the transaction price (the amount of consideration we expect to be entitled to from a customer in exchange for the promised goods or services) to each performance obligation and recognize the associated revenue when (or as) each performance obligation is satisfied. Our estimate of the transaction price for each contract includes all variable consideration to which we expect to be entitled. Amounts are recorded as accounts receivable when our right to consideration is unconditional. We do not assess whether a contract has a significant financing component if the expectation at contract inception is that the period between payment by the customer and the transfer of the promised goods or services to the customer will be one year or less. We expense incremental costs of obtaining a contract as and when 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 and 2018, we had not capitalized any costs to obtain any of our contracts. Net Product Revenues Our net product sales consist of sales of ONPATTRO and GIVLAARI and are recognized, net of variable consideration related to certain allowances and accruals, at the time the customer obtains control of our product. We use the expected value method, which is the sum of probability-weighted amounts in a range of possible consideration amounts, or the most likely amount method, which is the single most likely amount in a range of possible considerations, to estimate variable consideration related to our product sales. We use the expected value method to estimate variable consideration for certain rebates, chargebacks, product returns, and other incentives and we use the most likely amount method for certain rebates and trade discounts and allowances. The following are the components of variable consideration related to product revenues. We record reserves, based on contractual terms, for these components related to product sold during the reporting period, as well as our estimate of product that remains in the distribution channel inventory at the end of the reporting period that we expect will be sold to qualified healthcare providers. On a quarterly basis, we update our estimates and record any needed adjustments in the period we identify the adjustments. Chargebacks : We estimate obligations resulting from contractual commitments with the government and other entities to sell products to qualified healthcare providers at prices lower than the list prices charged to the customer who directly purchases from us. The customer charges us for the difference between what it pays to us for the product and the selling price to the qualified healthcare providers. Rebates : We are subject to discount obligations under government programs, including Medicaid in the U.S. and similar programs in certain other countries, including countries in which we are accruing for estimated rebates because final pricing has not yet been negotiated. We are also subject to potential rebates in connection with our value-based agreements with certain commercial payors. We record reserves for rebates in the same period the related product revenue is recognized, resulting in a reduction of product revenues and a current liability that is included in accrued expenses on our consolidated balance sheet. Our estimate for rebates is based on statutory discount rates, expected utilization or an estimated number of patients on treatment, as applicable. Trade discounts and allowances : We provide customary invoice discounts on product sales to our customers for prompt payment and we pay fees for distribution services, such as fees for certain data that customers provide to us. We estimate our customers will earn these discounts and fees, and deduct these discounts and fees in full from gross product revenues and accounts receivable at the time we recognize the related revenues. Product returns: We offer customers product return rights if products are damaged, defective or expired, with “expired” defined as within three months pre- or post-expiry. We estimate the amount of product that will be returned using a probability-weighted estimate based on our sales history. Other incentives: Other incentives include co-payment assistance we provide to patients with commercial insurance that have coverage and reside in states that allow co-payment assistance. We estimate the average co-payment assistance amounts for our products based on expected customer demographics and record any such amounts within accrued expenses on our consolidated balance sheet. Revenues from Collaborators We earn revenue in connection with collaboration agreements which allow our collaboration partners to utilize our technology platforms and develop product candidates. Our collaboration agreements are detailed in Note 4, Collaboration Agreements. For each collaboration partner, we discuss our revenue recognition, including our significant performance obligations under each agreement. At contract inception, we assess whether the collaboration arrangements are within the scope of ASC Topic 808, Collaborative Arrangements, or 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 dependent on the commercial success of such activities. This assessment is performed based on the responsibilities of all parties in the arrangement. For collaboration arrangements within the scope of ASC 808 that contain multiple elements, we first determine which elements of the arrangement are within the scope of ASC 808 and which elements are within the scope of ASC 606. For elements of collaboration arrangements that are accounted for pursuant to ASC 808, an appropriate recognition method is determined and applied consistently, either by analogy to authoritative accounting literature or by applying a reasonable and rational policy election. For elements of collaboration arrangements that are accounted for pursuant to ASC 606, we identify the performance obligations and allocate the total consideration we expect to receive on a relative standalone selling price basis to each performance obligation. Variable consideration such as performance-based milestones will be included in the total consideration if we expect to receive such consideration and if it is probable that the inclusion of the variable consideration will not result in a significant reversal in the cumulative amount of revenue recognized under the arrangement. Our estimate of the total consideration we expect to receive under each collaboration arrangement is updated for each reporting period, and any adjustments to revenue are recorded on a cumulative catch-up basis. We exclude sales-based royalty and milestone payments from the total consideration we expect to receive until the underlying sales occur because the license to our intellectual property is deemed to be the predominant item to which the royalties or milestones relate as it is the primary driver of value in our collaboration arrangements. Key assumptions to determine the standalone selling price may include forecasted revenues, development timelines, reimbursement rates for personnel costs, discount rates and probabilities of technical and regulatory success. We recognize revenue associated with each performance obligation as the control over the promised goods or services transfer to our collaboration partner which occurs either at a point in time or over time. If control transfers over time, revenue is recognized by using a method of measuring progress that best depicts the transfer of goods or services. We evaluate the measure of progress and related inputs each reporting period and any resulting adjustments to revenue are recorded on a cumulative catch-up basis. Consideration received that does not meet the requirements to satisfy ASC 808 or ASC 606 revenue recognition criteria is recorded as deferred revenue in the accompanying consolidated balance sheets, classified as either short-term or long-term deferred revenue based on our best estimate of when, less than 12 months (short-term) or more than 12 months (long-term), such revenue will be recognized. Cost of Goods Sold Cost of goods sold includes the cost of producing and distributing inventories that are related to product revenues during the respective period (including salary-related and stock-based compensation expenses for employees involved with production and distribution, freight and indirect overhead costs), third-party royalties payable on our net product revenues and amortization of intangible assets associated with the sale of our products. Cost of goods sold may also include costs related to excess or obsolete inventory adjustment charges, abnormal costs, unabsorbed manufacturing and overhead costs, and manufacturing variances. Income Taxes Uncertain tax positions, for which management's assessment is that there is a more than 50% probability of sustaining the position upon challenge by a taxing authority based upon its technical merits, are subject to certain recognition and measurement criteria. The nature of the uncertain tax positions is often very complex and subject to change, and the amounts at issue can be substantial. We develop our cumulative probability assessment of the measurement of uncertain tax positions using internal experience, judgment and assistance from professional advisors. We re-evaluate these uncertain tax positions on a quarterly basis based on a number of factors including, but not limited to, changes in facts or circumstances, changes in tax law, and effectively settled issues under audit and new audit activity. Any change in these factors could result in the recognition of a tax benefit or an additional charge to the tax provision. We account for income taxes under the asset and liability method. Under this method, deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to differences between financial statement carrying amounts of existing assets and liabilities and their respective tax basis. Deferred tax assets and liabilities are measured using enacted rates in effect for the year in which these temporary differences are expected to be recovered or settled. Valuation allowances are provided if, based on the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. As of December 31, 2019, we have recorded no interest and penalty expense related to uncertain tax positions. Research and Development Expenses We record research and development expenses as incurred. Included in research and development expenses are wages, stock-based compensation expenses, benefits and other operating costs, facilities, supplies, external services, clinical trial and manufacturing costs, costs related to our collaboration arrangements, and overhead directly related to our research and development operations, as well as costs to acquire technology licenses. We have entered into several license agreements for rights to utilize certain technologies. The terms of the licenses may provide for upfront payments, annual maintenance payments, milestone payments based upon certain specified events being achieved and royalties on product sales. We charge costs to acquire and maintain licensed technology that has not reached technological feasibility and does not have alternative future use to research and development expense as incurred. During the years ended December 31, 2019, 2018 and 2017, we charged to research and development expense costs associated with license fees of $37.0 million, $8.0 million and $7.7 million, respectively. Stock-Based Compensation We recognize stock-based compensation expense for grants under our stock incentive plans and employee stock purchase plan, as well as inducement stock grants outside of our stock incentive plans. We account for all stock-based awards granted to employees at their fair value and generally recognize compensation expense over the vesting period of the award. Determining the amount of stock-based compensation to be recorded requires us to develop estimates of fair values of stock options as of the grant date. We calculate the grant date fair values of stock options using the Black-Scholes valuation model, which requires the input of subjective assumptions, including but not limited to expected stock price volatility over the term of the awards and the expected term of stock options. The fair value of restricted stock awards granted to employees is based upon the quoted closing market price per share on the date of grant. We have performance conditions included in certain of our stock option and restricted stock awards that are based upon the achievement of pre-specified clinical development, regulatory and/or commercial events. As the outcome of each event has inherent risk and uncertainties, and a positive outcome may not be known until the event is achieved, we begin to recognize the value of the performance-based stock option and restricted stock awards when we determine the achievement of each performance condition is deemed probable, a determination which requires significant judgment by management. At the probable date, we record a cumulative expense catch-up, with remaining expense amortized over the remaining service period. Comprehensive Loss Comprehensive loss is comprised of net loss and certain changes in stockholders’ equity that are excluded from net loss. We include foreign currency translation adjustments in other comprehensive loss if the functional currency is not the U.S. dollar. We include unrealized gains and losses on certain marketable securities in other comprehensive loss, including changes in the value of our marketable debt securities. We include certain changes in the fair value of the plan assets and projected benefit obligation attributed to our defined benefit pension plan in other comprehensive loss. Net Loss Per Common Share We compute basic net loss per common share by dividing net loss by the weighted-average number of common shares outstanding. We compute diluted net loss per common share by dividing net loss by the weighted-average number of common shares and dilutive potential common share equivalents then outstanding. Potential common shares consist of shares issuable upon the exercise of stock options (the proceeds of which are then assumed to have been used to repurchase outstanding shares using the treasury stock method). Because the inclusion of potential common shares would be anti-dilutive for all periods presented, diluted net loss per common share is the same as basic net loss per common share. The following table sets forth for the periods presented the potential common shares (prior to consideration of the treasury stock method) excluded from the calculation of net loss per common share because their inclusion would be anti-dilutive, in thousands: As of December 31, 2019 2018 2017 Options to purchase common stock 13,069 12,573 11,239 Unvested restricted common stock 749 36 149 Total 13,818 12,609 11,388 Segment Information We operate in a single reporting segment, the discovery, development and commercialization of RNAi therapeutics. Consistent with our management reporting, results of our operations are reported on a consolidated basis for purposes of segment reporting. As of December 31, 2019 and 2018, substantially all of our consolidated property, plant and equipment, net was from U.S. operations. For the years ended December 31, 2019, 2018 and 2017, net revenues from collaborators were attributed to the U.S. Please read Note 3 for information regarding our net product sales by geography. Recently Adopted Accounting Pronouncements In February 2016, the Financial Accounting Standards Board, or FASB, issued a new leasing standard, ASC 842, which generally requires lessees to recognize operating and financing lease liabilities and corresponding right-of-use assets on the consolidated balance sheet and to provide enhanced disclosures surrounding the amount, timing and uncertainty of cash flows arising from leasing arrangements. We adopted the new standard on January 1, 2019, using a modified retrospective basis and did not restate comparative periods. In addition, we did not elect the package of practical expedients permitted under the transition guidance that permits companies to carry forward prior conclusions related to (1) whether any expired or existing contracts are, or contain, leases, (2) the lease classification for expired or existing leases, and (3) initial direct costs for existing leases. All our leases have been classified as operating leases under the new leasing standard. We elected to combine lease and non-lease components for our facility leases and to keep leases with an initial term of 12 months or less off the consolidated balance sheets and recognize the associated lease payments in the consolidated statements of operations and comprehensive loss on a straight-line basis over the lease term. Please read Note 8 for additional disclosures related to accounting for leases under this new standard. The adoption of ASC 842 had a material impact on our consolidated balance sheet as the standard requires us to measure and recognize a right of use asset and lease liability. As most leases do not provide an implicit rate, our incremental borrowing rate was determined based on the information available at the date of adoption to measure our lease liability. Costs determined to be variable and not based on an index or rate were not included in the measurement of the lease liability. We recognized approximately $290.0 million of operating lease liabilities and approximately $230.0 million of operating lease right-of-use assets on our consolidated balance sheet as of January 1, 2019. The adoption of the standard did not have a material impact on our consolidated statement of operations and comprehensive loss and there was no impact to cash from or used in operating, financing or investing activities on our consolidated statement of cash flows. In May 2014, the FASB issued new accounting guidance related to revenue recognition (ASC 606), which outlines a comprehensive revenue recognition model and supersedes most current revenue recognition accounting guidance and requires increased disclosures. The new accounting guidance defines a five-step approach that requires a company to recognize revenue as control of goods or services transfers to a customer at an amount that reflects the expected consideration to be received in exchange for those goods or services. We adopted ASC 606 in the first quarter of 2018 using the modified retrospective method to all contracts that were not completed as of January 1, 2018. We recognized the cumulative effect o

NET PRODUCT REVENUES

NET PRODUCT REVENUES	12 Months Ended
	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
NET PRODUCT REVENUES	NET PRODUCT REVENUES Net product revenues by geography consist of the following, in thousands: Years Ended December 31, 2019 2018 2017 United States $ 116,452 $ 8,589 $ — Rest of world 50,085 3,946 — Total net product revenues $ 166,537 $ 12,535 $ — As of December 31, 2019 and 2018, net product revenue-related receivables of $28.1 million and $13.1 million , respectively, were included in “Accounts receivable, net.” The following table summarizes balances and activity in each product revenue allowance and reserve category during the years ended December 31, 2019 and 2018, in thousands: As of December 31, 2019 Chargebacks and Rebates Trade Discounts and Allowances Returns Reserve and Other Incentives Total Beginning balance $ 3,441 $ 218 $ 321 $ 3,980 Provision related to current period sales 44,371 3,227 5,108 52,706 Credit or payments made during the period for current year sales (15,216) (2,817) (3,231) (21,264) Credit or payments made during the period for prior year sales (109) (218) (220) (547) Total $ 32,487 $ 410 $ 1,978 $ 34,875 As of December 31, 2018 Chargebacks and Rebates Trade Discounts and Allowances Returns Reserve and Other Incentives Total Beginning balance $ — $ — $ — $ — Provision related to current period sales 4,081 292 657 5,030 Credit or payments made during the period for current year sales (640) (74) (336) (1,050) Total $ 3,441 $ 218 $ 321 $ 3,980

