Alnylam Pharmaceuticals (ALNY) 10-K 11 Feb 21 2020 FY Annual report Financial data

Cover

Cover - USD ($)	12 Months Ended
	Dec. 31, 2020	Feb. 05, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Current Fiscal Year End Date	--12-31
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
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 16,993,028,024
Entity Common Stock, Shares Outstanding		117,002,019
Documents Incorporated by Reference	Portions of the registrant’s definitive proxy statement for its 2021 annual meeting of stockholders, which the registrant intends to file pursuant to Regulation 14A with the Securities and Exchange Commission not later than 120 days after the registrant’s fiscal year end of December 31, 2020, are incorporated by reference into Part II, Item 5 and Part III of this Form 10-K.
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Central Index Key	0001178670

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 496,580	$ 547,178
Marketable debt securities	1,333,182	975,017
Marketable equity securities	44,633	13,967
Accounts receivable, net	102,413	43,011
Inventory	75,202	56,348
Prepaid expenses and other current assets	62,767	80,343
Receivable related to the sale of future royalties	500,000	0
Total current assets	2,614,777	1,715,864
Property, plant and equipment, net	465,029	425,179
Operating lease right-of-use assets	241,485	221,197
Restricted investments	40,725	14,825
Other assets	45,045	18,069
Total assets	3,407,061	2,395,134
Current liabilities:
Accounts payable	51,966	49,884
Accrued expenses	355,909	197,201
Operating lease liability	36,872	27,688
Deferred revenue	127,207	77,821
Liability related to the sale of future royalties	13,316	0
Total current liabilities	585,270	352,594
Operating lease liability, net of current portion	293,039	276,135
Deferred revenue, net of current portion	225,094	318,383
Long-term debt	191,278	0
Liability related to the sale of future royalties, net of current portion	1,058,225	0
Other liabilities	37,908	9,330
Total liabilities	2,390,814	956,442
Commitments and contingencies (Note 12)
Stockholders’ equity:
Preferred stock, $0.01 par value per share, 5,000 shares authorized and no shares issued and outstanding as of December 31, 2020 and December 31, 2019	0	0
Common stock, $0.01 par value per share, 250,000 and 250,000 shares authorized as of December 31, 2020 and December 31, 2019, respectively; 116,427 shares issued and outstanding as of December 31, 2020; 112,188 shares issued and outstanding as of December 31, 2019	1,164	1,122
Additional paid-in capital	5,644,074	5,201,176
Accumulated other comprehensive loss	(43,622)	(36,518)
Accumulated deficit	(4,585,369)	(3,727,088)
Total stockholders’ equity	1,016,247	1,438,692
Total liabilities and stockholders’ equity	$ 3,407,061	$ 2,395,134

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
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	250,000,000
Common stock, shares issued (in shares)	116,427,000	112,188,000
Common stock, shares outstanding (in shares)	116,427,000	112,188,000

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues:
Revenues	$ 492,853	$ 219,750	$ 74,908
Operating costs and expenses:
Cost of goods sold	78,052	25,062	1,802
Research and development	654,819	655,114	505,420
Selling, general and administrative	588,420	479,005	382,359
Total operating costs and expenses	1,321,291	1,159,181	889,581
Loss from operations	(828,438)	(939,431)	(814,673)
Other (expense) income:
Interest expense	(84,496)	0	0
Interest income	11,809	33,448	29,262
Other income, net	45,525	20,730	24,737
Total other (expense) income	(27,162)	54,178	