COLLABORATION AGREEMENTS

COLLABORATION AGREEMENTS	12 Months Ended
	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
COLLABORATION AGREEMENTS	COLLABORATION AGREEMENTS The following table summarizes our total consolidated net revenues from collaborators, in thousands: Year Ended December 31, Description 2019 2018 2017 (1) Regeneron Pharmaceuticals $ 26,075 $ — $ — Vir Biotechnology 12,809 12,778 1,464 Sanofi Genzyme 10,976 46,000 54,625 The Medicines Company 2,315 2,789 30,217 Other 1,038 806 3,606 Total net revenues from collaborators $ 53,213 $ 62,373 $ 89,912 (1) As described in Note 2 above, prior period amounts have not been adjusted under the modified retrospective method. The following table presents the balance of our receivables and contract liabilities related to our collaboration agreements, in thousands: As of December 31, 2019 2018 Receivables included in "Accounts receivable, net" $ 14,929 $ 5,625 Contract liabilities included in "Deferred revenue" 153,117 3,954 The following table presents revenue recognized as a result of changes in contract liability related to our collaboration agreements, in thousands: Included in Deferred Revenue Contract liability as of January 1, 2018 $ 16,570 Consideration earned, not yet recognized as revenue 3,954 Revenue recognized on contract liability as of January 1, 2018 (16,570) Contract liability as of December 31, 2018 3,954 Consideration earned, not yet recognized as revenue 153,117 Revenue recognized on contract liability as of December 31, 2018 (3,954) Contract liability as of December 31, 2019 $ 153,117 In order to determine revenue recognized in the period from contract liabilities, 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 opposed to a portion applying to the new consideration for the period. The following table provides the research and development expenses incurred by type, for which we recognize net revenue, that are directly attributable to our collaboration agreements by collaboration partner, in thousands: Year Ended 2019 2018 2017 Clinical Trial and Manufacturing External Services Other Clinical Trial and Manufacturing External Services Other Clinical Trial and Manufacturing External Services Other Regeneron $ 2,793 $ 344 $ 21,779 $ — $ — $ — $ — $ — $ — Sanofi 11,505 334 2,017 36,600 5,340 1,279 174,901 4,475 5,327 Vir 10,353 381 4,745 7,272 8,251 548 1,134 926 — MDCO 2,025 — 696 1,664 2 203 5,421 — 106 Ionis — — — — 2,150 1,097 — 3,250 — Total $ 26,676 $ 1,059 $ 29,237 $ 45,536 $ 15,743 $ 3,127 $ 181,456 $ 8,651 $ 5,433 The research and development expenses incurred for each agreement listed in the table above consist of costs incurred for (i) clinical and manufacturing expenses, (ii) external services including consulting services and lab supplies and services, and (iii) other expenses, including professional services, facilities and overhead allocations, and a reasonable estimate of compensation and related costs as billed to our counterparties for which we recognize net revenue from collaborations. As part of our revenue recognition policy adopted January 1, 2018, the costs in the above table are considered an input in our determination of transaction price when they relate to consideration received for the delivery of goods or services. For the years ended December 31, 2019, 2018 and 2017, we did not incur material selling, general and administrative expenses related to our collaboration agreements. Product Alliances Regeneron Collaboration On April 8, 2019, we entered into a global, strategic collaboration with Regeneron Pharmaceuticals, Inc., or Regeneron, to discover, develop and commercialize RNAi therapeutics for a broad range of diseases by addressing therapeutic targets expressed in the eye and central nervous system, or CNS, in addition to a select number of targets expressed in the liver, which we refer to as the Regeneron Collaboration. The Regeneron Collaboration is governed by a Master Agreement, referred to as the Regeneron Master Agreement, which became effective on May 21, 2019, or the Effective Date. Upon closing of the Stock Purchase Agreement dated April 8, 2019, referred to as the Regeneron SPA, we issued 4,444,445 shares of common stock to Regeneron for aggregate cash consideration of $400.0 million, or $90.00 per share. Please read Note 10 for a full discussion regarding the Regeneron SPA. In connection with the Regeneron Master Agreement, we and Regeneron entered into (i) a binding co-co collaboration term sheet covering the continued development of cemdisiran, our C5 small interfering RNA, or siRNA, currently in Phase 2 development for C5 complement-mediated diseases, as a monotherapy and (ii) a binding license term sheet to evaluate anti-C5 antibody-siRNA combinations for C5 complement-mediated diseases including evaluating the combination of Regeneron’s pozelimab (REGN3918), currently in Phase 1 development, and cemdisiran. The C5 co-co collaboration and license agreements were executed in August 2019. Under the terms of the Regeneron Collaboration, we are working exclusively with Regeneron to discover RNAi therapeutics for eye and CNS diseases for an initial five-year research period, which we refer to as the Initial Research Term. Regeneron has an option to extend the Initial Research Term (referred to as the Research Term Extension Period, and together with the Initial Research Term, the Research Term) for up to an additional two years, for a research term extension fee of up to $400.0 million. The Regeneron Collaboration also covers a select number of RNAi therapeutic programs designed to target genes expressed in the liver, including our previously-announced collaboration with Regeneron to identify RNAi therapeutics for the chronic liver disease nonalcoholic steatohepatitis. We retain broad global rights to all of our other unpartnered liver-directed clinical and pre-clinical pipeline programs. The Regeneron Collaboration is governed by a joint steering committee that is comprised of an equal number of representatives from each party. Regeneron will lead development and commercialization for all programs targeting eye diseases (subject to limited exceptions), entitling us to certain potential milestone and royalty payments pursuant to the terms of a license agreement, the form of which has been agreed upon by the parties. We and Regeneron will alternate leadership on CNS and liver programs, with the lead party retaining global development and commercial responsibility. For CNS and liver programs, both we and Regeneron will have the option at lead candidate selection to enter into a co-co collaboration agreement, the form of which has been agreed upon by the parties, whereby both companies will share equally all costs of, and profits from, all development and commercialization activities under the program. If the non-lead party elects to not enter into a co-co collaboration agreement with respect to a given CNS or liver program, we and Regeneron will enter into a license agreement with respect to such program and the lead party will be the “Licensee” for the purposes of the license agreement. If the lead party for a CNS or liver program elects to not enter into the co-co collaboration agreement, then we and Regeneron will enter into a license agreement with respect to such program and leadership of the program will transfer to the other party and the former non-lead party will be the “Licensee” for the purposes of the license agreement. With respect to the programs directed to C5 complement-mediated diseases, we retain control of cemdisiran monotherapy development, and Regeneron is leading combination product development. Under the C5 co-co collaboration agreement, we and Regeneron equally share costs and potential future profits on any monotherapy program. Under the C5 license agreement, for cemdisiran to be used as part of a combination product, Regeneron is solely responsible for all development and commercialization costs and we will receive low double-digit royalties and commercial milestones of up to $325.0 million on any potential combination product sales. The C5 co-co collaboration agreement, the C5 license agreement, and the Master Agreement have been combined for accounting purposes and treated as a single agreement. In connection with the Regeneron Master Agreement, Regeneron made an upfront payment of $400.0 million. We are also eligible to receive up to an additional $200.0 million in milestone payments upon achievement of certain criteria during early clinical development for eye and CNS programs. We and Regeneron plan to advance programs directed to up to 30 targets under the Regeneron Collaboration during the Initial Research Term. For each program, Regeneron will provide us with $2.5 million in funding at program initiation and an additional $2.5 million at lead candidate identification, with the potential for approximately $30.0 million in annual discovery funding to us as the Regeneron Collaboration reaches steady state. Regeneron has the right to terminate the Regeneron Master Agreement for convenience upon ninety days’ notice. The termination of the Regeneron Master Agreement does not affect the term of any license agreement or co-co collaboration agreement then in effect. In addition, either party may terminate the Regeneron Master Agreement for a material breach by, or insolvency of, the other party. Unless earlier terminated pursuant to its terms, the Regeneron Master Agreement will remain in effect with respect to each program until (a) such program becomes a terminated program or (b) the parties enter into a license agreement or co-co collaboration agreement with respect to such program. The Regeneron Master Agreement includes various representations, warranties, covenants, dispute escalation and resolution mechanisms, indemnities and other provisions customary for transactions of this nature. For any license agreement subsequently entered into, the licensee will generally be responsible for its own costs and expenses incurred in connection with the development and commercialization of the collaboration products. The licensee will pay to the licensor certain development and/or commercialization milestone payments totaling up to $150.0 million for each collaboration product. In addition, following the first commercial sale of the applicable collaboration product under a license agreement, the licensee is required to make certain tiered royalty payments, ranging from low double-digits up to 20%, to the licensor based on the aggregate annual net sales of the collaboration product, subject to customary reductions. For any co-co collaboration agreement subsequently entered into, we and Regeneron will share equally all costs of, and profits from, development and commercialization activities. In the event that a party exercises its opt-out right, the lead party will be responsible for all costs and expenses incurred in connection with the development and commercialization of the collaboration products under the applicable co-co collaboration agreement, subject to continued sharing of costs through defined points. If a party exercises its opt-out right, following the first commercial sale of the applicable collaboration product under a co-co collaboration agreement, the lead party is required to make certain tiered royalty payments, ranging from low double-digits up to 20%, to the other party based on the aggregate annual net sales of the collaboration product and the timing of the exercise of the opt-out right, subject to customary reductions and a reduction for opt-out transition costs. Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with drug development, we may not receive any milestone or royalty payments from Regeneron under the Regeneron Master Agreement, the C5 license agreement, or any future license agreement, or under any co-co collaboration agreement in the event we exercise our opt-out right. Our obligations under the Regeneron Collaboration include: (i) a research license and research services, collectively referred to as the Research Services Obligation; (ii) a worldwide license to cemdisiran for combination therapies, and manufacturing and supply and development service obligations, collectively referred to as the C5 License Obligation; and (iii) development, manufacturing and commercialization activities for cemdisiran monotherapies, referred to as the C5 Co-Co Obligation. The research license is not distinct from the research services primarily as a result of Regeneron being unable to benefit on its own or with other resources reasonably available, as the license is providing access to specialized expertise, particularly as it relates to RNAi technology that is not available in the marketplace. Similarly, the worldwide license to cemdisiran for combination therapies is not distinct from the manufacturing and supply and development service obligations, as Regeneron cannot benefit on its own from the value of the license without receipt of supply. Separately, the cemdisiran monotherapy co-co collaboration agreement is under the scope of ASC 808 as we and Regeneron are both active participants in the development and manufacturing activities and are exposed to significant risks and rewards that are dependent on commercial success of the activities of the arrangement. The development and manufacturing activities are a combined unit of account under the scope of ASC 808 and are not deliverables under ASC 606. The total transaction price is comprised of the $400.0 million upfront payment and additional variable consideration related to research, development, manufacturing and supply activities related to the Research Services Obligation and the C5 License Obligation. We utilized the expected value method to determine the amount of reimbursement for these activities. We determined that any variable consideration related to sales-based royalties and milestones related to the worldwide license to cemdisiran for combination therapies is deemed to be constrained and therefore has been excluded from the transaction price. In addition, we are eligible to receive future milestones upon the achievement of certain criteria during early clinical development for the eye and CNS programs. We are also eligible to receive royalties on future commercial sales for certain eye, CNS or liver targets, if any; however, these amounts are excluded from variable consideration under the Regeneron Collaboration as we are only eligible to receive such amounts if, after a drug candidate is identified, the form of license agreement is subsequently executed resulting in a license that is granted to Regeneron. Any such subsequently granted license would represent a separate transaction under ASC 606. We allocated the initial transaction price to each unit of account based on the applicable accounting guidance as follows, in thousands: Performance Obligations Standalone Selling Price Transaction Price Allocated Accounting Guidance Research Services Obligation $ 130,700 $ 183,100 ASC 606 C5 License Obligation 97,600 92,500 ASC 606 C5 Co-Co Obligation 364,600 246,000 ASC 808 $ 521,600 The transaction price was allocated to the obligations based on the relative estimated standalone selling prices of each obligation, over which management has applied significant judgment. We developed the estimated standalone selling price for the licenses included in the Research Services Obligation and the C5 License Obligation primarily based on the probability-weighted present value of expected future cash flows associated with each license related to each specific program. In developing such estimate, we applied judgment in the determination of the forecasted revenues, taking into consideration the applicable market conditions and relevant entity-specific factors, the expected number of targets or indications expected to be pursued under each license, the probability of success, the time needed to develop a product candidate pursuant to the associated license and the discount rate. We developed the estimated standalone selling price for the services and/or manufacturing and supply included in each of the obligations, as applicable, primarily based on the nature of the services to be performed and/or goods to be manufactured and estimates of the associated costs. The estimated standalone selling price of the C5 Co-Co Obligation was developed by estimating the present value of expected future cash flows that Regeneron is entitled to receive. In developing such estimate, we applied judgment in determining the indications that will be pursued, the forecasted revenues for such indications, the probability of success and the discount rate. For the Research Services Obligation and the C5 License Obligation accounted for under ASC 606, we measure proportional performance over time using an input method based on cost incurred relative to the total estimated costs for each of the identified obligations, on a quarterly basis, by determining the proportion of effort incurred as a percentage of total effort we expect to expend. This ratio is applied to the transaction price allocated to each obligation. Management has applied significant judgment in the process of developing our estimates. Any changes to these estimates will be recognized in the period in which they change as a cumulative catch up. We re-evaluate the transaction price as of the end of each reporting period and during the period from the execution of the Regeneron Master Agreement through December 31, 2019, the transaction price increased $33.5 million from $521.6 million upon execution of the Regeneron Master Agreement to $555.1 million as of December 31, 2019, resulting from additional variable consideration we expect to receive. For the C5 Co-Co Obligation accounted for under ASC 808, the transaction price allocated to this obligation is recognized using a proportional performance method. Revenue recognized under this agreement, inclusive of the amount allocated to the C5 Co-Co Obligation and future cost reimbursements, is accounted for as collaboration revenue. The following table provides a summary of the transaction price allocated to each unit of account based on the applicable accounting guidance, in addition to revenue activity during the period, in thousands: Revenue Recognized During Deferred Revenue Performance Obligations Transaction Price Allocated Year Ended December 31, 2019 As of December 31, 2019 Accounting Guidance Research Services Obligation $ 200,600 $ 21,000 $ 84,800 ASC 606 C5 License Obligation 108,500 — 65,800 ASC 606 C5 Co-Co Obligation 246,000 2,900 243,000 ASC 808 Total $ 555,100 $ 23,900 $ 393,600 As of December 31, 2019, the aggregate amount of the transaction price allocated to the remaining Research Services Obligation and C5 License Obligation that was unsatisfied was $288.1 million, which is expected to be recognized through the term of the Regeneron Collaboration as the services are performed. This amount excludes the transaction price allocated to the C5 Co-Co Obligation accounted for under ASC 808. Deferred revenue related to the Regeneron Collaboration is classified as either current or non-current in the consolidated balance sheets based on the period the revenue is expected to be recognized. Sanofi Genzyme Collaboration 2014 Sanofi Genzyme Collaboration, as amended in January 2018 and further amended in April 2019 In January 2014, we entered into the 2014 Sanofi Genzyme collaboration. The 2014 Sanofi Genzyme collaboration superseded and replaced the 2012 Sanofi Genzyme agreement and was amended in January 2018, at which time we also entered into an Exclusive License Agreement, referred to as the Exclusive TTR License, as well as the ALN-AT3 Global License Terms, referred to as the AT3 License Terms, as described below. The 2014 Sanofi Genzyme collaboration and the AT3 License Terms were further amended in April 2019. Under the 2014 Sanofi Genzyme collaboration, certain of Sanofi Genzyme’s specific license rights and the programs which Sanofi Genzyme opted into prior to the 2018 amendment included the following: • Upon the effective date of the 2014 Sanofi Genzyme collaboration, Sanofi Genzyme opted into a broader regional license and collaboration for patisiran, which was originally established under the 2012 Sanofi Genzyme agreement, and a co-development/co-commercialization license for revusiran. As part of our TTR program, we are also developing vutrisiran. Sanofi Genzyme had a right to elect a co-development/co-commercialization license for vutrisiran. • In September 2015, Sanofi Genzyme elected to opt into our fitusiran clinical development program for the treatment of hemophilia under the regional license terms. Cost-sharing for the fitusiran program began in January 2016 under the regional license terms. Sanofi Genzyme also had the right to elect to co-develop and co-commercialize fitusiran in the U.S., Canada and Western Europe, referred to as the Alnylam Territory, pursuant to the co-development/co-commercialization license terms. Upon opt-in, we retained product rights in the Alnylam Territory, while Sanofi Genzyme obtained exclusive rights to develop and commercialize fitusiran in the rest of the world, referred to as the Sanofi Genzyme Territory, and to co-commercialize the product in the Alnylam Territory. In November 2016, Sanofi Genzyme exercised that right and elected to co-develop and co-commercialize fitusiran in the Alnylam Territory. Development costs for co-development/co-commercialization products, once Sanofi Genzyme exercised an option, were shared between Sanofi Genzyme and us, with Sanofi Genzyme responsible for 50% of the global development costs. In connection with the exercise of its co-development/co-commercialization rights for fitusiran, Sanofi Genzyme paid us approximately $6.0 million in January 2017 for its incremental share of co-development costs incurred from January 2016 through September 2016. Sanofi Genzyme was required to make certain milestone payments for fitusiran, and in December 2014, we earned a development milestone payment of $25.0 million based upon the initiation of the first global Phase 3 clinical trial for revusiran. Sanofi Genzyme was also obligated to pay us a milestone of $25.0 million upon the dosing of the first patient in our ATLAS Phase 3 program for fitusiran. In addition, Sanofi Genzyme was required to pay tiered double-digit royalties up to 20% for each co-development/co-commercialization product based on annual net sales, if any, in the Sanofi Genzyme Territory for such product by Sanofi Genzyme, its affiliates and sublicensees. The parties were to share profits equally and we expected to book product sales in the Alnylam Territory. • During 2016, Sanofi Genzyme elected not to opt into the development and commercialization of givosiran or cemdisiran in the Sanofi Genzyme Territory. Sanofi Genzyme’s rights with respect to patisiran and fitusiran were modified in connection with the 2018 amendment, the Exclusive TTR License and the AT3 License Terms, as described below. At such time, Sanofi Genzyme had the right to opt into our future rare genetic disease programs for development and commercialization in the Sanofi Genzyme Territory under the regional license terms. In connection with the 2018 amendment, the Exclusive TTR License and the AT3 License Terms, we and Sanofi Genzyme agreed to terminate the co-development and co-commercialization rights related to revusiran, vutrisiran and fitusiran under the 2014 Sanofi Genzyme collaboration, and further agreed that no future rights would be granted to Sanofi Genzyme for co-development and co-commercialization under the 2014 Sanofi Genzyme collaboration, as amended by the 2018 amendment. During the first quarter of 2018, Sanofi Genzyme elected not to exercise its global option for our lumasiran program. In April 2019, we and Sanofi Genzyme further amended the 2014 Sanofi Genzyme collaboration, which we refer to as the Collaboration Amendment. Under the Collaboration Amendment, we and Sanofi Genzyme agreed to conclude the research and option phase under our collaboration agreement. In connection and simultaneously with entering into the Collaboration Amendment, we and Sanofi Genzyme also entered into the Amended and Restated ALN-AT3 Global License Terms, with respect to ALN-AT3 (fitusiran) and certain back-up products, which we refer to as the A&R AT3 License Terms. The A&R AT3 License Terms amend and restate the original AT3 License Terms to modify certain of the business terms. The material collaboration terms for fitusiran continued unchanged. Such terms are described below. Exclusive TTR License and A&R AT3 License Terms As noted above, the 2018 amendment, together with the Exclusive TTR License and the original AT3 License Terms, revised the terms and conditions of the 2014 Sanofi Genzyme collaboration to (i) provide us the exclusive right to pursue the further global development and commercialization of all TTR products, including ONPATTRO, vutrisiran and any back-up products, (ii) provide Sanofi Genzyme the exclusive right to pursue the further global development and commercialization of fitusiran and any back-up products and (iii) terminate the previous co-development and co-commercialization rights related to revusiran, vutrisiran and fitusiran under the 2014 Sanofi Genzyme collaboration. As a result, we are funding all development and commercialization costs for ONPATTRO and vutrisiran. We also funded development and commercialization costs for fitusiran through the transition period, up to a cap of $50.0 million, after which Sanofi Genzyme became responsible for funding all development and commercialization costs for fitusiran. We completed the transition period relating to the transition of the fitusiran program to Sanofi Genzyme in 2018. Each party was responsible for its costs associated with the transfer of the respective program to the other party. Under the 2018 amendment and the Exclusive TTR License, Sanofi Genzyme is eligible to receive (i) royalties up to 25%, increasing over time, based on annual net sales of ONPATTRO in territories excluding the U.S., Canada and Western Europe, provided royalties on annual net sales of ONPATTRO in Japan will be 25% beginning as of the effective date of the Exclusive TTR License, (ii) tiered royalties of 15% to 30% based on global annual net sales of vutrisiran (consistent with the royalties due to us from Sanofi Genzyme on fitusiran), and (iii) tiered royalties of up to 15% based on global annual net sales of any back-up products, in each case by us, our affiliates and our sublicensees. The Collaboration Amendment entered into in April 2019 made no changes to the terms described in clauses (i)-(iii) above, which remain in full force and effect. Except as described below, there are no additional milestones due to either party with respect to ONPATTRO, vutrisiran or fitusiran. In consideration for the rights granted to Sanofi Genzyme under the 2018 amendment and the original AT3 License Terms, Sanofi Genzyme was required to make one milestone payment of $50.0 million following the dosing of the first patient in the ATLAS Phase 3 program for fitusiran. This milestone was achieved in the first quarter of 2018. In addition, under the A&R AT3 License Terms, we are eligible to receive tiered royalties of 15% to 30% based on global annual net sales of fitusiran and up to 15% based on global annual net sales of any back-up products controlled by Sanofi Genzyme, in each case by Sanofi Genzyme, its affiliates and its sublicensees. We intend to continue to work with Sanofi Genzyme to ensure continuity for the supply of fitusiran for ongoing clinical studies, and, at Sanofi Genzyme’s request, commercial sales. Sanofi Genzyme also has the right to manufacture fitusiran. Under the A&R AT3 License Terms, we agreed to advance, at our cost, a selected investigational asset in an undisclosed rare genetic disease through the end of Investigational New Drug-enabling studies. Following completion of such studies, we will transition, at our cost, such asset to Sanofi Genzyme. Thereafter, Sanofi Genzyme will fund all potential future development and commercialization costs for such asset. If this asset is developed and approved, we will be eligible to receive tiered double-digit royalties on global net sales. Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with drug development, we may not receive any royalty payments under the A&R AT3 License Terms. The 2014 Sanofi Genzyme collaboration, as amended, will continue to be governed by an alliance joint steering committee that is comprised of an equal number of representatives from each party. Additional committees oversee certain matters that may arise under the Exclusive TTR License and the A&R AT3 License Terms. The original master agreement (including the license terms appended thereto), as well as the Exclusive TTR License and the A&R AT3 License Terms, contain certain termination provisions, including for material breach by the other party. In addition, we have the right to terminate the Exclusive TTR License without cause with respect to any or all licensed products at any time upon six months’ prior written notice and Sanofi Genzyme has the right to terminate the A&R AT3 License Terms without cause with respect to any particular licensed product at any time upon six months’ prior written notice. The term of the Exclusive TTR License expires on a licensed product-by-licensed product and country-by-country basis upon expiration of the last royalty term to expire under the agreement, where a royalty term is defined as the latest to occur of (a) expiration of the last valid claim of patent rights covering a licensed product; (b) the expiration of Regulatory Exclusivity for a licensed product, as defined in the Exclusive TTR License; or (c) the twelfth anniversary of the first commercial sale of the licensed product in such country. The term of the A&R AT3 License Terms expires on a licensed product-by-licensed product and country-by-country basis upon expiration of the last royalty term to expire under the agreement, where a royalty term is defined as the latest to occur of (x) the expiration of the last valid claim of patent rights covering a licensed product; (y) the expiration of Regulatory Exclusivity for a licensed product, as defined in the A&R AT3 License Terms; or (z) the twelfth anniversary of the first commercial sale of the licensed product in such country. As noted above, the Sanofi Genzyme collaboration originally entered into in 2012 was materially modified during its term when the agreement was amended in 2014, prior to our adoption of the new revenue standard on January 1, 2018. In accordance with the new revenue standard, we evaluated the Sanofi Genzyme collaboration with the aggregate effect of all modifications when identifying performance obligations, determining the transaction price and allocating the transaction price. We determined that certain promises included in these agreements are within the scope of the new revenue standard since Sanofi Genzyme is a customer with respect to the license of the rights to its territories. We also determined, however, that certain aspects of these agreements are within the scope of the collaboration accounting guidance with respect to co-commercialization activities as these activities are joint risk-sharing and are not reflective of a vendor-customer relationship. We apply the new revenue standard to all promises associated with the transfer of goods and services to a customer. We concluded that Sanofi Genzyme meets the definition of a customer as we were delivering intellectual property and know-how rights as well as research and development activities for the TTR programs and fitusiran programs in support of territories in which we are not jointly sharing the risks and rewards. We concluded that the accounting for the original 2014 Sanofi Genzyme collaboration, and the collaboration, as amended in 2018, should be assessed as separate contracts for (i) the patisiran and revusiran (TTR) programs, upon the initiation of the 2014 Sanofi Genzyme collaboration, and (ii) the subsequent opt-in by Sanofi Genzyme for the fitusiran program. In addition, we determined that the Sanofi Genzyme collaboration met the requirements to be acc

OTHER BALANCE SHEET DETAILS

OTHER BALANCE SHEET DETAILS	12 Months Ended
	Dec. 31, 2019
Balance Sheet Related Disclosures [Abstract]
OTHER BALANCE SHEET DETAILS	OTHER BALANCE SHEET DETAILS The following table presents our inventory as of December 31, 2019 and 2018, in thousands: As of December 31, 2019 2018 Raw materials $ 15,418 $ 8,709 Work in process 38,275 15,262 Finished goods 2,655 97 Total inventory $ 56,348 $ 24,068 Property, plant and equipment, net consist of the following as of December 31, 2019 and 2018, in thousands: As of December 31, 2019 2018 Buildings $ 250,380 $ — Leasehold improvements 132,632 65,928 Construction in progress 54,195 260,975 Laboratory equipment 29,755 43,427 Computer equipment and software 14,956 13,495 Furniture and fixtures 10,339 7,238 Land 9,080 9,080 501,337 400,143 Less: accumulated depreciation (76,158) (79,485) Total $ 425,179 $ 320,658 Accrued expenses consist of the following as of December 31, 2019 and 2018, in thousands: As of December 31, 2019 2018 Compensation and related $ 68,304 $ 37,301 Pre-clinical, clinical trial and manufacturing 34,269 32,205 Product revenue allowances 32,670 2,696 Licensing and collaboration agreements 20,622 6,433 Consulting and professional services 14,251 10,450 Other 27,085 23,634 Total $ 197,201 $ 112,719 The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within our consolidated balance sheets that sum to the total of these amounts shown in the consolidated statements of cash flows, in thousands: As of December 31, 2019 2018 2017 Cash and cash equivalents $ 547,178 $ 420,146 $ 645,361 Restricted cash included in prepaid expenses and other current assets 4 225 — Restricted cash included in long-term other assets 2,446 2,260 1,471 Total cash, cash equivalents, and restricted cash shown in the consolidated $ 549,628 $ 422,631 $ 646,832 The following table summarizes the changes in accumulated other comprehensive (loss) income, by component, for the years ended December 31, 2019 and 2018, in thousands: Loss on Investment in Joint Venture Defined Benefit Pension Unrealized (Losses) Gains from Debt Securities Foreign Currency Translation Total Accumulated Other Comprehensive (Loss) Income Balance as of December 31, 2017 $ (32,792) $ — $ (1,641) $ — $ (34,433) Other comprehensive loss before reclassifications — — (411) — (411) Amounts reclassified from other comprehensive income — — 1,631 — 1,631 Net other comprehensive income — — 1,220 — 1,220 Balance as of December 31, 2018 (32,792) — (421) — (33,213) Other comprehensive (loss) income before reclassifications — (3,661) 22 (343) (3,982) Amounts reclassified from other comprehensive income — 141 536 — 677 Net other comprehensive (loss) income — (3,520) 558 (343) (3,305) Balance as of December 31, 2019 $ (32,792) $ (3,520) $ 137 $ (343) $ (36,518)

FAIR VALUE MEASUREMENTS

FAIR VALUE MEASUREMENTS	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	FAIR VALUE MEASUREMENTS The following tables present information about our assets that are measured at fair value on a recurring basis as of December 31, 2019 and 2018, and indicate the fair value hierarchy of the valuation techniques we utilized to determine such fair value, in thousands: Description As of December 31, Quoted Prices in Active Markets Significant Observable Inputs Significant Unobservable Inputs Cash equivalents: Commercial paper $ 3,439 $ — $ 3,439 $ — U.S. treasury securities 336,693 — 336,693 — Money market funds 119,882 119,882 — — Marketable debt securities: Certificates of deposit 4,301 — 4,301 — Commercial paper 36,474 — 36,474 — Corporate notes 146,040 — 146,040 — U.S. government-sponsored enterprise securities 32,488 — 32,488 — U.S. treasury securities 755,714 — 755,714 — Marketable equity securities 13,967 13,967 — — Restricted cash (money market funds) 1,482 1,482 — — Total $ 1,450,480 $ 135,331 $ 1,315,149 $ — Description As of December 31, Quoted Prices in Active Markets Significant Observable Inputs Significant Unobservable Inputs Cash equivalents: U.S. treasury securities $ 221,281 $ — $ 221,281 $ — Money market funds 102,445 102,445 — — Marketable debt securities: Certificates of deposit 8,951 — 8,951 — Commercial paper 57,197 — 57,197 — Corporate notes 232,410 — 232,410 — U.S. government-sponsored enterprise securities 39,018 — 39,018 — U.S. treasury securities 325,227 — 325,227 — Marketable equity securities 1,206 1,206 — — Restricted cash (money market funds) 1,477 1,477 — — Total $ 989,212 $ 105,128 $ 884,084 $ — For the years ended December 31, 2019 and 2018, there were no transfers between Level 1 and Level 2 financial assets. The carrying amounts reflected in our consolidated balance sheets for cash, accounts receivable, net, other current assets, accounts payable and accrued expenses approximate fair value due to their short-term maturities. The fair value of our long-term debt as of December 31, 2018, computed pursuant to a discounted cash flow technique using a market interest rate, was $30.1 million and is considered a Level 3 fair value measurement. The effective interest rate reflects the current market rate. As of December 31, 2019, we had no outstanding long-term debt. In October 2017, when we entered into the Vir agreement, described above, Vir was a private clinical-stage immunology company. As part of the upfront consideration, we received 5,000,000 shares of Vir common stock. On September 27, 2019, Vir implemented a 1-for-4.5 reverse split of its common stock, converting our 5,000,000 shares to 1,111,111 shares. On October 10, 2019, Vir completed its initial public offering, or IPO. As a result of the IPO, our Vir common stock is now publicly traded and subject to a 180-day lock-up period from the effective date of Vir's IPO. During the fourth quarter of 2019, we recognized a gain of $11.3 million to record the fair market value of our Vir common stock as of December 31, 2019.

MARKETABLE DEBT SECURITIES

MARKETABLE DEBT SECURITIES	12 Months Ended
	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
MARKETABLE DEBT SECURITIES	MARKETABLE DEBT SECURITIES The following tables summarize our marketable debt securities as of December 31, 2019 and 2018, in thousands: As of December 31, 2019 Amortized Gross Gross Fair Value Certificates of deposit $ 4,303 $ — $ (2) $ 4,301 Commercial paper 39,913 — — 39,913 Corporate notes 146,016 58 (34) 146,040 U.S. government-sponsored enterprise securities 32,487 3 (2) 32,488 U.S. treasury securities 1,092,293 185 (71) 1,092,407 Total $ 1,315,012 $ 246 $ (109) $ 1,315,149 As of December 31, 2018 Amortized Gross Gross Fair Value Certificates of deposit $ 8,951 $ — $ — $ 8,951 Commercial paper 57,197 — — 57,197 Corporate notes 232,695 — (285) 232,410 U.S. government-sponsored enterprise securities 39,031 — (13) 39,018 U.S. treasury securities 546,631 1 (124) 546,508 Total $ 884,505 $ 1 $ (422) $ 884,084 The fair values of our marketable debt securities by classification in the consolidated balance sheets were as follows, in thousands: December 31, 2019 December 31, 2018 Cash and cash equivalents $ 340,132 $ 221,281 Marketable debt securities 975,017 662,803 Total $ 1,315,149 $ 884,084

LEASES

LEASES	12 Months Ended
	Dec. 31, 2019
Leases [Abstract]
LEASES	LEASES Overview of Significant Leases We lease three facilities for office and laboratory space in Cambridge, Massachusetts that represent substantially all of our significant lease obligations. An overview of these significant leases are as follows: 675 West Kendall Street We lease office and laboratory space located at 675 West Kendall Street, Cambridge, Massachusetts from BMR-675 West Kendall Street, LLC, or BMR, under a non-cancelable real property lease. In September 2019, we moved our corporate headquarters and research facility to this location from our 300 Third Street location. The lease commenced on May 1, 2018 and monthly rent payments became due commencing on February 1, 2019 upon substantial completion of the building improvements, and continue for 15 years, with options to renew for two five-year terms each. Exercise of these options was not determined to be reasonably certain and thus was not included in the operating lease liability on the consolidated balance sheet as of December 31, 2019. Under the terms of the 675 West Kendall Lease, BMR agreed to contribute a total of $56.1 million toward the cost of base building and tenant improvements. As of December 31, 2019, in connection with base building and tenant improvements to date, we received substantially all of the funds from BMR. In connection with the 675 West Kendall Lease, we were required to provide a $14.8 million security deposit that is recorded as restricted investments on our consolidated balance sheet as of December 31, 2019. 300 Third Street We lease office and laboratory space located at 300 Third Street, Cambridge, Massachusetts under a non-cancelable real property lease agreement by and between us and ARE-MA Region No. 28, LLC, or ARE-MA, dated as of September 26, 2003, as amended. The term of the lease expires on January 31, 2034 with options to renew for two five-year terms each. Exercise of these options was not determined to be reasonably certain and thus was not included in the operating lease liability on the consolidated balance sheet as of December 31, 2019. 101 Main Street We lease office space on several floors at 101 Main Street, Cambridge, Massachusetts under non-cancelable real property lease agreements by and between us and RREEF America REIT II CORP. PPP, or RREEF, entered into in March 2015 and May 2015 that will expire in March 2024 and June 2021, respectively, each with an option to renew for one five-year term. Exercise of these options was not determined to be reasonably certain and thus was not included in the operating lease liability on the consolidated balance sheet as of December 31, 2019. Other Lease Disclosures Our facility leases described above generally contain customary provisions allowing the landlords to terminate the leases if we fail to remedy a breach of any of our obligations under any such lease within specified time periods, or upon our bankruptcy or insolvency. The leases do not include any restrictions or covenants that had to be accounted for under the lease guidance. Total rent expense, including operating expenses, under our real property leases was $52.4 million, $40.6 million and $18.7 million for the years ended December 31, 2019, 2018 and 2017, respectively. The below table summarizes our costs included in operating expenses related to right of use lease assets we have entered into through December 31, 2019, in thousands: Description Year Ended Operating lease cost $ 38,613 Variable lease cost 15,209 Total $ 53,822 Short-term lease costs were not material for the year ended December 31, 2019. Net cash paid for the amounts included in the measurement of the operating lease liability in our consolidated balance sheet and included in operating lease liability within operating activities in our consolidated statement of cash flow was $33.7 million for the year ended December 31, 2019. The weighted-average remaining lease term and weighted-average discount rate for all leases as of December 31, 2019 was 13.2 years and 8.2%, respectively. Future lease payments for non-cancellable operating leases and a reconciliation to the carrying amount of the operating lease liability presented in the consolidated balance sheet as of December 31, 2019 were as follows, in thousands: Year Ending December 31 2020 $ 29,157 2021 37,597 2022 37,754 2023 36,250 2024 35,381 2025 and thereafter 356,567 Total undiscounted lease liability 532,706 Less imputed interest (225,873) Less impact of future leases not yet commenced (3,010) Total discounted lease liability $ 303,823 Current operating lease liability $ 27,688 Non-current operating lease liability 276,135 Total $ 303,823 Under the prior lease guidance, minimum payments under our non-cancelable facility leases, as of December 31, 2018, were as follows, in thousands: Year Ending December 31 2019 $ 32,228 2020 34,826 2021 34,410 2022 34,826 2023 35,270 Thereafter 390,455 Total $ 562,015

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	12 Months Ended
	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES Technology License and Other Commitments We have licensed from third parties the rights to use certain technologies and information in our research processes as well as in any other products we may develop. In accordance with the related license or technology agreements, we are required to make certain fixed payments to the licensor or a designee of the licensor over various agreement terms. Many of these agreement terms are consistent with the remaining lives of the underlying intellectual property that we have licensed. As of December 31, 2019, our commitments over the next five years to make fixed and cancellable payments under existing license agreements were not material. We in-license technology from a number of sources, including Ionis and Merck Sharp & Dohme Corp, or Merck. In addition, we have collaboration agreements relating to the research, development and commercialization of certain of our product candidates. Pursuant to these agreements, we will be required to make additional payments, including in some cases milestone payments if and when we achieve specified development, regulatory and commercialization events, as well as royalty payments on sales of our approved products. Amounts related to contingent milestone payments are not considered contractual obligations as they are contingent upon the successful achievement of such milestones. Based on our current development plans, during the next 12 months from the filing of this annual report on Form 10-K, potential milestone payments due to third parties, could be approximately $18.4 million, including $12.8 million in regulatory and development milestones and $5.6 million in commercial milestones, in connection with our various collaborations and license agreements. These milestones generally become due and payable upon achievement. Because the achievement of these milestones was not considered probable as of December 31, 2019, such contingencies have not been recorded in our consolidated financial statements. Credit Agreements On April 29, 2016, we entered into a Credit Agreement, or the Credit Agreement, by and among Alnylam U.S., Inc., as the borrower, us, as a guarantor, and Wells Fargo Bank, National Association, as the lender. The Credit Agreement was entered into in connection with the planned build out of our new drug substance manufacturing facility. The Credit Agreement provided for a $30.0 million term loan facility and was scheduled to mature on April 29, 2021. On September 27, 2019, we repaid in full the $30.0 million outstanding principal amount under the Credit Agreement and the Credit Agreement terminated in accordance with its terms upon repayment of the outstanding indebtedness. Interest on borrowings under the Credit Agreement was calculated based on LIBOR plus 0.45%, except in the event of default. The obligations of the borrower and us under the Credit Agreement were secured by cash collateral in an amount equal to, at any given time, at least 100% of the principal amount of all term loans outstanding under such Credit Agreement at such time. As of December 31, 2018, we recorded $30.0 million of cash collateral in connection with the Credit Agreement as restricted investments on our consolidated balance sheets. Litigation From time to time, we are a party to legal proceedings in the course of our business, including the matters described below. The claims and legal proceedings in which we could be involved include challenges to the scope, validity or enforceability of patents relating to our commercially approved products and other product candidates, and challenges by us to the scope, validity or enforceability of the patents held by others. These include claims by third parties that we infringe their patents. The outcome of any such legal proceedings, regardless of the merits, is inherently uncertain. In addition, litigation and related matters are costly and may divert the attention of our management and other resources that would otherwise be engaged in other activities. If we were unable to prevail in any such legal proceedings, our business, results of operations, liquidity and financial condition could be adversely affected. Our accounting policy for accrual of legal costs is to recognize such expenses as incurred. Securities Litigation On September 26, 2018, Caryl Hull Leavitt, individually and on behalf of all others similarly situated, filed a class action complaint for violation of federal securities laws against us, our Chief Executive Officer and our former Chief Financial Officer in the United States District Court for the Southern District of New York. By stipulation of the parties and Order of the Court dated November 20, 2018, the action was transferred to the United States District Court for the District of Massachusetts. On May 8, 2019, the Court entered an order appointing a lead plaintiff, and on July 3, 2019, lead plaintiff filed a consolidated class action complaint, or the Complaint. In addition to the originally named defendants, the Complaint also names as defendants certain of our other executive officers, and purports to be brought on behalf of a class of persons who acquired our securities between September 20, 2017 and September 12, 2018 and seeks to recover damages caused by defendants’ alleged violations of the federal securities laws and to pursue remedies under Sections 10(b) and 20(a) of the Securities Exchange Act of 1934, as amended, and Rule 10b-5 promulgated thereunder. The Complaint alleges, among other things, that the defendants made materially false and misleading statements related to the efficacy and safety of our product, ONPATTRO. The plaintiff seeks, among other things, the designation of this action as a class action, an award of unspecified compensatory damages, interest, costs and expenses, including counsel fees and expert fees, and other relief as the court deems appropriate. All defendants filed a motion to dismiss the Complaint in its entirety on July 31, 2019. The motion to dismiss was fully briefed on September 30, 2019. On September 12, 2019, the Chester County Employees Retirement Fund, individually and on behalf of all others similarly situated, filed a purported securities class action complaint for violation of federal securities laws against us, certain of our current and former directors and officers, and the underwriters of our November 14, 2017 public stock offering, in the Supreme Court of the State of New York, New York County. On November 7, 2019, plaintiff filed an amended complaint, or the New York Complaint. The New York Complaint is brought on behalf of an alleged class of those who purchased our securities pursuant and/or traceable to our November 14, 2017 public stock offering. The New York Complaint purports to allege claims arising under Sections 11, 12(a)(2) and 15 of the Securities Act of 1933, as amended, and generally alleges that the defendants violated the federal securities laws by, among other things, making material misstatements or omissions concerning the results of our APOLLO Phase 3 clinical trial of patisiran. The plaintiff seeks, among other things, the designation of the action as a class action, an award of unspecified compensatory damages, rescissory damages, interest, costs and expenses, including counsel fees and expert fees, and other relief as the court deems appropriate. All defendants filed a joint motion to dismiss the New York Complaint in its entirety on December 20, 2019. Plaintiff’s response to that motion was filed on February 3, 2020. We believe that the allegations contained in these complaints are without merit and intend to defend the cases vigorously. We cannot predict at this point the length of time that these actions will be ongoing or the liabilities, if any, which may arise therefrom. Dicerna Litigation On June 10, 2015, we filed a trade secret misappropriation lawsuit against Dicerna in the Superior Court of Middlesex County, Massachusetts seeking to stop misappropriation by Dicerna of our confidential, proprietary and trade secret information related to the RNAi assets we purchased from Merck, including certain N-acetylgalactosamine conjugate technology. In addition to permanent injunctive relief, we were also seeking monetary damages from Dicerna. On April 18, 2018, we and Dicerna entered into a Settlement Agreement resolving all ongoing litigation between the companies. Under the terms of the Settlement Agreement, Dicerna was required to pay us an aggregate of $25.0 million, all of which we had received as of January 2019. Indemnifications In connection with license agreements we may enter with companies to obtain rights to intellectual property, we may be required to indemnify such companies for certain damages arising in connection with the intellectual property rights licensed under the agreements. Under such agreements, we may be responsible for paying the costs of any litigation relating to the license agreements or the underlying intellectual property rights, including the costs associated with certain litigation regarding the licensed intellectual property. We are also a party to a number of agreements entered into in the ordinary course of business, which contain typical provisions that obligate us to indemnify the other parties to such agreements upon the occurrence of certain events, including litigation. For example, under the underwriting agreement entered into in connection with our November 2017 public offering, we have an obligation to indemnify the underwriters and each person, if any, who controls the underwriters, for certain costs and expenses arising in connection with the class action complaint filed against us and such underwriters in New York state court, described above. These indemnification costs are charged to selling, general and administrative expense. Our maximum potential future liability under any such indemnification provisions is uncertain. However, to date, other than certain costs associated with certain previously settled litigation, we have not incurred material costs to defend lawsuits or settle claims related to these indemnification provisions. We have determined that the estimated aggregate fair value of our potential liabilities under all such indemnification provisions is minimal and had not recorded any liability related to such indemnification provisions as of December 31, 2019 or 2018.

STOCKHOLDERS' EQUITY

STOCKHOLDERS' EQUITY	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
STOCKHOLDERS' EQUITY	STOCKHOLDERS’ EQUITY Preferred Stock We have authorized up to 5,000,000 shares of preferred stock, $0.01 par value per share, for issuance. The preferred stock will have such rights, preferences, privileges and restrictions, including voting rights, dividend rights, conversion rights, redemption privileges and liquidation preferences, as shall be determined by our board of directors upon its issuance. As of December 31, 2019 and 2018, there were no shares of preferred stock outstanding. Public Offerings In January 2019, we sold an aggregate of 5,000,000 shares of our common stock through an underwritten public offering at a price to the public of $77.50 per share. As a result of the offering, we received aggregate net proceeds of $381.9 million, after deducting underwriting discounts and commissions and other estimated offering expenses of $5.6 million. In November 2017, we sold an aggregate of 6,440,000 shares of our common stock through an underwritten public offering at a price to the public of $125.00 per share. As a result of the offering, which included the full exercise of the underwriters’ option to purchase additional shares, we received aggregate net proceeds of $784.5 million, after deducting underwriting discounts and commissions and other offering expenses of $20.5 million. In May 2017, we sold an aggregate of 5,000,000 shares of our common stock through an underwritten public offering at a price to the public of $71.87 per share. As a result of the offering, we received aggregate net proceeds of $355.2 million, after deducting underwriting discounts and commissions and other offering expenses of $4.2 million. Regeneron Equity Placement On April 8, 2019, we executed the Regeneron SPA with Regeneron to sell 4,444,445 shares of our common stock for aggregate cash consideration of $400.0 million, or $90.00 per share, which we refer to as the Equity Transaction. As a condition to consummating the transactions contemplated by the Regeneron SPA, we and Regeneron entered into an Investor Agreement dated April 8, 2019, or the Investor Agreement. Under the Investor Agreement, until the expiration or termination of the Research Term under the Regeneron Master Agreement subject to extension by one year if the Research Term or Regeneron Master Agreement is terminated by Regeneron at will, or by up to two years if as of the expiration or termination of the Research Term Regeneron owns more than 19.99% of our outstanding shares, Regeneron and its affiliates will be bound by certain “standstill” provisions. The standstill provisions include agreements not to acquire more than 30% of our outstanding shares of common stock, call stockholder meetings, nominate directors other than those approved by our board of directors, subject to certain limited exceptions, or propose or support a proposal to acquire us. Further, under the Investor Agreement, Regeneron agreed to vote, and cause its affiliates to vote, all shares of our voting securities Regeneron is entitled to vote in a manner as recommended by our board of directors, except with respect to certain change of control transactions, liquidation or dissolution of our company, or, after the standstill term, any contested election of directors. Under the Investor Agreement, Regeneron agreed not to dispose of any of the purchased shares or any shares of common stock beneficially owned by it immediately after the closing of the Regeneron Master Agreement, until the earlier of (i) the four-year anniversary of the closing of the Equity Transaction and (ii) the termination of the Regeneron Collaboration, subject to limited exceptions, which we refer to as the Lock-Up Period. Following the expiration of the Lock-Up Period, if at any time Regeneron beneficially owns at least 9.9% of our outstanding shares, then until such time as Regeneron beneficially owns less than 5% of our outstanding shares, Regeneron will not dispose of any shares except (a) pursuant to a registered underwritten public offering pursuant to the Investor Agreement, (b) in a manner consistent with the volume limitations set forth in Rule 144 under the Securities Act, or (c) as otherwise approved by us. Under the Investor Agreement, following the Lock-Up Period, Regeneron will have three demand rights to require us to conduct a registered underwritten public offering with respect to the shares of common stock beneficially owned by Regeneron immediately after the closing of the Equity Transaction. In addition, following the Lock-Up Period, subject to certain conditions, Regeneron will be entitled to participate in registered underwritten public offerings by us if other selling stockholders are included in the registration. The rights and restrictions under the Investor Agreement are subject to termination upon the occurrence of certain events. Under the terms of the Regeneron SPA, if at the time of closing of the Equity Transaction a sufficient number of authorized shares of common stock under our Restated Certificate of Incorporation was not available, the Equity Transaction would have been settled in the form of our Series A redeemable convertible preferred stock. On April 25, 2019, following the receipt of stockholder approval at our annual meeting, a Certificate of Amendment was filed to our Restated Certificate of Incorporation to increase the number of authorized shares of common stock from 125,000,000 to 250,000,000 shares, providing for a sufficient number of authorized shares of common stock available to be issued to Regeneron pursuant to the Equity Transaction. On May 21, 2019, subsequent to the expiration of the applicable pre-merger waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended, Regeneron purchased 4,444,445 shares of our common stock for aggregate cash consideration of $400.0 million. Because we had an obligation to Regeneron as of April 8, 2019 that may have resulted in the issuance of redeemable convertible preferred stock, we were required to follow the guidance in ASC 480 and mark-to-market the obligation to potentially issue this redeemable security until April 25, 2019, when it became known that the obligation would be fulfilled in common stock. The final mark-to-market adjustment of this obligation under ASC 480 resulted in us recording a gain of $9.4 million included in other income in the consolidated statements of operations and comprehensive loss with the offsetting adjustment to equity netting against the $400.0 million proceeds that were received upon closing.

STOCK-BASED COMPENSATION

STOCK-BASED COMPENSATION	12 Months Ended
	Dec. 31, 2019
Share-based Payment Arrangement [Abstract]
STOCK-BASED COMPENSATION	STOCK-BASED COMPENSATIONStock Plans In May 2017, our stockholders approved a second amendment and restatement of the 2009 Stock Incentive Plan, or the Amended 2009 Plan, which increased the number of shares of common stock authorized for issuance from 11,700,000 to 15,480,000. On May 10, 2018, our stockholders approved our 2018 Stock Incentive Plan, or the 2018 Plan, which provided for the issuance of up to 3,500,000 shares in addition to the shares that remained available for grant under our Amended 2009 Plan as of the date of approval of the 2018 Plan by stockholders, and any shares underlying any awards granted under the Amended 2009 Plan that expire, or are terminated, surrendered or canceled without having been fully exercised or are forfeited in whole or in part after the date of our 2018 annual meeting. In 2019, we amended the 2018 Plan to increase the shares of common stock authorized for issuance thereunder by 3,290,000. The 2018 Plan, as amended, provides for the granting of stock options, restricted stock and restricted stock units (together, restricted stock awards), stock appreciation rights and other stock-based awards, and has a fungible share pool. Any award that is not a full value award is counted against the authorized share limits specified as one share for each share of common stock subject to the award, and all full value awards, defined as restricted stock awards or other stock-based awards, are counted as one and a half shares for each one share of common stock subject to such full value award. As of December 31, 2019, an aggregate of 18,887,759 shares of common stock were reserved for issuance under our stock plans, including outstanding stock options to purchase 13,069,165 shares of common stock, 749,071 outstanding restricted stock units, 4,729,971 of common stock available for additional equity awards and 339,552 shares available for future grant under our Amended and Restated 2004 Employee Stock Purchase Plan, or the Amended and Restated ESPP. Each stock option shall expire within 10 years of issuance. Time-based stock options granted to employees generally vest as to 25% of the shares on the first anniversary of the grant date and 6.25% of the shares at the end of each successive three-month period thereafter until fully vested. Change in Control Agreements On November 7, 2017, we entered into a Change in Control, or CIC, Agreement with each member of our management board. If a member of our management board is terminated by us without Cause, as defined in the CIC Agreement, or if a management board member terminates his or her employment for Good Reason, as defined in the CIC Agreement, in either case, within 12 months following a CIC, such management board member will be entitled to receive certain benefits, including the immediate acceleration of all outstanding unvested stock options and other stock-based awards. In accordance with accounting guidance for stock-based compensation expense, we expect to record the modification date fair value for any equity grants that were not considered probable of vesting as of November 7, 2017 that ultimately vest. During the years ended December 31, 2019, 2018, and 2017, we recorded $2.2 million, $20.8 million and $1.1 million, respectively, related to this modification. Inducement Equity Grants In May 2017, our compensation committee approved the grant of 125,000 non-qualified stock options and 25,000 performance-based stock options to a newly hired executive. In February 2017, our compensation committee approved the grant of 50,000 non-qualified stock options to a newly hired vice president. The non-qualified stock options vest as to 25% of the shares on the first anniversary of the respective grant date and as to 6.25% of the shares at the end of each successive three-month period thereafter until fully vested. The performance-based stock options vested upon the later of the one-year anniversary of the respective grant date and the launch of our first internally developed product, the achievement of which occurred in the third quarter of 2018. These awards were made as inducement grants outside of our stockholder approved stock plans in accordance with Nasdaq Listing Rule 5635(c)(4). Stock-Based Compensation The following table summarizes stock-based compensation expenses included in operating costs and expenses, in thousands: Year Ended December 31, 2019 2018 2017 Research and development $ 88,930 $ 80,509 $ 51,872 Selling, general and administrative 85,911 77,243 40,947 Total $ 174,841 $ 157,752 $ 92,819 The following table summarizes stock-based compensation expense, in thousands: Year Ended December 31, 2019 2018 2017 Stock-based compensation expense by type of award: Time-based stock options $ 99,097 $ 83,403 $ 61,802 Performance-based stock options 48,207 56,419 23,260 Time-based restricted stock units 2,351 538 541 Performance-based restricted stock units 22,123 13,144 — ESPP share issuances 3,482 2,842 2,155 Other equity programs 1,709 1,345 1,946 Non-employee stock options 1,275 1,485 3,115 Less: Stock-based compensation expense capitalized to inventory (3,403) (1,424) — Total $ 174,841 $ 157,752 $ 92,819 The following table summarizes our unrecognized stock-based compensation expense, net of estimated forfeitures, as of December 31, 2019 by type of awards, and the weighted-average period over which that expense is expected to be recognized: At December 31, 2019 Unrecognized Weighted- (in thousands) (in years) Type of award: Time-based stock options $ 184,062 2.62 Performance-based stock options 5,997 * Time-based restricted stock units 7,014 1.08 Performance-based restricted stock units 12,217 * ESPP share issuances 903 0.33 __________________________________________ * Performance-based stock options and performance-based restricted stock units are recorded as expense beginning when vesting events are determined to be probable. Valuation Assumptions for Stock Options The fair value of stock options, at date of grant, based on the following assumptions, was estimated using the Black-Scholes option-pricing model. Our expected stock-price volatility assumption is based on the historical volatility of our publicly traded stock. The expected life assumption is based on our historical data. The dividend yield assumption is based on the fact that we have never paid cash dividends and have no present intention to pay cash dividends. The risk-free interest rate used for each grant is equal to the zero coupon rate for instruments with a similar expected life. The following table summarizes the Black-Scholes valuation assumption inputs for stock options granted during the years indicated: 2019 2018 2017 Risk-free interest rate 1.4 - 2.6% 2.7 - 2.9% 1.9 -2.3% Expected dividend yield — — — Expected option life 5.6 - 7.3 years 5.7 - 7.2 years 5.7 - 7.2 years Expected volatility 63 - 66% 64 - 67% 61 - 67% Stock Option Activity The following table summarizes the activity of our stock option plans and the inducement grants described above, excluding performance-based stock options: Number of Weighted- Weighted- Aggregate Outstanding, December 31, 2018 9,829 $ 72.48 Granted 3,073 83.09 Exercised (1,105) 43.84 Cancelled (919) 89.94 Outstanding, December 31, 2019 10,878 $ 76.92 6.67 $ 428,124 Exercisable as of December 31, 2019 6,196 $ 69.44 5.21 $ 290,437 Vested or expected to vest as of December 31, 2019 10,434 $ 76.47 6.57 $ 415,443 The weighted-average fair value of stock options granted was $49.27, $66.49 and $44.76 per share for the years ended December 31, 2019, 2018 and 2017, respectively. The intrinsic value of stock options exercised was $55.4 million, $87.1 million and $111.3 million for the years ended December 31, 2019, 2018 and 2017, respectively. We satisfy stock option exercises with newly issued shares of our common stock. Performance-Based Stock Options With respect to the performance-based portion of the annual stock option awards, a portion of the shares subject to the performance-based stock option will vest upon the later of the one-year anniversary of the date of grant and the achievement of specific clinical development, regulatory and/or commercial events, as approved by our compensation committee. During each of the years ended December 31, 2018 and 2017, we also granted an option to purchase 25,000 shares of common stock to a newly hired executive that were subject to vest upon the later of the one The following table summarizes the activity of our performance-based stock options granted under our equity plans and the performance-based portion of the inducement grants described above: Number of Weighted- Weighted- Aggregate Outstanding, December 31, 2018 2,744 $ 84.75 Granted — — Exercised (271) 56.18 Cancelled (282) 96.57 Outstanding, December 31, 2019 2,191 $ 86.77 6.35 $ 65,009 Exercisable as of December 31, 2019 1,956 $ 82.89 6.17 $ 65,009 During the years ended December 31, 2019, 2018 and 2017, there were 889,896, 763,982 and 614,796 performance-based stock options that vested, respectively. The intrinsic value of performance-based stock options exercised was $11.0 million, $8.0 million and $1.8 million for the years ended December 31, 2019, 2018 and 2017, respectively. We satisfy performance-based stock option exercises with newly issued shares of our common stock. Restricted Stock Units and Awards The following table summarizes the activity of our restricted stock units and awards, excluding performance-based stock units: Number of Weighted- Outstanding, December 31, 2018 15 $ 89.44 Granted 127 78.01 Vested (14) 95.66 Cancelled (1) 89.82 Outstanding, December 31, 2019 127 $ 77.23 Performance-Based Restricted Stock Units The following table summarizes the activity of our performance-based restricted stock units granted under our equity plans: Number of Weighted- Outstanding, December 31, 2018 22 $ 91.72 Granted 685 85.01 Vested (13) 79.14 Cancelled (71) 85.00 Outstanding, December 31, 2019 623 $ 85.36 The performance-based restricted stock units granted in 2019 will vest upon the later of the one-year anniversary of the date of grant and the achievement of specific clinical development, regulatory and/or commercial events, as approved by our compensation committee. During the year ended December 31, 2019, we recognized $20.7 million in expense related to these performance awards. In August 2018, we recorded stock-based compensation expense of $11.8 million for 124,833 performance-based restricted stock units that fully vested upon the launch of ONPATTRO. The total fair value of these performance-based restricted stock units that vested (measured on the date of vesting) was $11.4 million. Employee Stock Purchase Plan In 2004, we adopted the 2004 Employee Stock Purchase Plan and in May 2017, our stockholders approved the Amended and Restated ESPP, providing the authorization of 1,215,789 shares for issuance. Under the Amended and Restated ESPP, each offering period is six months, at the end of which employees may purchase shares of common stock through payroll deductions made over the term of the offering. The per-share purchase price at the end of each offering period is equal to the lesser of 85% of the closing price of our common stock at the beginning or end of the offering period. We issued 109,590, 78,085 and 103,666 shares during the years ended December 31, 2019, 2018 and 2017, respectively, and as of December 31, 2019, we had 339,552 shares available for issuance under the Amended and Restated ESPP. We estimate the fair value of shares to be issued under the Amended and Restated ESPP using the Black-Scholes option-pricing model on the date of grant, or first day of the offering period. The following table summarizes information pertaining to stock purchase rights granted under the employee stock purchase plan, during the years indicated: 2019 2018 2017 Weighted-average fair value, per share $ 25.51 $ 35.67 $ 17.10 Weighted-average stock price volatility 52 % 63 % 91 % Expected option life 6 months 6 months 6 months Risk-free interest rate 2.5 % 2.0 % 0.7 %

INCOME TAXES

INCOME TAXES	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES The domestic and foreign components of loss before income taxes are as follows, in thousands: 2019 2018 2017 Domestic $ (597,602) $ (573,245) $ (378,293) Foreign (288,514) (187,429) (112,581) Loss before income taxes $ (886,116) $ (760,674) $ (490,874) The provision for income taxes for each of the years ended December 31, 2019, 2018 and 2017 consisted of the following: 2019 2018 2017 Current provision: Domestic $ (394) $ — $ — Foreign 3,232 1,611 — Total current provision 2,838 1,611 — Deferred benefit: Federal 394 (788) — Foreign (2,369) — — Total deferred benefit (1,975) (788) — Total provision for income taxes $ 863 $ 823 $ — Deferred income taxes reflect the tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting and income tax purposes. We establish a valuation allowance when uncertainty exists as to whether all or a portion of the net deferred tax assets will be realized. Components of the net deferred tax (liability) asset as of December 31, 2019 and 2018 are as follows, in thousands: 2019 2018 Deferred tax assets: Net operating loss carryforwards $ 627,466 $ 506,574 Research and development and ODC credits 261,616 220,776 AMT credits 394 788 Lease liability 69,334 — Deferred compensation 75,058 57,268 Intangible assets 66,615 62,260 Other 13,266 20,660 Total deferred tax assets 1,113,749 868,326 Deferred tax liabilities: Property, plant and equipment, net (10,077) (6,550) Unrealized gain on marketable securities (3,932) (903) Right of use assets (50,294) — Deferred tax asset valuation allowance (1,046,013) (860,085) Net deferred tax asset $ 3,433 $ 788 Our effective income tax rate differs from the statutory federal income tax rate as follows for the years ended December 31, 2019, 2018 and 2017: 2019 2018 2017 At U.S. federal statutory rate 21.0 % 21.0 % 35.0 % State taxes, net of federal effect 3.6 2.1 3.8 Stock-based compensation — 0.8 3.3 Tax credits 3.7 4.2 9.9 Orphan drug credit — — (3.4) Other permanent items (0.3) (0.3) (1.0) Foreign rate differential (6.9) (5.3) (8.1) Tax reform change — — (46.5) Revaluation of deferred credits due to rate change — (3.5) — Other (0.1) — (0.9) Valuation allowance (21.0) (19.0) 7.9 Effective income tax rate — % — % — % We have evaluated the positive and negative evidence bearing upon the realizability of our deferred tax assets. We have concluded, in accordance with the applicable accounting standards, that it is more likely than not that we may not realize the benefit of all of our deferred tax assets, with the exception of the deferred assets related to certain foreign subsidiaries, and alternative minimum tax credits as these have become refundable through 2021. Accordingly, we have recorded a valuation allowance against the deferred tax assets that management believes will not be realized. We re-evaluate the positive and negative evidence on a quarterly basis. The valuation allowance increased by $185.9 million, $171.8 million and $112.9 million for the years ended December 31, 2019, 2018 and 2017, respectively, primarily due to additional operating losses. During the year ended December 31, 2019, we recorded a net provision for income taxes of $0.9 million. This is comprised of $3.2 million in current foreign tax expense offset by $2.4 million in deferred tax benefit in the foreign jurisdictions. As of December 31, 2019, we had federal and state net operating loss carryforwards of $2.4 billion and $2.1 billion, respectively, to reduce future taxable income. As of December 31, 2019, approximately $0.9 billion of our federal net operating loss carryforward can be carried forward indefinitely while the remaining federal net operating loss of $1.5 billion expires at various dates through 2037. As of December 31, 2019, we had federal and state research and development, including Orphan Drug, and state investment tax credit carryforwards of $240.3 million and $27.0 million, respectively, available to reduce future tax liabilities that expire at various dates through 2039. As of December 31, 2019, we had alternative minimum tax credits of $0.4 million that will either be available to reduce future regular tax liabilities or be fully refundable in 2021. We have a valuation allowance against the net operating loss and deferred tax assets related to tax credits as it is unlikely that we will realize these assets. Ownership changes, as defined in the Internal Revenue Code, including those resulting from the issuance of common stock in connection with our public offerings, may limit the amount of net operating loss and tax credit carryforwards that can be utilized to offset future taxable income or tax liability. The amount of the limitation is determined in accordance with Section 382 of the Internal Revenue Code. We have performed an analysis of ownership changes through December 31, 2019. Based on this analysis, we do not believe that any of our tax attributes will expire unutilized due to Section 382 limitations. We apply the accounting guidance in ASC 740 related to accounting for uncertainty in income taxes. Our reserves related to taxes are based on a determination of whether, and how much of, a tax benefit taken by us in our tax filings or positions is more likely than not to be realized following resolution of any potential contingencies present related to tax benefit. The following table presents our unrecognized tax benefits activity for the years ended December 31, 2019 and 2018, in thousands: Description 2019 2018 Unrecognized tax benefits at the beginning of the year $ — $ — Gross increases - current period tax positions 305 — Unrecognized tax benefits at the end of the year $ 305 $ — The uncertain tax position does not impact our effective income tax rate due to the full valuation allowance. The tax years 2016 through 2019 remain open to examination by major taxing jurisdictions to which we are subject, which are primarily in the U.S., although carryforward attributes generated prior to 2016 may still be adjusted upon examination by the Internal Revenue Service or state tax authorities if they have or will be used in a future period.

DEFINED BENEFIT PLANS

DEFINED BENEFIT PLANS	12 Months Ended
	Dec. 31, 2019
Retirement Benefits [Abstract]
DEFINED BENEFIT PLANS	DEFINED BENEFIT PLANSWe maintain defined benefit plans for employees in certain countries outside the U.S., including retirement benefit plans required by applicable local law. The unfunded benefit obligation corresponds to the projected benefit obligations of which the discounted net present value is calculated based on years of employment, expected salary increases and pension adjustments, offset by the fair value of the assets held by the plan. The unfunded benefit obligation was approximately $4.3 million as of December 31, 2019 and is recorded in other liabilities on the consolidated balance sheet. The unfunded benefit obligation as of December 31, 2018 and the total net periodic benefit cost for the years ended December 31, 2019, 2018 and 2017 were not material.

QUARTERLY FINANCIAL DATA (UNAUD

QUARTERLY FINANCIAL DATA (UNAUDITED)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
QUARTERLY FINANCIAL DATA (UNAUDITED)	QUARTERLY FINANCIAL DATA (UNAUDITED)The following information has been derived from unaudited consolidated financial statements that, in the opinion of management, include all recurring adjustments necessary for a fair statement of such information. Three Months Ended March 31, June 30, September 30, December 31, (In thousands, except per share data) Revenues $ 33,294 $ 44,714 $ 70,061 $ 71,681 Operating costs and expenses 222,082 280,985 286,360 369,754 Net loss $ (181,915) $ (219,481) $ (208,535) $ (276,185) Net loss per common share — basic and diluted $ (1.73) $ (2.02) $ (1.92) $ (2.47) Weighted-average common shares — basic and diluted 105,400 108,576 108,701 111,750 Three Months Ended March 31, June 30, September 30, December 31, (In thousands, except per share data) Revenues $ 21,899 $ 29,907 $ 2,069 $ 21,033 Operating costs and expenses 169,304 222,261 256,627 241,389 Net loss $ (141,214) $ (163,560) $ (245,282) $ (211,441) Net loss per common share — basic and diluted $ (1.41) $ (1.63) $ (2.43) $ (2.09) Weighted-average common shares — basic and diluted 99,979 100,519 100,783 101,066

SUMMARY OF SIGNIFICANT ACCOUN_2

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	Basis of Presentation and Principles of Consolidation The accompanying consolidated financial statements reflect the operations of Alnylam and our wholly-owned subsidiaries. All intercompany accounts and transactions have been eliminated.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America, or GAAP, requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. In our consolidated financial statements, there are significant estimates and assumptions related to our inventory valuation and related reserves, income taxes, revenue recognition, research and development expenses, and stock-based compensation. We base our estimates on historical experience and on various other assumptions that we believe to be reasonable. Actual results could differ from those estimates.
Reclassification	Reclassification Certain prior period amounts in the consolidated financial statements have been reclassified to conform to the current period presentation.
Liquidity	Liquidity Based on our current operating plan, we believe that our cash, cash equivalents and marketable debt and equity securities as of December 31, 2019, together with the cash we expect to generate from product sales and under our current alliances, will be sufficient to enable us to advance our long-term strategic goals for at least the next 12 months from the filing of this annual report on Form 10-K.
Concentrations of Credit Risk and Significant Customers	Concentrations of Credit Risk and Significant Customers Financial instruments that potentially expose us to concentrations of credit risk consist primarily of cash, cash equivalents and marketable debt securities. As of December 31, 2019 and 2018, substantially all of our cash, cash equivalents and marketable debt securities were invested in money market funds, certificates of deposit, commercial paper, corporate notes, U.S. government-sponsored enterprise securities and U.S. treasury securities through highly rated financial institutions. Corporate notes may also include foreign bonds denominated in U.S. dollars. Investments are restricted, in accordance with our investment policy, to a concentration limit per issuer.
Fair Value Measurements	Fair Value Measurements The fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. In general, fair values determined by Level 1 inputs utilize quoted prices (unadjusted) in active markets for identical assets or liabilities. Fair values determined by Level 2 inputs utilize data points that are observable, such as quoted prices (adjusted), interest rates and yield curves. Fair values determined by Level 3 inputs utilize unobservable data points for the asset or liability, and include situations where there is little, if any, market activity for the asset or liability. The fair value hierarchy level is determined by the lowest level of significant input.
Investments in Marketable Securities and Cash Equivalents	Investments in Marketable Securities and Cash Equivalents We invest our excess cash balances in marketable debt securities and classify our investments as either held-to-maturity or available-for-sale based on facts and circumstances present at the time we purchased the securities. At each balance sheet date presented, we classified all of our investments in debt securities as available-for-sale and as current assets as they represent the investment of funds available for current operations. We report available-for-sale debt securities 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 (loss) income, a component of stockholders’ equity. Realized gains and losses are determined using the specific identification method and are included in other income (expense). If any adjustment to fair value reflects a decline in the value of the marketable debt securities, we consider all available evidence to evaluate the extent to which the decline is “other than temporary,” including our intention to sell and, if so, mark the investment to market through a charge to our consolidated statements of operations and comprehensive loss. We did not record any impairment charges related to our marketable debt securities during the years ended December 31, 2019, 2018 or 2017. Our marketable debt securities are classified as cash equivalents if the original maturity, from the date of purchase, is 90 days or less, and as marketable debt securities if the original maturity, from the date of purchase, is in excess of 90 days. Our cash equivalents are generally composed of commercial paper, corporate notes, U.S. government-sponsored enterprise securities, U.S. treasury securities and money market funds. We measure marketable equity investments (except those accounted for under the equity method of accounting or those that result in consolidation of an investee), which have readily available prices, at fair value with changes in fair value recognized in other income (expense) on our consolidated statements of operations and comprehensive loss. We obtain fair value measurement data for our marketable debt securities from independent pricing services. We perform validation procedures to ensure the reasonableness of this data. This includes meeting with the independent pricing services to understand the methods and data sources used. For our marketable debt securities, we perform our own review of prices received from the independent pricing services by comparing these prices to other sources and for our marketable equity securities, we confirm those securities are trading in active markets. Prior to January 1, 2018, we recognized unrealized gains and losses on our marketable equity securities through accumulated other comprehensive income (loss) on our consolidated balance sheets.
Accounts Receivable	Accounts Receivable We record accounts receivable net of customer allowances for distribution services, prompt payment discounts and chargebacks based on contractual terms. As of December 31, 2019 and 2018, we determined an allowance for doubtful accounts was not required based upon our review of contractual payment terms and individual customer circumstances. We have standard payment terms that generally require payment within approximately 30 to 90 days. Accounts receivable, net on our consolidated balance sheets also includes billed and unbilled collaboration receivables.
Inventory	Inventory Inventory is measured at the lower of cost or estimated net realizable value. We use a standard cost basis, which approximates cost determined on a first-in, first-out basis. Inventory costs include all raw materials, direct conversion costs and overhead. Raw and intermediate materials that may be used for either research and development or commercial purposes are classified as inventory until the material is consumed or otherwise allocated for research and development. If the material is used for research and development, it is expensed as research and development once that determination is made. We capitalize inventory costs that are expected to be sold commercially once we determine it is probable that the inventory costs will be recovered through commercial sale based on the review of several factors, including (i) the likelihood that all required regulatory approvals will be received, considering any special filing status, (ii) the expected timing of validation (if not yet completed) of manufacturing processes in the associated facility, (iii) the expected expiration of the inventory, (iv) logistical or commercial constraints that may impede the timely distribution and sale of the product, including transport requirements and reimbursement status, (v) current market factors, including competitive landscape and pricing, (vi) threatened or anticipated litigation challenges, (vii) history of approvals of similar products or formulations and (viii) FDA (or other appropriate regulatory agencies) correspondence regarding the safety and efficacy of the product. Prior to the capitalization of inventory costs, we record such costs as research and development expenses on our consolidated statements of operations and comprehensive loss. We reduce our inventory to net realizable value for potentially excess, dated or obsolete inventory based on our quarterly assessment of the recoverability of our capitalized inventory. We periodically review inventory levels to identify what may expire prior to expected sale or has a cost basis in excess of its estimated realizable value and write-down such inventories as appropriate.
Property, Plant and Equipment	Property, Plant and Equipment Property, plant and equipment are stated at cost, net of accumulated depreciation. Depreciation expense is recorded on a straight-line basis over the estimated useful life of the asset. Leasehold improvements are amortized over the shorter of the asset’s estimated useful life or the lease term. Construction in progress reflects amounts incurred for construction or improvements of property, plant or equipment that have not been placed in service. Costs of construction of certain long-lived assets include capitalized interest, which is amortized over the estimated useful life of the related asset. The cost and accumulated depreciation of assets retired or sold are removed from the respective asset category, and any gain or loss is recognized in our consolidated statements of operations and comprehensive loss. During the years ended December 31, 2019, 2018 and 2017, we recorded $16.6 million, $12.8 million and $11.9 million, respectively, of depreciation expense related to our property, plant and equipment. The estimated useful lives of property, plant and equipment are as follows: Asset Category Useful Life Laboratory equipment 5 Computer equipment and software 3-10 years Furniture and fixtures 5 Leasehold improvements Shorter of asset life or lease term Manufacturing Equipment 7-15 years Buildings 40 years Land — Construction in progress —
Research and Development Accruals	Research and Development Accruals We record accrued liabilities related to products we have received or services that we have incurred, specifically related to ongoing pre-clinical studies and clinical trials, for which service providers have not yet billed us, or when billing terms under these contracts do not coincide with the timing of when the work is performed, as of our period-end. These costs primarily relate to third-party clinical management costs, laboratory and analysis costs, toxicology studies and investigator fees. The
Revenue Recognition	Revenue Recognition We recognize revenue when control of promised goods or services is transferred to a customer at an amount that reflects the consideration to which we expect to be entitled in exchange for those goods or services. To determine revenue recognition, we perform the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price, including variable consideration, if any; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) we satisfy a performance obligation. We only apply the five-step model to contracts when collectability of the consideration to which we are entitled in exchange for the goods or services we transfer to the customer is determined to be probable. At contract inception, once the contract is determined to be within the scope of Accounting Standards Codification Topic 606, Revenue from Contracts with Customers, or ASC 606, we assess whether the goods or services promised within each contract are distinct and, therefore, represent a separate performance obligation. Goods and services that are determined not to be distinct are combined with other promised goods and services until a distinct bundle is identified. We then allocate the transaction price (the amount of consideration we expect to be entitled to from a customer in exchange for the promised goods or services) to each performance obligation and recognize the associated revenue when (or as) each performance obligation is satisfied. Our estimate of the transaction price for each contract includes all variable consideration to which we expect to be entitled. Amounts are recorded as accounts receivable when our right to consideration is unconditional. We do not assess whether a contract has a significant financing component if the expectation at contract inception is that the period between payment by the customer and the transfer of the promised goods or services to the customer will be one year or less. We expense incremental costs of obtaining a contract as and when 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 and 2018, we had not capitalized any costs to obtain any of our contracts. Net Product Revenues Our net product sales consist of sales of ONPATTRO and GIVLAARI and are recognized, net of variable consideration related to certain allowances and accruals, at the time the customer obtains control of our product. We use the expected value method, which is the sum of probability-weighted amounts in a range of possible consideration amounts, or the most likely amount method, which is the single most likely amount in a range of possible considerations, to estimate variable consideration related to our product sales. We use the expected value method to estimate variable consideration for certain rebates, chargebacks, product returns, and other incentives and we use the most likely amount method for certain rebates and trade discounts and allowances. The following are the components of variable consideration related to product revenues. We record reserves, based on contractual terms, for these components related to product sold during the reporting period, as well as our estimate of product that remains in the distribution channel inventory at the end of the reporting period that we expect will be sold to qualified healthcare providers. On a quarterly basis, we update our estimates and record any needed adjustments in the period we identify the adjustments. Chargebacks : We estimate obligations resulting from contractual commitments with the government and other entities to sell products to qualified healthcare providers at prices lower than the list prices charged to the customer who directly purchases from us. The customer charges us for the difference between what it pays to us for the product and the selling price to the qualified healthcare providers. Rebates : We are subject to discount obligations under government programs, including Medicaid in the U.S. and similar programs in certain other countries, including countries in which we are accruing for estimated rebates because final pricing has not yet been negotiated. We are also subject to potential rebates in connection with our value-based agreements with certain commercial payors. We record reserves for rebates in the same period the related product revenue is recognized, resulting in a reduction of product revenues and a current liability that is included in accrued expenses on our consolidated balance sheet. Our estimate for rebates is based on statutory discount rates, expected utilization or an estimated number of patients on treatment, as applicable. Trade discounts and allowances : We provide customary invoice discounts on product sales to our customers for prompt payment and we pay fees for distribution services, such as fees for certain data that customers provide to us. We estimate our customers will earn these discounts and fees, and deduct these discounts and fees in full from gross product revenues and accounts receivable at the time we recognize the related revenues. Product returns: We offer customers product return rights if products are damaged, defective or expired, with “expired” defined as within three months pre- or post-expiry. We estimate the amount of product that will be returned using a probability-weighted estimate based on our sales history. Other incentives: Other incentives include co-payment assistance we provide to patients with commercial insurance that have coverage and reside in states that allow co-payment assistance. We estimate the average co-payment assistance amounts for our products based on expected customer demographics and record any such amounts within accrued expenses on our consolidated balance sheet. Revenues from Collaborators We earn revenue in connection with collaboration agreements which allow our collaboration partners to utilize our technology platforms and develop product candidates. Our collaboration agreements are detailed in Note 4, Collaboration Agreements. For each collaboration partner, we discuss our revenue recognition, including our significant performance obligations under each agreement. At contract inception, we assess whether the collaboration arrangements are within the scope of ASC Topic 808, Collaborative Arrangements, or 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 dependent on the commercial success of such activities. This assessment is performed based on the responsibilities of all parties in the arrangement. For collaboration arrangements within the scope of ASC 808 that contain multiple elements, we first determine which elements of the arrangement are within the scope of ASC 808 and which elements are within the scope of ASC 606. For elements of collaboration arrangements that are accounted for pursuant to ASC 808, an appropriate recognition method is determined and applied consistently, either by analogy to authoritative accounting literature or by applying a reasonable and rational policy election. For elements of collaboration arrangements that are accounted for pursuant to ASC 606, we identify the performance obligations and allocate the total consideration we expect to receive on a relative standalone selling price basis to each performance obligation. Variable consideration such as performance-based milestones will be included in the total consideration if we expect to receive such consideration and if it is probable that the inclusion of the variable consideration will not result in a significant reversal in the cumulative amount of revenue recognized under the arrangement. Our estimate of the total consideration we expect to receive under each collaboration arrangement is updated for each reporting period, and any adjustments to revenue are recorded on a cumulative catch-up basis. We exclude sales-based royalty and milestone payments from the total consideration we expect to receive until the underlying sales occur because the license to our intellectual property is deemed to be the predominant item to which the royalties or milestones relate as it is the primary driver of value in our collaboration arrangements. Key assumptions to determine the standalone selling price may include forecasted revenues, development timelines, reimbursement rates for personnel costs, discount rates and probabilities of technical and regulatory success. We recognize revenue associated with each performance obligation as the control over the promised goods or services transfer to our collaboration partner which occurs either at a point in time or over time. If control transfers over time, revenue is recognized by using a method of measuring progress that best depicts the transfer of goods or services. We evaluate the measure of progress and related inputs each reporting period and any resulting adjustments to revenue are recorded on a cumulative catch-up basis. Consideration received that does not meet the requirements to satisfy ASC 808 or ASC 606 revenue recognition criteria is recorded as deferred revenue in the accompanying consolidated balance sheets, classified as either short-term or long-term deferred revenue based on our best estimate of when, less than 12 months (short-term) or more than 12 months (long-term), such revenue will be recognized.
Cost of Goods Sold	Cost of Goods Sold Cost of goods sold includes the cost of producing and distributing inventories that are related to product revenues during the respective period (including salary-related and stock-based compensation expenses for employees involved with production and distribution, freight and indirect overhead costs), third-party royalties payable on our net product revenues and amortization of intangible assets associated with the sale of our products. Cost of goods sold may also include costs related to excess or obsolete inventory adjustment charges, abnormal costs, unabsorbed manufacturing and overhead costs, and manufacturing variances.
Income Taxes	Income Taxes Uncertain tax positions, for which management's assessment is that there is a more than 50% probability of sustaining the position upon challenge by a taxing authority based upon its technical merits, are subject to certain recognition and measurement criteria. The nature of the uncertain tax positions is often very complex and subject to change, and the amounts at issue can be substantial. We develop our cumulative probability assessment of the measurement of uncertain tax positions using internal experience, judgment and assistance from professional advisors. We re-evaluate these uncertain tax positions on a quarterly basis based on a number of factors including, but not limited to, changes in facts or circumstances, changes in tax law, and effectively settled issues under audit and new audit activity. Any change in these factors could result in the recognition of a tax benefit or an additional charge to the tax provision. We account for income taxes under the asset and liability method. Under this method, deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to differences between financial statement carrying amounts of existing assets and liabilities and their respective tax basis. Deferred tax assets and liabilities are measured using enacted rates in effect for the year in which these temporary differences are expected to be recovered or settled. Valuation allowances are provided if, based on the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. As of December 31, 2019, we have recorded no interest and penalty expense related to uncertain tax positions.
Research and Development Expenses	Research and Development Expenses We record research and development expenses as incurred. Included in research and development expenses are wages, stock-based compensation expenses, benefits and other operating costs, facilities, supplies, external services, clinical trial and manufacturing costs, costs related to our collaboration arrangements, and overhead directly related to our research and development operations, as well as costs to acquire technology licenses.
Stock-Based Compensation	Stock-Based Compensation We recognize stock-based compensation expense for grants under our stock incentive plans and employee stock purchase plan, as well as inducement stock grants outside of our stock incentive plans. We account for all stock-based awards granted to employees at their fair value and generally recognize compensation expense over the vesting period of the award. Determining the amount of stock-based compensation to be recorded requires us to develop estimates of fair values of stock options as of the grant date. We calculate the grant date fair values of stock options using the Black-Scholes valuation model, which requires the input of subjective assumptions, including but not limited to expected stock price volatility over the term of the awards and the expected term of stock options. The fair value of restricted stock awards granted to employees is based upon the quoted closing market price per share on the date of grant. We have performance conditions included in certain of our stock option and restricted stock awards that are based upon the achievement of pre-specified clinical development, regulatory and/or commercial events. As the outcome of each event has inherent risk and uncertainties, and a positive outcome may not be known until the event is achieved, we begin to recognize the value of the performance-based stock option and restricted stock awards when we determine the achievement of each performance condition is deemed probable, a determination which requires significant judgment by management. At the probable date, we record a cumulative expense catch-up, with remaining expense amortized over the remaining service period.
Comprehensive Loss	Comprehensive Loss Comprehensive loss is comprised of net loss and certain changes in stockholders’ equity that are excluded from net loss. We include foreign currency translation adjustments in other comprehensive loss if the functional currency is not the U.S. dollar. We include unrealized gains and losses on certain marketable securities in other comprehensive loss, including changes in the value of our marketable debt securities. We include certain changes in the fair value of the plan assets and projected benefit obligation attributed to our defined benefit pension plan in other comprehensive loss.
Net Loss Per Common Share	Net Loss Per Common Share We compute basic net loss per common share by dividing net loss by the weighted-average number of common shares outstanding. We compute diluted net loss per common share by dividing net loss by the weighted-average number of common shares and dilutive potential common share equivalents then outstanding. Potential common shares consist of shares issuable upon the exercise of stock options (the proceeds of which are then assumed to have been used to repurchase outstanding shares using the treasury stock method). Because the inclusion of potential common shares would be anti-dilutive for all periods presented, diluted net loss per common share is the same as basic net loss per common share.
Segment Information	Segment Information We operate in a single reporting segment, the discovery, development and commercialization of RNAi therapeutics. Consistent with our management reporting, results of our operations are reported on a consolidated basis for purposes of segment reporting. As of December 31, 2019 and 2018, substantially all of our consolidated property, plant and equipment, net was from U.S. operations. For the years ended December 31, 2019, 2018 and 2017, net revenues from collaborators were attributed to the U.S. Please read Note 3 for information regarding our net product sales by geography.
Recent Accounting Pronouncements	Recently Adopted Accounting Pronouncements In February 2016, the Financial Accounting Standards Board, or FASB, issued a new leasing standard, ASC 842, which generally requires lessees to recognize operating and financing lease liabilities and corresponding right-of-use assets on the consolidated balance sheet and to provide enhanced disclosures surrounding the amount, timing and uncertainty of cash flows arising from leasing arrangements. We adopted the new standard on January 1, 2019, using a modified retrospective basis and did not restate comparative periods. In addition, we did not elect the package of practical expedients permitted under the transition guidance that permits companies to carry forward prior conclusions related to (1) whether any expired or existing contracts are, or contain, leases, (2) the lease classification for expired or existing leases, and (3) initial direct costs for existing leases. All our leases have been classified as operating leases under the new leasing standard. We elected to combine lease and non-lease components for our facility leases and to keep leases with an initial term of 12 months or less off the consolidated balance sheets and recognize the associated lease payments in the consolidated statements of operations and comprehensive loss on a straight-line basis over the lease term. Please read Note 8 for additional disclosures related to accounting for leases under this new standard. The adoption of ASC 842 had a material impact on our consolidated balance sheet as the standard requires us to measure and recognize a right of use asset and lease liability. As most leases do not provide an implicit rate, our incremental borrowing rate was determined based on the information available at the date of adoption to measure our lease liability. Costs determined to be variable and not based on an index or rate were not included in the measurement of the lease liability. We recognized approximately $290.0 million of operating lease liabilities and approximately $230.0 million of operating lease right-of-use assets on our consolidated balance sheet as of January 1, 2019. The adoption of the standard did not have a material impact on our consolidated statement of operations and comprehensive loss and there was no impact to cash from or used in operating, financing or investing activities on our consolidated statement of cash flows. In May 2014, the FASB issued new accounting guidance related to revenue recognition (ASC 606), which outlines a comprehensive revenue recognition model and supersedes most current revenue recognition accounting guidance and requires increased disclosures. The new accounting guidance defines a five-step approach that requires a company to recognize revenue as control of goods or services transfers to a customer at an amount that reflects the expected consideration to be received in exchange for those goods or services. We adopted ASC 606 in the first quarter of 2018 using the modified retrospective method to all contracts that were not completed as of January 1, 2018. We recognized the cumulative effect of initially applying the new revenue accounting guidance as an adjustment to opening retained earnings. Prior period results have not been restated and continue to be reported in accordance with the accounting guidance in effect for those periods. There was no impact to cash from or used in operating, financing or investing activities on our consolidated statement of cash flows as a result of the adoption of the new revenue standard. The following table summarizes the cumulative effect to our consolidated balance sheet upon the adoption of the new revenue standard on January 1, 2018, in thousands: Consolidated Balance Sheet Balance as of December 31, Adjustments Balance as of January 1, 2018 Deferred revenue, current portion $ 41,705 $ (34,463) $ 7,242 Deferred revenue, net of current portion $ 43,075 $ (33,747) $ 9,328 Accumulated deficit $ (2,147,685) $ 68,210 $ (2,079,475) The adoption of the new revenue standard resulted in a cumulative reduction of $68.2 million of deferred revenue with a corresponding adjustment to the opening balance of accumulated deficit recorded in the first quarter of 2018. As a result of the cumulative reduction in deferred revenue, our corresponding deferred tax asset was reduced by $13.6 million, which was offset by a corresponding decrease to our valuation allowance. These offsetting adjustments were recorded to our accumulated deficit in the first quarter of 2018. In accordance with the new revenue standard requirements, the following tables summarize the impact of adoption on our consolidated balance sheet and consolidated statement of operations and comprehensive loss, in thousands: At December 31, 2018 Consolidated Balance Sheet As Reported Balances Effect of Deferred revenue, current portion $ 3,496 $ 3,496 $ — Deferred revenue, net of current portion $ 458 $ 18,451 $ (17,993) Accumulated deficit $ (2,840,972) $ (2,859,049) $ (18,077) Year Ended December 31, 2018 Consolidated Statement of Operations and Comprehensive Loss As Reported Balances Effect of Net revenues from collaborators $ 62,373 $ 112,506 $ (50,133) Net loss $ (761,497) $ (711,364) $ 50,133 Net loss per common share - basic and diluted $ (7.57) $ (7.07) $ 0.50 The impact of our adoption of the new revenue standard did not have a material impact on the amount of net product revenues recognized during the year ended December 31, 2018 . In March 2017, the FASB issued a new standard that amends the amortization period for certain purchased callable debt securities held at a premium by shortening the amortization period for the premium to the earliest call date. The new standard became effective for us on January 1, 2019. This standard did not have a significant impact on our consolidated financial statements and related disclosures. Recently Issued Accounting Pronouncements, Not Yet Adopted In June 2016, the FASB issued new accounting guidance which requires entities to record expected credit losses for certain financial instruments, including trade receivables, as an allowance that reflects the entity's current estimate of credit losses expected to be incurred. For available-for-sale debt securities in unrealized loss positions, the new standard requires allowances to be recorded instead of reducing the amortized cost of the investment. The new standard became effective for us on January 1, 2020. We currently do not expect this guidance to have a significant impact on our consolidated financial statements and related disclosures. In August 2018, the FASB issued amendments to accounting guidance that eliminate, add and modify certain disclosure requirements on fair value measurements. The new standard became effective for us on January 1, 2020. We currently do not expect this guidance to have a significant impact on our consolidated financial statements and related disclosures. In August 2018, the FASB issued new accounting guidance to clarify the accounting for implementation costs in cloud computing arrangements (hosting arrangements). The new standard requires a customer in a cloud computing arrangement to determine which implementation costs to capitalize as assets or expense as incurred. Capitalized implementation costs related to a hosting arrangement that is a service contract will be amortized over the term of the hosting arrangement, beginning when the module or component of the hosting arrangement is ready for its intended use. The new standard became effective for us on January 1, 2020. We currently do not expect this guidance to have a significant impact on our consolidated financial statements and related disclosures. In August 2018, the FASB issued amendments to accounting guidance to modify the disclosure requirements for employers that sponsor a defined benefit pension plan. The amendments become effective for our fiscal year, on a retrospective basis, beginning January 1, 2021, however early adoption is permitted. We are currently evaluating the timing of our adoption and the expected impact this guidance could have on our consolidated financial statements and related disclosures. In November 2018, the FASB issued new accounting guidance to clarify the interaction between the accounting guidance for collaborative arrangements and revenue from contracts with customers. The new standard became effective for us on January 1, 2020 using a retrospective transition method. We currently do not expect this guidance to have a significant impact on our consolidated financial statements and related disclosures. In December 2019, the FASB issued amendments to accounting guidance that simplifies the accounting for income taxes, as part of its initiative to reduce complexity in the accounting standards. The amendments eliminate certain exceptions related to the approach for intraperiod tax allocation, the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. The amendments also clarify and simplify other aspects of the accounting for income taxes. The amendments become effective for our fiscal year, including interim periods, beginning January 1, 2021, however early adoption is permitted. We are currently evaluating the timing of our adoption and the expected impact this guidance could have on our consolidated financial statements and related disclosures.
Subsequent Events	Subsequent EventsWe did not have any material recognized or unrecognized subsequent events.

SUMMARY OF SIGNIFICANT ACCOUN_3

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Concentrations of Credit Risk and Significant Customers	The following table summarizes customers that represent 10% or greater of our consolidated total gross revenues for the periods indicated: Year Ended December 31, 2019 2019 2018 2017 Distributor A 44 % 13 % N/A Sanofi Genzyme * 58 % 61 % Vir Biotechnology * 16 % * The Medicines Company * * 34 % __________________________________________ * Represents less than 10% The following table summarizes customers with amounts due that represent 10% or greater of our consolidated gross accounts receivable balance, at the periods indicated: As of December 31, 2019 2019 2018 Distributor A 28 % 44 % Sanofi Genzyme 14 % 29 % Vir Biotechnology 13 % * Distributor B 10 % * __________________________________________ * Represents less than 10%
Estimated Useful Lives of Property, Plant and Equipment	The estimated useful lives of property, plant and equipment are as follows: Asset Category Useful Life Laboratory equipment 5 Computer equipment and software 3-10 years Furniture and fixtures 5 Leasehold improvements Shorter of asset life or lease term Manufacturing Equipment 7-15 years Buildings 40 years Land — Construction in progress —
Common Shares Excluded from the Calculation of Net Loss Per Common Share	The following table sets forth for the periods presented the potential common shares (prior to consideration of the treasury stock method) excluded from the calculation of net loss per common share because their inclusion would be anti-dilutive, in thousands: As of December 31, 2019 2018 2017 Options to purchase common stock 13,069 12,573 11,239 Unvested restricted common stock 749 36 149 Total 13,818 12,609 11,388
Cumulative Effect and Impact on Consolidated Balance Sheet and Consolidated Statement of Comprehensive Loss for Adoption of New Revenue Standard	The following table summarizes the cumulative effect to our consolidated balance sheet upon the adoption of the new revenue standard on January 1, 2018, in thousands: Consolidated Balance Sheet Balance as of December 31, Adjustments Balance as of January 1, 2018 Deferred revenue, current portion $ 41,705 $ (34,463) $ 7,242 Deferred revenue, net of current portion $ 43,075 $ (33,747) $ 9,328 Accumulated deficit $ (2,147,685) $ 68,210 $ (2,079,475) In accordance with the new revenue standard requirements, the following tables summarize the impact of adoption on our consolidated balance sheet and consolidated statement of operations and comprehensive loss, in thousands: At December 31, 2018 Consolidated Balance Sheet As Reported Balances Effect of Deferred revenue, current portion $ 3,496 $ 3,496 $ — Deferred revenue, net of current portion $ 458 $ 18,451 $ (17,993) Accumulated deficit $ (2,840,972) $ (2,859,049) $ (18,077) Year Ended December 31, 2018 Consolidated Statement of Operations and Comprehensive Loss As Reported Balances Effect of Net revenues from collaborators $ 62,373 $ 112,506 $ (50,133) Net loss $ (761,497) $ (711,364) $ 50,133 Net loss per common share - basic and diluted $ (7.57) $ (7.07) $ 0.50

NET PRODUCT REVENUES (Tables)

NET PRODUCT REVENUES (Tables)	12 Months Ended
	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
Net Product Revenues by Geography	Net product revenues by geography consist of the following, in thousands: Years Ended December 31, 2019 2018 2017 United States $ 116,452 $ 8,589 $ — Rest of world 50,085 3,946 — Total net product revenues $ 166,537 $ 12,535 $ —
Summary of Balances and Activity in Each Product Revenue Allowance and Reserve Category	The following table summarizes balances and activity in each product revenue allowance and reserve category during the years ended December 31, 2019 and 2018, in thousands: As of December 31, 2019 Chargebacks and Rebates Trade Discounts and Allowances Returns Reserve and Other Incentives Total Beginning balance $ 3,441 $ 218 $ 321 $ 3,980 Provision related to current period sales 44,371 3,227 5,108 52,706 Credit or payments made during the period for current year sales (15,216) (2,817) (3,231) (21,264) Credit or payments made during the period for prior year sales (109) (218) (220) (547) Total $ 32,487 $ 410 $ 1,978 $ 34,875 As of December 31, 2018 Chargebacks and Rebates Trade Discounts and Allowances Returns Reserve and Other Incentives Total Beginning balance $ — $ — $ — $ — Provision related to current period sales 4,081 292 657 5,030 Credit or payments made during the period for current year sales (640) (74) (336) (1,050) Total $ 3,441 $ 218 $ 321 $ 3,980

COLLABORATION AGREEMENTS (Table

COLLABORATION AGREEMENTS (Tables)	12 Months Ended
	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Revenue from Collaborators	The following table summarizes our total consolidated net revenues from collaborators, in thousands: Year Ended December 31, Description 2019 2018 2017 (1) Regeneron Pharmaceuticals $ 26,075 $ — $ — Vir Biotechnology 12,809 12,778 1,464 Sanofi Genzyme 10,976 46,000 54,625 The Medicines Company 2,315 2,789 30,217 Other 1,038 806 3,606 Total net revenues from collaborators $ 53,213 $ 62,373 $ 89,912 (1) As described in Note 2 above, prior period amounts have not been adjusted under the modified retrospective method.
Balance and Change in Contract Liabilities Related to Collaboration Agreements	The following table presents the balance of our receivables and contract liabilities related to our collaboration agreements, in thousands: As of December 31, 2019 2018 Receivables included in "Accounts receivable, net" $ 14,929 $ 5,625 Contract liabilities included in "Deferred revenue" 153,117 3,954 The following table presents revenue recognized as a result of changes in contract liability related to our collaboration agreements, in thousands: Included in Deferred Revenue Contract liability as of January 1, 2018 $ 16,570 Consideration earned, not yet recognized as revenue 3,954 Revenue recognized on contract liability as of January 1, 2018 (16,570) Contract liability as of December 31, 2018 3,954 Consideration earned, not yet recognized as revenue 153,117 Revenue recognized on contract liability as of December 31, 2018 (3,954) Contract liability as of December 31, 2019 $ 153,117
Schedule of Research and Development Expenses Incurred by Type that are Directly Attributable to Collaboration Agreements	The following table provides the research and development expenses incurred by type, for which we recognize net revenue, that are directly attributable to our collaboration agreements by collaboration partner, in thousands: Year Ended 2019 2018 2017 Clinical Trial and Manufacturing External Services Other Clinical Trial and Manufacturing External Services Other Clinical Trial and Manufacturing External Services Other Regeneron $ 2,793 $ 344 $ 21,779 $ — $ — $ — $ — $ — $ — Sanofi 11,505 334 2,017 36,600 5,340 1,279 174,901 4,475 5,327 Vir 10,353 381 4,745 7,272 8,251 548 1,134 926 — MDCO 2,025 — 696 1,664 2 203 5,421 — 106 Ionis — — — — 2,150 1,097 — 3,250 — Total $ 26,676 $ 1,059 $ 29,237 $ 45,536 $ 15,743 $ 3,127 $ 181,456 $ 8,651 $ 5,433
Schedule of Allocated Transaction Price	We allocated the initial transaction price to each unit of account based on the applicable accounting guidance as follows, in thousands: Performance Obligations Standalone Selling Price Transaction Price Allocated Accounting Guidance Research Services Obligation $ 130,700 $ 183,100 ASC 606 C5 License Obligation 97,600 92,500 ASC 606 C5 Co-Co Obligation 364,600 246,000 ASC 808 $ 521,600
Schedule of Revenue Recognized Based on Accounting Guidance	The following table provides a summary of the transaction price allocated to each unit of account based on the applicable accounting guidance, in addition to revenue activity during the period, in thousands: Revenue Recognized During Deferred Revenue Performance Obligations Transaction Price Allocated Year Ended December 31, 2019 As of December 31, 2019 Accounting Guidance Research Services Obligation $ 200,600 $ 21,000 $ 84,800 ASC 606 C5 License Obligation 108,500 — 65,800 ASC 606 C5 Co-Co Obligation 246,000 2,900 243,000 ASC 808 Total $ 555,100 $ 23,900 $ 393,600

OTHER BALANCE SHEET DETAILS (Ta

OTHER BALANCE SHEET DETAILS (Tables)	12 Months Ended
	Dec. 31, 2019
Balance Sheet Related Disclosures [Abstract]
Schedule of Inventory	The following table presents our inventory as of December 31, 2019 and 2018, in thousands: As of December 31, 2019 2018 Raw materials $ 15,418 $ 8,709 Work in process 38,275 15,262 Finished goods 2,655 97 Total inventory $ 56,348 $ 24,068
Property, Plant and Equipment, Net	Property, plant and equipment, net consist of the following as of December 31, 2019 and 2018, in thousands: As of December 31, 2019 2018 Buildings $ 250,380 $ — Leasehold improvements 132,632 65,928 Construction in progress 54,195 260,975 Laboratory equipment 29,755 43,427 Computer equipment and software 14,956 13,495 Furniture and fixtures 10,339 7,238 Land 9,080 9,080 501,337 400,143 Less: accumulated depreciation (76,158) (79,485) Total $ 425,179 $ 320,658
Accrued Expenses	Accrued expenses consist of the following as of December 31, 2019 and 2018, in thousands: As of December 31, 2019 2018 Compensation and related $ 68,304 $ 37,301 Pre-clinical, clinical trial and manufacturing 34,269 32,205 Product revenue allowances 32,670 2,696 Licensing and collaboration agreements 20,622 6,433 Consulting and professional services 14,251 10,450 Other 27,085 23,634 Total $ 197,201 $ 112,719
Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within our consolidated balance sheets that sum to the total of these amounts shown in the consolidated statements of cash flows, in thousands: As of December 31, 2019 2018 2017 Cash and cash equivalents $ 547,178 $ 420,146 $ 645,361 Restricted cash included in prepaid expenses and other current assets 4 225 — Restricted cash included in long-term other assets 2,446 2,260 1,471 Total cash, cash equivalents, and restricted cash shown in the consolidated $ 549,628 $ 422,631 $ 646,832
Summary of Changes in Accumulated Other Comprehensive Income (Loss)	The following table summarizes the changes in accumulated other comprehensive (loss) income, by component, for the years ended December 31, 2019 and 2018, in thousands: Loss on Investment in Joint Venture Defined Benefit Pension Unrealized (Losses) Gains from Debt Securities Foreign Currency Translation Total Accumulated Other Comprehensive (Loss) Income Balance as of December 31, 2017 $ (32,792) $ — $ (1,641) $ — $ (34,433) Other comprehensive loss before reclassifications — — (411) — (411) Amounts reclassified from other comprehensive income — — 1,631 — 1,631 Net other comprehensive income — — 1,220 — 1,220 Balance as of December 31, 2018 (32,792) — (421) — (33,213) Other comprehensive (loss) income before reclassifications — (3,661) 22 (343) (3,982) Amounts reclassified from other comprehensive income — 141 536 — 677 Net other comprehensive (loss) income — (3,520) 558 (343) (3,305) Balance as of December 31, 2019 $ (32,792) $ (3,520) $ 137 $ (343) $ (36,518)

FAIR VALUE MEASUREMENTS (Tables

FAIR VALUE MEASUREMENTS (Tables)	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value of Assets Measured on a Recurring Basis	The following tables present information about our assets that are measured at fair value on a recurring basis as of December 31, 2019 and 2018, and indicate the fair value hierarchy of the valuation techniques we utilized to determine such fair value, in thousands: Description As of December 31, Quoted Prices in Active Markets Significant Observable Inputs Significant Unobservable Inputs Cash equivalents: Commercial paper $ 3,439 $ — $ 3,439 $ — U.S. treasury securities 336,693 — 336,693 — Money market funds 119,882 119,882 — — Marketable debt securities: Certificates of deposit 4,301 — 4,301 — Commercial paper 36,474 — 36,474 — Corporate notes 146,040 — 146,040 — U.S. government-sponsored enterprise securities 32,488 — 32,488 — U.S. treasury securities 755,714 — 755,714 — Marketable equity securities 13,967 13,967 — — Restricted cash (money market funds) 1,482 1,482 — — Total $ 1,450,480 $ 135,331 $ 1,315,149 $ — Description As of December 31, Quoted Prices in Active Markets Significant Observable Inputs Significant Unobservable Inputs Cash equivalents: U.S. treasury securities $ 221,281 $ — $ 221,281 $ — Money market funds 102,445 102,445 — — Marketable debt securities: Certificates of deposit 8,951 — 8,951 — Commercial paper 57,197 — 57,197 — Corporate notes 232,410 — 232,410 — U.S. government-sponsored enterprise securities 39,018 — 39,018 — U.S. treasury securities 325,227 — 325,227 — Marketable equity securities 1,206 1,206 — — Restricted cash (money market funds) 1,477 1,477 — — Total $ 989,212 $ 105,128 $ 884,084 $ —

MARKETABLE DEBT SECURITIES (Tab

MARKETABLE DEBT SECURITIES (Tables)	12 Months Ended
	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Summary of Company's Marketable Debt Securities	The following tables summarize our marketable debt securities as of December 31, 2019 and 2018, in thousands: As of December 31, 2019 Amortized Gross Gross Fair Value Certificates of deposit $ 4,303 $ — $ (2) $ 4,301 Commercial paper 39,913 — — 39,913 Corporate notes 146,016 58 (34) 146,040 U.S. government-sponsored enterprise securities 32,487 3 (2) 32,488 U.S. treasury securities 1,092,293 185 (71) 1,092,407 Total $ 1,315,012 $ 246 $ (109) $ 1,315,149 As of December 31, 2018 Amortized Gross Gross Fair Value Certificates of deposit $ 8,951 $ — $ — $ 8,951 Commercial paper 57,197 — — 57,197 Corporate notes 232,695 — (285) 232,410 U.S. government-sponsored enterprise securities 39,031 — (13) 39,018 U.S. treasury securities 546,631 1 (124) 546,508 Total $ 884,505 $ 1 $ (422) $ 884,084
Summary of Fair Value of Marketable Debt Securities	The fair values of our marketable debt securities by classification in the consolidated balance sheets were as follows, in thousands: December 31, 2019 December 31, 2018 Cash and cash equivalents $ 340,132 $ 221,281 Marketable debt securities 975,017 662,803 Total $ 1,315,149 $ 884,084

LEASES (Tables)

LEASES (Tables)	12 Months Ended
	Dec. 31, 2019
Leases [Abstract]
Summary of Costs Included in Operating Expenses Related to Leases	The below table summarizes our costs included in operating expenses related to right of use lease assets we have entered into through December 31, 2019, in thousands: Description Year Ended Operating lease cost $ 38,613 Variable lease cost 15,209 Total $ 53,822
Summary of Future Lease Payments for Non-cancellable Operating Leases and Reconciliation to Carrying Amount of Operating Lease Liability	Future lease payments for non-cancellable operating leases and a reconciliation to the carrying amount of the operating lease liability presented in the consolidated balance sheet as of December 31, 2019 were as follows, in thousands: Year Ending December 31 2020 $ 29,157 2021 37,597 2022 37,754 2023 36,250 2024 35,381 2025 and thereafter 356,567 Total undiscounted lease liability 532,706 Less imputed interest (225,873) Less impact of future leases not yet commenced (3,010) Total discounted lease liability $ 303,823 Current operating lease liability $ 27,688 Non-current operating lease liability 276,135 Total $ 303,823
Future Minimum Payments Under Non-cancelable Leases	Under the prior lease guidance, minimum payments under our non-cancelable facility leases, as of December 31, 2018, were as follows, in thousands: Year Ending December 31 2019 $ 32,228 2020 34,826 2021 34,410 2022 34,826 2023 35,270 Thereafter 390,455 Total $ 562,015

STOCK-BASED COMPENSATION (Table

STOCK-BASED COMPENSATION (Tables)	12 Months Ended
	Dec. 31, 2019
Share-based Payment Arrangement [Abstract]
Stock-based Compensation Expenses Included in Operating Costs and Expenses	The following table summarizes stock-based compensation expenses included in operating costs and expenses, in thousands: Year Ended December 31, 2019 2018 2017 Research and development $ 88,930 $ 80,509 $ 51,872 Selling, general and administrative 85,911 77,243 40,947 Total $ 174,841 $ 157,752 $ 92,819
Summary of Stock-Based Compensation Expense	The following table summarizes stock-based compensation expense, in thousands: Year Ended December 31, 2019 2018 2017 Stock-based compensation expense by type of award: Time-based stock options $ 99,097 $ 83,403 $ 61,802 Performance-based stock options 48,207 56,419 23,260 Time-based restricted stock units 2,351 538 541 Performance-based restricted stock units 22,123 13,144 — ESPP share issuances 3,482 2,842 2,155 Other equity programs 1,709 1,345 1,946 Non-employee stock options 1,275 1,485 3,115 Less: Stock-based compensation expense capitalized to inventory (3,403) (1,424) — Total $ 174,841 $ 157,752 $ 92,819
Summary of Unrecognized Stock-Based Compensation Expense, Net of Estimated Forfeitures	The following table summarizes our unrecognized stock-based compensation expense, net of estimated forfeitures, as of December 31, 2019 by type of awards, and the weighted-average period over which that expense is expected to be recognized: At December 31, 2019 Unrecognized Weighted- (in thousands) (in years) Type of award: Time-based stock options $ 184,062 2.62 Performance-based stock options 5,997 * Time-based restricted stock units 7,014 1.08 Performance-based restricted stock units 12,217 * ESPP share issuances 903 0.33 __________________________________________ * Performance-based stock options and performance-based restricted stock units are recorded as expense beginning when vesting events are determined to be probable.
Valuation Assumptions for Stock Options	2019 2018 2017 Risk-free interest rate 1.4 - 2.6% 2.7 - 2.9% 1.9 -2.3% Expected dividend yield — — — Expected option life 5.6 - 7.3 years 5.7 - 7.2 years 5.7 - 7.2 years Expected volatility 63 - 66% 64 - 67% 61 - 67%
Stock Option and Inducement Grant Activity	The following table summarizes the activity of our stock option plans and the inducement grants described above, excluding performance-based stock options: Number of Weighted- Weighted- Aggregate Outstanding, December 31, 2018 9,829 $ 72.48 Granted 3,073 83.09 Exercised (1,105) 43.84 Cancelled (919) 89.94 Outstanding, December 31, 2019 10,878 $ 76.92 6.67 $ 428,124 Exercisable as of December 31, 2019 6,196 $ 69.44 5.21 $ 290,437 Vested or expected to vest as of December 31, 2019 10,434 $ 76.47 6.57 $ 415,443 The following table summarizes the activity of our performance-based stock options granted under our equity plans and the performance-based portion of the inducement grants described above: Number of Weighted- Weighted- Aggregate Outstanding, December 31, 2018 2,744 $ 84.75 Granted — — Exercised (271) 56.18 Cancelled (282) 96.57 Outstanding, December 31, 2019 2,191 $ 86.77 6.35 $ 65,009 Exercisable as of December 31, 2019 1,956 $ 82.89 6.17 $ 65,009
Restricted Stock Units and Performance-Based Restricted Stock Units Activity	The following table summarizes the activity of our restricted stock units and awards, excluding performance-based stock units: Number of Weighted- Outstanding, December 31, 2018 15 $ 89.44 Granted 127 78.01 Vested (14) 95.66 Cancelled (1) 89.82 Outstanding, December 31, 2019 127 $ 77.23 Performance-Based Restricted Stock Units The following table summarizes the activity of our performance-based restricted stock units granted under our equity plans: Number of Weighted- Outstanding, December 31, 2018 22 $ 91.72 Granted 685 85.01 Vested (13) 79.14 Cancelled (71) 85.00 Outstanding, December 31, 2019 623 $ 85.36
Stock Purchase Rights Granted Under the Employee Stock Purchase Plan	The following table summarizes information pertaining to stock purchase rights granted under the employee stock purchase plan, during the years indicated: 2019 2018 2017 Weighted-average fair value, per share $ 25.51 $ 35.67 $ 17.10 Weighted-average stock price volatility 52 % 63 % 91 % Expected option life 6 months 6 months 6 months Risk-free interest rate 2.5 % 2.0 % 0.7 %

INCOME TAXES (Tables)

INCOME TAXES (Tables)	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Schedule of Domestic and Foreign Components of Loss before Income Taxes	The domestic and foreign components of loss before income taxes are as follows, in thousands: 2019 2018 2017 Domestic $ (597,602) $ (573,245) $ (378,293) Foreign (288,514) (187,429) (112,581) Loss before income taxes $ (886,116) $ (760,674) $ (490,874)
Schedule of Provision for Income Taxes	The provision for income taxes for each of the years ended December 31, 2019, 2018 and 2017 consisted of the following: 2019 2018 2017 Current provision: Domestic $ (394) $ — $ — Foreign 3,232 1,611 — Total current provision 2,838 1,611 — Deferred benefit: Federal 394 (788) — Foreign (2,369) — — Total deferred benefit (1,975) (788) — Total provision for income taxes $ 863 $ 823 $ —
Schedule of Components of Net Deferred Tax (Liability) Asset	Components of the net deferred tax (liability) asset as of December 31, 2019 and 2018 are as follows, in thousands: 2019 2018 Deferred tax assets: Net operating loss carryforwards $ 627,466 $ 506,574 Research and development and ODC credits 261,616 220,776 AMT credits 394 788 Lease liability 69,334 — Deferred compensation 75,058 57,268 Intangible assets 66,615 62,260 Other 13,266 20,660 Total deferred tax assets 1,113,749 868,326 Deferred tax liabilities: Property, plant and equipment, net (10,077) (6,550) Unrealized gain on marketable securities (3,932) (903) Right of use assets (50,294) — Deferred tax asset valuation allowance (1,046,013) (860,085) Net deferred tax asset $ 3,433 $ 788
Schedule of Effective Income Tax Rate Differs from Statutory Federal Income Tax Rate	Our effective income tax rate differs from the statutory federal income tax rate as follows for the years ended December 31, 2019, 2018 and 2017: 2019 2018 2017 At U.S. federal statutory rate 21.0 % 21.0 % 35.0 % State taxes, net of federal effect 3.6 2.1 3.8 Stock-based compensation — 0.8 3.3 Tax credits 3.7 4.2 9.9 Orphan drug credit — — (3.4) Other permanent items (0.3) (0.3) (1.0) Foreign rate differential (6.9) (5.3) (8.1) Tax reform change — — (46.5) Revaluation of deferred credits due to rate change — (3.5) — Other (0.1) — (0.9) Valuation allowance (21.0) (19.0) 7.9 Effective income tax rate — % — % — %
Schedule of Unrecognized Tax Benefits Activity	The following table presents our unrecognized tax benefits activity for the years ended December 31, 2019 and 2018, in thousands: Description 2019 2018 Unrecognized tax benefits at the beginning of the year $ — $ — Gross increases - current period tax positions 305 — Unrecognized tax benefits at the end of the year $ 305 $ —

QUARTERLY FINANCIAL DATA (UNA_2

QUARTERLY FINANCIAL DATA (UNAUDITED) (Tables)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information	The following information has been derived from unaudited consolidated financial statements that, in the opinion of management, include all recurring adjustments necessary for a fair statement of such information. Three Months Ended March 31, June 30, September 30, December 31, (In thousands, except per share data) Revenues $ 33,294 $ 44,714 $ 70,061 $ 71,681 Operating costs and expenses 222,082 280,985 286,360 369,754 Net loss $ (181,915) $ (219,481) $ (208,535) $ (276,185) Net loss per common share — basic and diluted $ (1.73) $ (2.02) $ (1.92) $ (2.47) Weighted-average common shares — basic and diluted 105,400 108,576 108,701 111,750 Three Months Ended March 31, June 30, September 30, December 31, (In thousands, except per share data) Revenues $ 21,899 $ 29,907 $ 2,069 $ 21,033 Operating costs and expenses 169,304 222,261 256,627 241,389 Net loss $ (141,214) $ (163,560) $ (245,282) $ (211,441) Net loss per common share — basic and diluted $ (1.41) $ (1.63) $ (2.43) $ (2.09) Weighted-average common shares — basic and diluted 99,979 100,519 100,783 101,066

Customers that Represent Greate

Customers that Represent Greater than Ten Percent of Gross Revenues (Detail) - Revenue from Rights Concentration Risk - Gross Revenues	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Distributor A
Concentration Risk [Line Items]
Concentration risk percentage	44.00%	13.00%
Sanofi Genzyme
Concentration Risk [Line Items]
Concentration risk percentage		58.00%	61.00%
Vir Biotechnology
Concentration Risk [Line Items]
Concentration risk percentage		16.00%
The Medicines Company
Concentration Risk [Line Items]
Concentration risk percentage			34.00%

Customers that Represent Grea_2

Customers that Represent Greater than Ten Percent of Gross Accounts Receivable (Detail) - Accounts Receivable - Credit Concentration Risk	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Distributor A
Concentration Risk [Line Items]
Concentration risk percentage	28.00%	44.00%
Sanofi Genzyme
Concentration Risk [Line Items]
Concentration risk percentage	14.00%	29.00%
Vir Biotechnology
Concentration Risk [Line Items]
Concentration risk percentage	13.00%
Distributor B
Concentration Risk [Line Items]
Concentration risk percentage	10.00%

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Investments in Marketable Securities and Cash Equivalents - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Marketable securities classified as cash equivalents, maximum original maturity	90 days
Policy for marketable securities	90 days

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Accounts Receivable - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019
Minimum
Significant Accounting Policies [Line Items]
Accounts receivable payment term	30 days
Maximum
Significant Accounting Policies [Line Items]
Accounts receivable payment term	90 days

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Estimated Useful Lives of Property, Plant and Equipment (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 16.6	$ 12.8	$ 11.9
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years
Computer equipment and software | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Computer equipment and software | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	10 years
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	Shorter of asset life or lease term
Manufacturing Equipment | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	7 years
Manufacturing Equipment | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	15 years
Buildings
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	40 years

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Revenue Recognition - Additional Information (Detail) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Capitalized cost	$ 0	$ 0

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Net Product Revenues - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Product returns, description	We offer customers product return rights if products are damaged, defective or expired, with “expired” defined as within three months pre- or post-expiry.

Summary of Significant Accoun_9

Summary of Significant Accounting Policies - Income Taxes - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019USD ($)
Accounting Policies [Abstract]
Interest and penalty expense related to uncertain tax positions	$ 0

Summary of Significant Accou_10

Summary of Significant Accounting Policies - Research and Development Expenses - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Research and development
Significant Accounting Policies [Line Items]
Research and development expense costs associated with license fees	$ 37	$ 8	$ 7.7

Potential Common Shares Exclude

Potential Common Shares Excluded from Calculation of Net Loss Per Common Share (Detail) - shares shares in Thousands	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 excluded from computation of earnings per share	13,818	12,609	11,388
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share	13,069	12,573	11,239
Unvested restricted common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share	749	36	149

Summary of Significant Accou_11

Summary of Significant Accounting Policies - Segment Information - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019Segment
Accounting Policies [Abstract]
Number of reporting segments	1

Summary of Significant Accou_12

Summary of Significant Accounting Policies - Recently Adopted Accounting Pronouncements - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019	Jan. 01, 2018
Significant Accounting Policies [Line Items]
Operating lease liabilities	$ 303,823
Operating lease right-of-use assets	$ 221,197
Accounting Standards Update 2016-02
Significant Accounting Policies [Line Items]
Operating lease liabilities		$ 290,000
Operating lease right-of-use assets		$ 230,000
ASU 2014-09 Revenue from Contracts with Customers | Reduction In Deferred Revenue
Significant Accounting Policies [Line Items]
Offsetting adjustments to accumulated deficit			$ (68,200)
ASU 2014-09 Revenue from Contracts with Customers | Reduction In Deferred Tax Asset
Significant Accounting Policies [Line Items]
Offsetting adjustments to accumulated deficit			$ 13,600

Cumulative Effect to Consolidat

Cumulative Effect to Consolidated Balance Sheet for Adoption of New Revenue Standard (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current portion	$ 77,821	$ 3,496		$ 41,705
Deferred revenue, net of current portion	318,383	458		43,075
Accumulated deficit	$ (3,727,088)	(2,840,972)		$ (2,147,685)
ASU 2014-09 Revenue from Contracts with Customers
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current portion			$ 7,242
Deferred revenue, net of current portion			9,328
Accumulated deficit			(2,079,475)
ASU 2014-09 Revenue from Contracts with Customers | Effect of Change Higher/(Lower)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current portion			(34,463)
Deferred revenue, net of current portion		(17,993)	(33,747)
Accumulated deficit		$ (18,077)	$ 68,210

Impact on Consolidated Balance

Impact on Consolidated Balance Sheet and Consolidated Statement of Comprehensive Loss for Adoption of New Revenue Standard (Detail) - USD ($) $ / shares in Units, $ in Thousands	Jan. 01, 2018	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
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current portion		$ 77,821				$ 3,496				$ 77,821	$ 3,496	$ 41,705
Deferred revenue, net of current portion		318,383				458				318,383	458	43,075
Accumulated deficit		(3,727,088)				(2,840,972)				(3,727,088)	(2,840,972)	(2,147,685)
Revenues	$ 2,900	71,681	$ 70,061	$ 44,714	$ 33,294	21,033	$ 2,069	$ 29,907	$ 21,899	219,750	74,908	89,912
Net loss		$ (276,185)	$ (208,535)	$ (219,481)	$ (181,915)	$ (211,441)	$ (245,282)	$ (163,560)	$ (141,214)	$ (886,116)	$ (761,497)	$ (490,874)
Net loss per common share - basic and diluted (in dollars per share)		$ (2.47)	$ (1.92)	$ (2.02)	$ (1.73)	$ (2.09)	$ (2.43)	$ (1.63)	$ (1.41)	$ (8.11)	$ (7.57)	$ (5.42)
Net Revenues from Collaborators
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues										$ 53,213	$ 62,373	$ 89,912
ASU 2014-09 Revenue from Contracts with Customers
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current portion	7,242
Deferred revenue, net of current portion	9,328
Accumulated deficit	(2,079,475)
ASU 2014-09 Revenue from Contracts with Customers | Balances Without Adoption of New Revenue Standard
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current portion						$ 3,496					3,496
Deferred revenue, net of current portion						18,451					18,451
Accumulated deficit						(2,859,049)					(2,859,049)
Net loss											$ (711,364)
Net loss per common share - basic and diluted (in dollars per share)											$ (7.07)
ASU 2014-09 Revenue from Contracts with Customers | Balances Without Adoption of New Revenue Standard | Net Revenues from Collaborators
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues											$ 112,506
ASU 2014-09 Revenue from Contracts with Customers | Effect of Change Higher/(Lower)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current portion	(34,463)
Deferred revenue, net of current portion	(33,747)					(17,993)					(17,993)
Accumulated deficit	$ 68,210					$ (18,077)					(18,077)
Net loss											$ 50,133
Net loss per common share - basic and diluted (in dollars per share)											$ 0.50
ASU 2014-09 Revenue from Contracts with Customers | Effect of Change Higher/(Lower) | Net Revenues from Collaborators
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues											$ (50,133)

Net Product Revenues - Summary

Net Product Revenues - Summary of Net Product Revenues by Geography (Detail) - USD ($) $ in Thousands	Jan. 01, 2018	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
Disaggregation of Revenue [Line Items]
Revenues	$ 2,900	$ 71,681	$ 70,061	$ 44,714	$ 33,294	$ 21,033	$ 2,069	$ 29,907	$ 21,899	$ 219,750	$ 74,908	$ 89,912
Product Revenues, Net
Disaggregation of Revenue [Line Items]
Revenues										166,537	12,535	0
Product Revenues, Net | United States
Disaggregation of Revenue [Line Items]
Revenues										116,452	8,589	0
Product Revenues, Net | Rest of world
Disaggregation of Revenue [Line Items]
Revenues										$ 50,085	$ 3,946	$ 0

Net Product Revenues - Addition

Net Product Revenues - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Accounts receivable, net	$ 43,011	$ 18,760
Product Revenues, Net
Disaggregation of Revenue [Line Items]
Accounts receivable, net	$ 28,100	$ 13,100

Net Product Revenues - Summar_2

Net Product Revenues - Summary of Balances and Activity in Each Product Revenue Allowance and Reserve Category (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balance	$ 3,980	$ 0
Provision related to current period sales	52,706	5,030
Credit or payments made during the period for current year sales	(21,264)	(1,050)
Credit or payments made during the period for prior year sales	(547)
Total	34,875	3,980
Chargebacks and Rebates
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balance	3,441	0
Provision related to current period sales	44,371	4,081
Credit or payments made during the period for current year sales	(15,216)	(640)
Credit or payments made during the period for prior year sales	(109)
Total	32,487	3,441
Trade Discounts and Allowances
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balance	218	0
Provision related to current period sales	3,227	292
Credit or payments made during the period for current year sales	(2,817)	(74)
Credit or payments made during the period for prior year sales	(218)
Total	410	218
Returns Reserve and Other Incentives
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balance	321	0
Provision related to current period sales	5,108	657
Credit or payments made during the period for current year sales	(3,231)	(336)
Credit or payments made during the period for prior year sales	(220)
Total	$ 1,978	$ 321

Collaboration Agreements - Net

Collaboration Agreements - Net Revenue from Collaborators (Detail) - USD ($) $ in Thousands	Jan. 01, 2018	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
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	$ 2,900	$ 71,681	$ 70,061	$ 44,714	$ 33,294	$ 21,033	$ 2,069	$ 29,907	$ 21,899	$ 219,750	$ 74,908	$ 89,912
Net Revenues from Collaborators
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										53,213	62,373	89,912
Net Revenues from Collaborators | Regeneron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										26,075	0	0
Net Revenues from Collaborators | Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										12,809	12,778	1,464
Net Revenues from Collaborators | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										10,976	46,000	54,625
Net Revenues from Collaborators | MDCO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										2,315	2,789	30,217
Net Revenues from Collaborators | Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										$ 1,038	$ 806	$ 3,606

Collaboration Agreements - Bala

Collaboration Agreements - Balance of Receivables and Contract Liabilities Related to Collaboration Agreements (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Receivables included in "Accounts receivable, net"	$ 14,929	$ 5,625
Contract liabilities included in "Deferred revenue"	$ 153,117	$ 3,954	$ 16,570

Collaboration Agreements - Chan

Collaboration Agreements - Change in Contract Liability Balance Related to Collaboration Agreements (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Changes in contract liability
Contract liability, beginning of period	$ 3,954	$ 16,570
Consideration earned, not yet recognized as revenue	153,117	3,954
Revenue recognized on contract liability	(3,954)	(16,570)
Contract liability, end of period	$ 153,117	$ 3,954

Collaboration Agreements - Sche

Collaboration Agreements - Schedule of Research and Development Expenses Incurred by Type that are Directly Attributable to Collaboration Agreements (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	$ 655,114	$ 505,420	$ 390,635
Clinical Trial and Manufacturing
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	26,676	45,536	181,456
Clinical Trial and Manufacturing | Regeneron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	2,793	0	0
Clinical Trial and Manufacturing | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	11,505	36,600	174,901
Clinical Trial and Manufacturing | Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	10,353	7,272	1,134
Clinical Trial and Manufacturing | MDCO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	2,025	1,664	5,421
Clinical Trial and Manufacturing | Ionis
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	0	0	0
External Services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	1,059	15,743	8,651
External Services | Regeneron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	344	0	0
External Services | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	334	5,340	4,475
External Services | Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	381	8,251	926
External Services | MDCO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	0	2	0
External Services | Ionis
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	0	2,150	3,250
Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	29,237	3,127	5,433
Other | Regeneron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	21,779	0	0
Other | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	2,017	1,279	5,327
Other | Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	4,745	548	0
Other | MDCO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	696	203	106
Other | Ionis
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	$ 0	$ 1,097	$ 0

Collaboration Agreements - Rege

Collaboration Agreements - Regeneron Collaboration (Detail)	Dec. 31, 2019USD ($)	May 21, 2019USD ($)shares	Apr. 08, 2019USD ($)Program$ / sharesshares	Jan. 31, 2019$ / sharesshares	Nov. 30, 2017$ / sharesshares	May 31, 2017$ / sharesshares	Dec. 31, 2019USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Issuance of common stock, net of offering costs (in shares) | shares				5,000,000	6,440,000	5,000,000
Offering proceeds, net of costs								$ 381,900,000	$ 0	$ 1,139,625,000
Underwritten public offering amount per share (in dollars per share) | $ / shares				$ 77.50	$ 125	$ 71.87
Increase (decrease) in transaction price							$ 33,500,000
Global Strategic Collaboration | Regeneron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Issuance of common stock, net of offering costs (in shares) | shares		4,444,445	4,444,445
Offering proceeds, net of costs		$ 400,000,000	$ 400,000,000
Underwritten public offering amount per share (in dollars per share) | $ / shares			$ 90
Discovery period of programs development			5 years
Extended additional discovery period of programs development			2 years
Maximum royalties and commercial milestone payments upon potential product sale			$ 325,000,000
Upfront fee received			400,000,000
Maximum additional milestone payments to be receive upon achievement of certain criteria			$ 200,000,000
Number of targeted programs | Program			30
Percentage of maximum royalty payments			20.00%
Transaction price	$ 555,100,000		$ 521,600,000
Transactional price remaining performance obligation	$ 288,100,000						$ 288,100,000	$ 288,100,000
Global Strategic Collaboration | Regeneron | Funding At Program Initiation
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential proceeds from collaboration arrangement			2,500,000
Global Strategic Collaboration | Regeneron | Funding At Lead Candidate Identification
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential proceeds from collaboration arrangement			2,500,000
Global Strategic Collaboration | Regeneron | Funding An Annual Discovery
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential proceeds from collaboration arrangement			30,000,000
Global Strategic Collaboration | Regeneron | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research term extension fee			400,000,000
Collaborative arrangement milestone payments			$ 150,000,000
Royalty rate			20.00%

Collaboration Agreements - Sc_2

Collaboration Agreements - Schedule of Transaction Price Allocated (Details) - Global Strategic Collaboration - Regeneron - USD ($) $ in Thousands	Dec. 31, 2019	Apr. 08, 2019
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction price	$ 555,100	$ 521,600
Research Services Obligation [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Standalone Selling Price	130,700
Transaction price	200,600	183,100
C5 License Obligation
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Standalone Selling Price	97,600
Transaction price	108,500	92,500
C5 Co Co Obligation
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Standalone Selling Price	364,600
Transaction price	$ 246,000	$ 246,000

Collaboration Agreements - Sc_3

Collaboration Agreements - Schedule of Revenue Recognized by Accounting Guidance (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Apr. 08, 2019	Jan. 01, 2018	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
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues			$ 2,900	$ 71,681	$ 70,061	$ 44,714	$ 33,294	$ 21,033	$ 2,069	$ 29,907	$ 21,899	$ 219,750	$ 74,908	$ 89,912
Deferred revenue	$ 77,821			77,821				$ 3,496				77,821	$ 3,496	$ 41,705
Global Strategic Collaboration | Regeneron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction price	555,100	$ 521,600
Revenues												23,900
Deferred revenue	393,600			393,600								393,600
Research Services Obligation [Member] | Global Strategic Collaboration | Regeneron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction price	200,600	183,100
Revenues												21,000
Deferred revenue	84,800			84,800								84,800
C5 License Obligation | Global Strategic Collaboration | Regeneron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction price	108,500	92,500
Revenues												0
Deferred revenue	65,800			65,800								65,800
C5 Co Co Obligation | Global Strategic Collaboration | Regeneron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction price	246,000	$ 246,000
Revenue from Collaborative Arrangement, Excluding Revenue from Contract with Customer												2,900
Deferred revenue	$ 243,000			$ 243,000								$ 243,000

Collaboration Agreements - Addi

Collaboration Agreements - Additional Information (Detail)

Jan. 01, 2018USD ($)

Jan. 14, 2014USD ($)

Jan. 31, 2019$ / sharesshares

Jan. 31, 2018USD ($)MilestonePayment

Nov. 30, 2017$ / sharesshares

Oct. 31, 2017USD ($)shares

May 31, 2017USD ($)$ / sharesshares

Jan. 31, 2017USD ($)

Nov. 30, 2016

Jan. 31, 2015USD ($)Program

Dec. 31, 2014USD ($)

Feb. 28, 2014USD ($)$ / shares

Feb. 28, 2013USD ($)

Dec. 31, 2019USD ($)shares

Sep. 30, 2019USD ($)

Jun. 30, 2019USD ($)

Mar. 31, 2019USD ($)

Dec. 31, 2018USD ($)shares

Sep. 30, 2018USD ($)

Jun. 30, 2018USD ($)

Mar. 31, 2018USD ($)

Dec. 31, 2015USD ($)

Dec. 31, 2019USD ($)Licenseshares

Dec. 31, 2018USD ($)shares

Dec. 31, 2017USD ($)

Dec. 31, 2014USD ($)shares

Dec. 31, 2017USD ($)

Dec. 31, 2019USD ($)shares

May 03, 2019shares

Jan. 17, 2019shares

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Revenues recognized

$ 2,900,000

$ 71,681,000

$ 70,061,000

$ 44,714,000

$ 33,294,000

$ 21,033,000

$ 2,069,000

$ 29,907,000

$ 21,899,000

$ 219,750,000

$ 74,908,000

$ 89,912,000

Deferred revenue

$ 153,117,000

$ 3,954,000

$ 153,117,000

$ 3,954,000

16,570,000

$ 16,570,000

$ 153,117,000

Issuance of common stock, net of offering costs (in shares) | shares

5,000,000

6,440,000

5,000,000

Underwritten public offering amount per share (in dollars per share) | $ / shares

$ 77.50

$ 125

$ 71.87

Common stock shares owned (in shares) | shares

112,188,000

101,177,000

112,188,000

101,177,000

112,188,000

Research and development

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

License fee

$ 37,000,000

$ 8,000,000

7,700,000

Vir Biotechnology

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Upfront fee received

$ 10,000,000

Shares of common stock (in shares) | shares

1,111,111

Future share payments on achievement of specified development milestones (in shares) | shares

1,111,111

Ionis

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Collaborative agreement termination notice period

90 days

Prior Revenue Standard | ASU 2014-09 Revenue from Contracts with Customers

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Deferred revenue

23,400,000

Product Alliances

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Upfront fee received

25,000,000

Product Alliances | MDCO

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Amount earned upon achievement of milestone

30,000,000

$ 10,000,000

$ 30,000,000

Transaction price

72,900,000

Upfront fee received

$ 25,000,000

$ 25,000,000

Cost-share reimbursements, net of payments included in transaction price

17,900,000

Maximum number of potential future milestones

180,000,000

Potential future payment for the achievement of certain development milestones

30,000,000

25,000,000

Potential future payment for the achievement of specified regulatory milestones

50,000,000

Potential future payment for the achievement of specified commercialization milestones

$ 100,000,000

Total costs expected

$ 17,900,000

$ 17,900,000

$ 17,900,000

Cost incurred

$ 17,800,000

Product Alliances | Maximum | MDCO

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Estimated range of expiration for fundamental patents

2028

Product Alliances | Minimum | MDCO

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Estimated range of expiration for fundamental patents

2016

Product Alliances | Prior Revenue Standard | ASU 2014-09 Revenue from Contracts with Customers | MDCO

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Deferred revenue

5,700,000

2018 Restructured Agreement Related to TTR Programs | Prior Revenue Standard | ASU 2014-09 Revenue from Contracts with Customers

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Deferred revenue

25,800,000

2018 Restructured Agreement | Fitusiran

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Total Cost Incurred in Transition

38,000,000

2018 Restructured Agreement | Prior Revenue Standard | Fitusiran | ASU 2014-09 Revenue from Contracts with Customers

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Incremental to transaction price

22,800,000

Discovery and Development Alliances | Ionis

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Number of programs Providing exclusive RNA therapeutic license rights | Program

4

Number of licensed products | License

10

Additional number of licensed products | License

1

Discovery and Development Alliances | Ionis | Research and development

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

License fee

$ 21,900,000

Discovery and Development Alliances | Ionis | Up Front Payment

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

License fee

$ 5,000,000

Discovery and Development Alliances | Ionis | Milestone Payments | Research and development

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Potential future payment for the achievement of specified regulatory milestones

3,400,000

Discovery and Development Alliances | Ionis | Therapeutic target | Research and development

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

License fee

$ 500,000

Collaboration Amendment | Minimum | Sanofi Genzyme

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Percentage ownership interest owned by noncontrolling owners

5.00%

5.00%

5.00%

Sanofi Genzyme

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Amount earned upon achievement of milestone

11,000,000

Upfront fee received

22,500,000

Excess of fair value over cash received for stock issuance

51,500,000

$ 51,500,000

Issuance of common stock, net of offering costs (in shares) | shares

8,766,338

Proceeds from issuance of common stock to Sanofi Genzyme

$ 21,400,000

$ 700,000,000

$ 0

0

$ 21,381,000

Voting percentage

20.00%

Agreement lock-up period, extension term

2 years

Agreement lock-up period

6 months

Underwritten public offering amount per share (in dollars per share) | $ / shares

$ 85.72

Fair value of shares issued

$ 751,500,000

Provision for (Benefit from) income taxes

$ 15,200,000

Common stock shares owned (in shares) | shares

0

10,554,134

Sanofi Genzyme | Concurrent Private Placement

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Issuance of common stock, net of offering costs (in shares) | shares

297,501

Sanofi Genzyme | Maximum

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Agreement to acquire outstanding shares of common stock percentage

30.00%

Sanofi Genzyme | Minimum

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Agreement to acquire outstanding shares of common stock percentage

5.00%

Sanofi Genzyme | Revusiran

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Amount earned upon achievement of milestone

$ 25,000,000

Sanofi Genzyme | Fitusiran

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Cost-share reimbursements, net of payments included in transaction price

147,300,000

Sanofi Genzyme | Regional Collaboration Product

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Potential future payment for the achievement of specified commercialization milestones

$ 20,000,000

Sanofi Genzyme | Co-developed/ Co-commercialized Collaboration Product

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Percentage of sharing in development cost

50.00%

Received incremental share of co-development costs for exercise of right, fitusiran

$ 6,000,000

Expected first milestone payment to be received fitusiran

$ 25,000,000

25,000,000

Sanofi Genzyme | Co-developed/ Co-commercialized Collaboration Product | Revusiran

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Amount earned upon achievement of milestone

$ 25,000,000

Sanofi Genzyme | Global Collaboration Product

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Royalty rate

20.00%

20.00%

Sanofi Genzyme | TTR License

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Transaction price

127,600,000

Sanofi Genzyme | TTR License | ONPATTRO | Japan

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Royalty rate

25.00%

Sanofi Genzyme | TTR License | ONPATTRO | Maximum | Excluding U.S., Canada and Western Europe

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Royalty rate

25.00%

Sanofi Genzyme | TTR License | Revusiran

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Cost-share reimbursements, net of payments included in transaction price

57,000,000

Sanofi Genzyme | TTR License | Fitusiran

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Amount earned upon achievement of milestone

50,000,000

Sanofi Genzyme | TTR License | Patisiran

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Cost-share reimbursements, net of payments included in transaction price

63,600,000

Sanofi Genzyme | TTR License | T T R S C Zero Two | Maximum

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Royalty rate

30.00%

Sanofi Genzyme | TTR License | T T R S C Zero Two | Minimum

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Royalty rate

15.00%

Sanofi Genzyme | TTR License | Back Up Products [Member] | Maximum

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Royalty rate

15.00%

Sanofi Genzyme | AT3 License Terms

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Number of milestone payments required | MilestonePayment

1

Expected milestone payment to be received fitusiran

$ 50,000,000

Sanofi Genzyme | AT3 License Terms | Fitusiran | Maximum

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Royalty rate

30.00%

Sanofi Genzyme | AT3 License Terms | Fitusiran | Minimum

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Royalty rate

15.00%

Sanofi Genzyme | AT3 License Terms | Back Up Products [Member] | Maximum

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Royalty rate

15.00%

Sanofi Genzyme | 2018 Restructured Agreement

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Revenues recognized

37,600,000

Sanofi Genzyme | 2018 Restructured Agreement | Fitusiran

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Amount earned upon achievement of milestone

$ 50,000,000

Revenues recognized

37,600,000

Deferred revenue

$ 600,000

Sanofi Genzyme | 2018 Restructured Agreement | Prior Revenue Standard | Fitusiran | ASU 2014-09 Revenue from Contracts with Customers

Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]

Revenues recognized

$ 60,300,000

Other Balance Sheet Details - S

Other Balance Sheet Details - Schedule of Inventory (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Balance Sheet Related Disclosures [Abstract]
Raw materials	$ 15,418	$ 8,709
Work in process	38,275	15,262
Finished goods	2,655	97
Total inventory	$ 56,348	$ 24,068

Other Balance Sheet Details -_2

Other Balance Sheet Details - Schedule of Property, Plant, and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 501,337	$ 400,143
Less: accumulated depreciation	(76,158)	(79,485)
Total	425,179	320,658
Buildings
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	250,380	0
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	132,632	65,928
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	54,195	260,975
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	29,755	43,427
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	14,956	13,495
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	10,339	7,238
Land
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 9,080	$ 9,080

Other Balance Sheet Details -_3

Other Balance Sheet Details - Schedule of Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Balance Sheet Related Disclosures [Abstract]
Compensation and related	$ 68,304	$ 37,301
Pre-clinical, clinical trial and manufacturing	34,269	32,205
Product revenue allowances	32,670	2,696
Licensing and collaboration agreements	20,622	6,433
Consulting and professional services	14,251	10,450
Other	27,085	23,634
Total	$ 197,201	$ 112,719

Other Balance Sheet Details -_4

Other Balance Sheet Details - Schedule of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Balance Sheet Related Disclosures [Abstract]
Cash and cash equivalents	$ 547,178	$ 420,146	$ 645,361
Restricted cash included in prepaid expenses and other current assets	4	225	0
Restricted cash included in long-term other assets	2,446	2,260	1,471
Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows	$ 549,628	$ 422,631	$ 646,832	$ 195,088

Other Balance Sheet Details -_5

Other Balance Sheet Details - Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 1,301,965	$ 1,766,431	$ 920,221
Other comprehensive loss before reclassifications	(3,982)	(411)
Amounts reclassified from other comprehensive income	677	1,631
Net other comprehensive income	(3,305)	1,220	(992)
Ending Balance	1,438,692	1,301,965	1,766,431
Loss on Investment in Joint Venture
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(32,792)	(32,792)
Other comprehensive loss before reclassifications	0	0
Amounts reclassified from other comprehensive income	0	0
Net other comprehensive income	0	0
Ending Balance	(32,792)	(32,792)	(32,792)
Defined Benefit Pension Plans, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	0	0
Other comprehensive loss before reclassifications	(3,661)	0
Amounts reclassified from other comprehensive income	141	0
Net other comprehensive income	(3,520)	0
Ending Balance	(3,520)	0	0
Unrealized (Losses) Gains from Debt Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(421)	(1,641)
Other comprehensive loss before reclassifications	22	(411)
Amounts reclassified from other comprehensive income	536	1,631
Net other comprehensive income	558	1,220
Ending Balance	137	(421)	(1,641)
Foreign Currency Translation Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	0	0
Other comprehensive loss before reclassifications	(343)	0
Amounts reclassified from other comprehensive income	0	0
Net other comprehensive income	(343)	0
Ending Balance	(343)	0	0
Total Accumulated Other Comprehensive (Loss) Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(33,213)	(34,433)	(33,441)
Net other comprehensive income	(3,305)	1,220	(992)
Ending Balance	$ (36,518)	$ (33,213)	$ (34,433)

Fair Value Measurements - Fair

Fair Value Measurements - Fair Value of Assets Measured on a Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 340,132	$ 221,281
Marketable debt securities	1,315,149	884,084
Marketable equity securities	13,967	1,206
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable equity securities	13,967	1,206
Total	1,450,480	989,212
Recurring | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	3,439
Marketable debt securities	36,474	57,197
Recurring | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	336,693	221,281
Marketable debt securities	755,714	325,227
Recurring | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	119,882	102,445
Restricted cash (money market funds)	1,482	1,477
Recurring | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	4,301	8,951
Recurring | Corporate notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	146,040	232,410
Recurring | U.S. government-sponsored enterprise securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	32,488	39,018
Recurring | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable equity securities	13,967	1,206
Total	135,331	105,128
Recurring | Quoted Prices in Active Markets (Level 1) | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0
Marketable debt securities	0	0
Recurring | Quoted Prices in Active Markets (Level 1) | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Marketable debt securities	0	0
Recurring | Quoted Prices in Active Markets (Level 1) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	119,882	102,445
Restricted cash (money market funds)	1,482	1,477
Recurring | Quoted Prices in Active Markets (Level 1) | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	0	0
Recurring | Quoted Prices in Active Markets (Level 1) | Corporate notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	0	0
Recurring | Quoted Prices in Active Markets (Level 1) | U.S. government-sponsored enterprise securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	0	0
Recurring | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable equity securities	0	0
Total	1,315,149	884,084
Recurring | Significant Observable Inputs (Level 2) | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	3,439
Marketable debt securities	36,474	57,197
Recurring | Significant Observable Inputs (Level 2) | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	336,693	221,281
Marketable debt securities	755,714	325,227
Recurring | Significant Observable Inputs (Level 2) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Restricted cash (money market funds)	0	0
Recurring | Significant Observable Inputs (Level 2) | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	4,301	8,951
Recurring | Significant Observable Inputs (Level 2) | Corporate notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	146,040	232,410
Recurring | Significant Observable Inputs (Level 2) | U.S. government-sponsored enterprise securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	32,488	39,018
Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable equity securities	0	0
Total	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0
Marketable debt securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Marketable debt securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Restricted cash (money market funds)	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Corporate notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | U.S. government-sponsored enterprise securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable debt securities	$ 0	$ 0

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail)	Sep. 27, 2019shares	Dec. 31, 2019USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Sep. 26, 2019shares	Oct. 31, 2017shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers between Level 1 and Level 2 financial assets		$ 0	$ 0	$ 0
Long-term debt		0	0	30,000,000
Gain on Vir common stock			$ 11,288,000	3,564,000	$ (1,894,000)
Vir Biotechnology
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shares of common stock (in shares) | shares	1,111,111					5,000,000	5,000,000
Reverse stock split conversion ratio	0.2222
Gain on Vir common stock		$ 11,300,000
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of long-term debt				$ 30,100,000

Marketable Debt Securities - Su

Marketable Debt Securities - Summary of Company's Marketable Debt Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,315,012	$ 884,505
Gross Unrealized Gains	246	1
Gross Unrealized Losses	(109)	(422)
Fair Value	1,315,149	884,084
Corporate notes
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	146,016	232,695
Gross Unrealized Gains	58	0
Gross Unrealized Losses	(34)	(285)
Fair Value	146,040	232,410
U.S. government-sponsored enterprise securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	32,487	39,031
Gross Unrealized Gains	3	0
Gross Unrealized Losses	(2)	(13)
Fair Value	32,488	39,018
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,092,293	546,631
Gross Unrealized Gains	185	1
Gross Unrealized Losses	(71)	(124)
Fair Value	1,092,407	546,508
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,303	8,951
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2)	0
Fair Value	4,301	8,951
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	39,913	57,197
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 39,913	$ 57,197

Marketable Debt Securities - _2

Marketable Debt Securities - Summary of Fair Value of Marketable Debt Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Cash and cash equivalents	$ 340,132	$ 221,281
Marketable debt securities	975,017	662,803
Total	$ 1,315,149	$ 884,084

Leases - Additional Information

Leases - Additional Information (Detail) $ in Thousands	May 01, 2018USD ($)	Sep. 26, 2003	May 31, 2015	Mar. 31, 2015	Dec. 31, 2019USD ($)RenewalOption	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Operating Leased Assets [Line Items]
Security deposit					$ 14,825	$ 44,825
Rent expense					52,400	$ 40,600	$ 18,700
Net cash paid included in operating activities in cash flow					$ 33,703
Operating lease, weighted average remaining lease term					13 years 2 months 12 days
Operating lease, weighted-average discount rate					8.20%
BMR-675 West Kendall Lease
Operating Leased Assets [Line Items]
Lease term	15 years
Number of lease extension options | RenewalOption					2
Operating lease renewal options period					5 years
Operating lease option to extend					options to renew for two five-year terms each
Operating lease, existence of option to terminate					true
Cost of base building and tenant improvement	$ 56,100
Security deposit					$ 14,800
Third Street Lease
Operating Leased Assets [Line Items]
Number of lease extension options | RenewalOption					2
Operating lease renewal options period					5 years
Operating lease option to extend					options to renew for two five-year terms each
Operating lease, existence of option to terminate					true
Lease extended expiration date		Jan. 31, 2034
101 Main Street Leases
Operating Leased Assets [Line Items]
Number of lease extension options | RenewalOption					1
Operating lease renewal options period					5 years
Operating lease option to extend					option to renew for one five-year term
Operating lease, existence of option to terminate					true
Extended lease expiration month and year			2021-06	2024-03

Leases - Summary of Costs Inclu

Leases - Summary of Costs Included in Operating Expenses Related to Leases (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Leases [Abstract]
Operating lease cost	$ 38,613
Variable lease cost	15,209
Total	$ 53,822

Leases - Summary of Future Leas

Leases - Summary of Future Lease Payments for Non-cancellable Operating Leases and Reconciliation to Carrying Amount of Operating Lease Liability (Detail) $ in Thousands	Dec. 31, 2019USD ($)
Leases [Abstract]
2020	$ 29,157
2021	37,597
2022	37,754
2023	36,250
2024	35,381
2025 and thereafter	356,567
Total undiscounted lease liability	532,706
Less imputed interest	(225,873)
Less impact of future leases not yet commenced	(3,010)
Total discounted lease liability	303,823
Current operating lease liability	27,688
Non-current operating lease liability	$ 276,135

Leases - Future Minimum Payment

Leases - Future Minimum Payments Under Non-cancelable Leases (Detail) $ in Thousands	Dec. 31, 2018USD ($)
Leases [Abstract]
2019	$ 32,228
2020	34,826
2021	34,410
2022	34,826
2023	35,270
Thereafter	390,455
Total	$ 562,015

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 27, 2019	Apr. 18, 2018	Apr. 29, 2016	Apr. 29, 2016	Dec. 31, 2019	Dec. 31, 2018
Commitments And Contingencies [Line Items]
Maximum potential future milestone payments to third parties					$ 18,400
Restricted investments					14,825	$ 44,825
Dicerna Pharmaceuticals, Inc
Commitments And Contingencies [Line Items]
Litigation settlement amount		$ 25,000
Term Loan Facility
Commitments And Contingencies [Line Items]
Line of credit facility, expiration date			Apr. 29, 2021
Wells Fargo Bank, National Association
Commitments And Contingencies [Line Items]
Cash collateral required for principal amount outstanding, percentage			100.00%
Restricted investments						$ 30,000
Wells Fargo Bank, National Association | Term Loan Facility
Commitments And Contingencies [Line Items]
Term loan facility principal			$ 30,000	$ 30,000
Repayment of term loan	$ 30,000
Wells Fargo Bank, National Association | Term Loan Facility | LIBOR
Commitments And Contingencies [Line Items]
Debt instrument, basis spread on variable rate				0.45%
Regulatory and Development Milestones
Commitments And Contingencies [Line Items]
Maximum potential future milestone payments to third parties					12,800
Commercial Milestones
Commitments And Contingencies [Line Items]
Maximum potential future milestone payments to third parties					$ 5,600

Stockholders Equity - Additiona

Stockholders Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	May 21, 2019	Apr. 25, 2019	Apr. 08, 2019	Jan. 31, 2019	Nov. 30, 2017	May 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Equity [Line Items]
Preferred stock, shares authorized (in shares)							5,000,000	5,000,000
Preferred stock, par value (in dollars per share)							$ 0.01	$ 0.01
Preferred stock, shares outstanding (in shares)							0	0
Issuance of common stock, net of offering costs (in shares)				5,000,000	6,440,000	5,000,000
Underwritten public offering amount per share (in dollars per share)				$ 77.50	$ 125	$ 71.87
Issuance of common stock, net of offering costs				$ 381,900	$ 784,500	$ 355,200	$ 772,477		$ 1,139,625
Underwriting discounts and commissions and other offering expenses				$ 5,600	$ 20,500	$ 4,200
Offering proceeds, net of costs							$ 381,900	$ 0	$ 1,139,625
Common stock, shares authorized (in shares)							250,000,000	125,000,000
Global Strategic Collaboration | Regeneron
Equity [Line Items]
Issuance of common stock, net of offering costs (in shares)	4,444,445		4,444,445
Underwritten public offering amount per share (in dollars per share)			$ 90
Offering proceeds, net of costs	$ 400,000		$ 400,000
Date of agreement							Apr. 8, 2019
Agreement extension period			2 years
Ownership percentage, shares of stock outstanding upper limit			30.00%
Mark-to-market adjustment gain		$ 9,400
Global Strategic Collaboration | Regeneron | Minimum
Equity [Line Items]
Ownership percentage of shares outstanding			19.99%
Beneficial ownership percentage following the expiration of lock-up period			9.90%
Common stock, shares authorized (in shares)		125,000,000
Global Strategic Collaboration | Regeneron | Maximum
Equity [Line Items]
Beneficial ownership percentage of shares outstanding			5.00%
Common stock, shares authorized (in shares)		250,000,000

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($)	1 Months Ended			12 Months Ended
	Aug. 31, 2018	May 31, 2017	Feb. 28, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	May 10, 2018	Apr. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock reserved for future issuance under stock options (in shares)				18,887,759
Stock-based compensation				$ 174,841,000	$ 157,752,000	$ 92,819,000
Total stock-based compensation expense				174,841,000	157,752,000	92,819,000
Employee stock purchase plan, offering period		6 months
Change in Control Agreements | Expense Associated with Equity Grants Modifications
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation				$ 2,200,000	$ 20,800,000	$ 1,100,000
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock reserved for future issuance under stock options (in shares)				13,069,165
Equity awards, term				10 years
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock reserved for future issuance under stock options (in shares)				749,071
Restricted stock awards vested				14,000
Weighted average fair value awards granted				$ 78.01
Additional Equity Awards Available for Future Grant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock reserved for future issuance under stock options (in shares)				4,729,971
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock incentive plan, shares authorized (in shares)		1,215,789
Shares of common stock reserved for future issuance under stock options (in shares)				339,552
Employee stock purchase plan, purchase price as a percentage of common stock closing price		85.00%
Employee stock purchase plan, shares issued				109,590	78,085	103,666
Weighted average fair value awards granted				$ 25.51	$ 35.67	$ 17.10
Expected volatility				52.00%	63.00%	91.00%
Expected option life				6 months	6 months	6 months
Risk-free interest rate, approximate				2.50%	2.00%	0.70%
Performance-based stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares)		25,000
Aggregate intrinsic value of stock option exercised				$ 11,000,000	$ 8,000,000	$ 1,800,000
Stock options vested (in shares)				889,896	763,982	614,796
Performance-based stock options | On-hire executive grant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares)					25,000	25,000
Vesting period					1 year
Non-qualified stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option vesting percentage on the first anniversary of grant date				25.00%
Option vesting percentage at the end of each successive three-month period				6.25%
Stock options granted (in shares)		125,000	50,000
Time-based stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option vesting percentage on the first anniversary of grant date				25.00%
Option vesting percentage at the end of each successive three-month period				6.25%
Weighted average fair value of stock options granted (in dollars per share)				$ 49.27	$ 66.49	$ 44.76
Aggregate intrinsic value of stock option exercised				$ 55,400,000	$ 87,100,000	$ 111,300,000
January 2016 Performance-Based Restricted Stock Unit
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 11,800,000
Restricted stock awards vested	124,833
Restricted stock awards vested, fair value	$ 11,400,000
Performance-based restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock awards vested				13,000
Weighted average fair value awards granted				$ 85.01
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected option life				5 years 7 months 6 days	5 years 8 months 12 days	5 years 8 months 12 days
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected option life				7 years 3 months 18 days	7 years 2 months 12 days	7 years 2 months 12 days
Stock Incentive Plan 2009
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock incentive plan, shares authorized (in shares)		15,480,000						11,700,000
Stock Incentive Plan 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock incentive plan, shares authorized (in shares)							3,500,000
Stock incentive plan, additional shares authorized (in shares)				3,290,000
Stock Incentive Plan 2018 | Performance-based restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense				$ 20,700,000

Stock-Based Compensation - Expe

Stock-Based Compensation - Expenses included in operating costs and expenses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 174,841	$ 157,752	$ 92,819
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	88,930	80,509	51,872
Selling, general and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 85,911	$ 77,243	$ 40,947

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Less: Stock-based compensation expense capitalized to inventory	$ (3,403)	$ (1,424)	$ 0
Total stock-based compensation expense	174,841	157,752	92,819
Time-based stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expensed and capitalized	99,097	83,403	61,802
Performance-based stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expensed and capitalized	48,207	56,419	23,260
Time-based restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expensed and capitalized	2,351	538	541
Performance-based restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expensed and capitalized	22,123	13,144	0
ESPP share issuances
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expensed and capitalized	3,482	2,842	2,155
Other Equity Programs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expensed and capitalized	1,709	1,345	1,946
Non-employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expensed and capitalized	$ 1,275	$ 1,485	$ 3,115

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Unrecognized Stock-Based Compensation Expense, Net of Estimated Forfeitures (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Time-based stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized expense, net of estimated forfeitures	$ 184,062
Weighted-average Recognition Period	2 years 7 months 13 days
Performance-based stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized expense, net of estimated forfeitures	$ 5,997
Time-based restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized expense, net of estimated forfeitures	$ 7,014
Weighted-average Recognition Period	1 year 29 days
Performance-based restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized expense, net of estimated forfeitures	$ 12,217
ESPP share issuances
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized expense, net of estimated forfeitures	$ 903
Weighted-average Recognition Period	3 months 29 days

Stock-Based Compensation - Valu

Stock-Based Compensation - Valuation Assumptions for Stock Options (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum	1.40%	2.70%	1.90%
Risk-free interest rate, maximum	2.60%	2.90%	2.30%
Expected dividend yield	0.00%	0.00%	0.00%
Expected volatility, minimum	63.00%	64.00%	61.00%
Expected volatility, maximum	66.00%	67.00%	67.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected option life	5 years 7 months 6 days	5 years 8 months 12 days	5 years 8 months 12 days
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected option life	7 years 3 months 18 days	7 years 2 months 12 days	7 years 2 months 12 days

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock Option Activity (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)$ / sharesshares
Time-based stock options
Number of Options
Outstanding, beginning balance (in shares) | shares	9,829
Granted (in shares) | shares	3,073
Exercised (in shares) | shares	(1,105)
Cancelled (in shares) | shares	(919)
Outstanding, ending balance (in shares) | shares	10,878
Exercisable at December 31, 2019 (in shares) | shares	6,196
Vested or expected to vest at December 31, 2019 (in shares) | shares	10,434
Weighted-average Exercise Price
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 72.48
Granted (in dollars per share) | $ / shares	83.09
Exercised (in dollars per share) | $ / shares	43.84
Cancelled (in dollars per share) | $ / shares	89.94
Outstanding, ending balance (in dollars per share) | $ / shares	76.92
Exercisable at December 31, 2019 (in dollars per share) | $ / shares	69.44
Vested or expected to vest at December 31, 2019 (in dollars per share) | $ / shares	$ 76.47
Weighted-average Remaining Contractual Term
Outstanding	6 years 8 months 1 day
Exercisable	5 years 2 months 15 days
Vested or expected to vest	6 years 6 months 25 days
Aggregate Intrinsic Value
Outstanding | $	$ 428,124
Exercisable | $	290,437
Vested or expected to vest | $	$ 415,443
Performance-based stock options
Number of Options
Outstanding, beginning balance (in shares) | shares	2,744
Granted (in shares) | shares	0
Exercised (in shares) | shares	(271)
Cancelled (in shares) | shares	(282)
Outstanding, ending balance (in shares) | shares	2,191
Exercisable at December 31, 2019 (in shares) | shares	1,956
Weighted-average Exercise Price
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 84.75
Granted (in dollars per share) | $ / shares	0
Exercised (in dollars per share) | $ / shares	56.18
Cancelled (in dollars per share) | $ / shares	96.57
Outstanding, ending balance (in dollars per share) | $ / shares	86.77
Exercisable at December 31, 2019 (in dollars per share) | $ / shares	$ 82.89
Weighted-average Remaining Contractual Term
Outstanding	6 years 4 months 6 days
Exercisable	6 years 2 months 1 day
Aggregate Intrinsic Value
Outstanding | $	$ 65,009
Exercisable | $	$ 65,009

Stock-Based Compensation - Acti

Stock-Based Compensation - Activity of Restricted Stock Units and Performance-Based Restricted Stock Units (Details) shares in Thousands	12 Months Ended
	Dec. 31, 2019$ / sharesshares
Restricted stock units
Number of Units
Outstanding, beginning balance (in shares) | shares	15
Granted (in shares) | shares	127
Vested (in shares) | shares	(14)
Cancelled (in shares) | shares	(1)
Outstanding, ending balance (in shares) | shares	127
Weighted-average Grant Date Fair Value
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 89.44
Granted (in dollars per share) | $ / shares	78.01
Vested (in dollars per share) | $ / shares	95.66
Cancelled (in dollars per share) | $ / shares	89.82
Outstanding, ending balance (in dollars per share) | $ / shares	$ 77.23
Performance-based restricted stock units
Number of Units
Outstanding, beginning balance (in shares) | shares	22
Granted (in shares) | shares	685
Vested (in shares) | shares	(13)
Cancelled (in shares) | shares	(71)
Outstanding, ending balance (in shares) | shares	623
Weighted-average Grant Date Fair Value
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 91.72
Granted (in dollars per share) | $ / shares	85.01
Vested (in dollars per share) | $ / shares	79.14
Cancelled (in dollars per share) | $ / shares	85
Outstanding, ending balance (in dollars per share) | $ / shares	$ 85.36

Income Taxes - Schedule of Dome

Income Taxes - Schedule of Domestic and Foreign Components of Loss before Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Domestic	$ (597,602)	$ (573,245)	$ (378,293)
Foreign	(288,514)	(187,429)	(112,581)
Loss before income taxes	$ (886,116)	$ (760,674)	$ (490,874)

Income Taxes - Schedule of Prov

Income Taxes - Schedule of Provision for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current provision:
Domestic	$ 394	$ 0	$ 0
Foreign	(3,232)	(1,611)	0
Total current provision	(2,838)	(1,611)	0
Deferred benefit:
Federal	(394)	788	0
Foreign	2,369	0	0
Total deferred benefit	1,975	788	0
Total provision for income taxes	$ (863)	$ (823)	$ 0

Income Taxes - Schedule of Comp

Income Taxes - Schedule of Components of Net Deferred Tax (Liability) Asset (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforwards	$ 627,466	$ 506,574
Research and development and ODC credits	261,616	220,776
AMT credits	394	788
Lease liability	69,334	0
Deferred compensation	75,058	57,268
Intangible assets	66,615	62,260
Other	13,266	20,660
Total deferred tax assets	1,113,749	868,326
Deferred tax liabilities:
Property, plant and equipment, net	(10,077)	(6,550)
Unrealized gain on marketable securities	(3,932)	(903)
Right of use assets	(50,294)	0
Deferred tax asset valuation allowance	(1,046,013)	(860,085)
Net deferred tax asset	$ 3,433	$ 788

Income Taxes - Schedule of Effe

Income Taxes - Schedule of Effective Income Tax Rate Differs from Statutory Federal Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, Percent
At U.S. federal statutory rate	21.00%	21.00%	35.00%
State taxes, net of federal effect	3.60%	2.10%	3.80%
Stock-based compensation	0.00%	0.80%	3.30%
Tax credits	3.70%	4.20%	9.90%
Orphan drug credit	0.00%	0.00%	(3.40%)
Other permanent items	(0.30%)	(0.30%)	(1.00%)
Foreign rate differential	(6.90%)	(5.30%)	(8.10%)
Tax reform change	0.00%	0.00%	(46.50%)
Revaluation of deferred credits due to rate change	0.00%	(3.50%)	0.00%
Other	(0.10%)	0.00%	(0.90%)
Valuation allowance	(21.00%)	(19.00%)	7.90%
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes [Line Items]
Increase (decrease) in valuation allowance	$ 185,900	$ 171,800	$ 112,900
Provision for income taxes	863	823	0
Foreign income taxes	3,232	1,611	0
Deferred tax benefit in foreign jurisdictions	2,369	$ 0	$ 0
Federal
Income Taxes [Line Items]
Net operating loss carryforward	2,400,000
Net operating loss carryforwards, not subject to expiration	900,000
Net operating loss carryforwards, subject to expiration	1,500,000
State and local jurisdiction
Income Taxes [Line Items]
Net operating loss carryforward	$ 2,100,000
Earliest tax year
Income Taxes [Line Items]
Open tax year	2016
Latest tax year
Income Taxes [Line Items]
Open tax year	2019
Research and Development, Including Orphan Drug, and State Investments Tax Credit | Federal
Income Taxes [Line Items]
Tax credit carryforwards	$ 240,300
Research and Development, Including Orphan Drug, and State Investments Tax Credit | State and local jurisdiction
Income Taxes [Line Items]
Tax credit carryforwards	27,000
Alternative minimum tax
Income Taxes [Line Items]
Tax credit carryforwards	$ 400
Tax credits refundable year	2021

Income Taxes - Unrecognized Tax

Income Taxes - Unrecognized Tax Benefit Activity (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]
Unrecognized tax benefits at the beginning of the year	$ 0	$ 0
Gross increases - current period tax positions	305	0
Unrecognized tax benefits at the end of the year	$ 305	$ 0

Defined Benefit Plans - Additio

Defined Benefit Plans - Additional Information (Details) $ in Millions	Dec. 31, 2019USD ($)
Other Liabilities | Unfunded Plan
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	$ 4.3

Schedule of Quarterly Financial

Schedule of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	Jan. 01, 2018	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
Selected Quarterly Financial Information [Abstract]
Revenues	$ 2,900	$ 71,681	$ 70,061	$ 44,714	$ 33,294	$ 21,033	$ 2,069	$ 29,907	$ 21,899	$ 219,750	$ 74,908	$ 89,912
Operating costs and expenses		369,754	286,360	280,985	222,082	241,389	256,627	222,261	169,304	1,159,181	889,581	590,000
Net loss		$ (276,185)	$ (208,535)	$ (219,481)	$ (181,915)	$ (211,441)	$ (245,282)	$ (163,560)	$ (141,214)	$ (886,116)	$ (761,497)	$ (490,874)
Net loss per common share - basic and diluted (in dollars per share)		$ (2.47)	$ (1.92)	$ (2.02)	$ (1.73)	$ (2.09)	$ (2.43)	$ (1.63)	$ (1.41)	$ (8.11)	$ (7.57)	$ (5.42)
Weighted-average common shares — basic and diluted (in shares)		111,750	108,701	108,576	105,400	101,066	100,783	100,519	99,979	109,264	100,590	90,554

Uncategorized Items - alny-2019

Label	Element	Value
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 68,210,000
Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 68,210,000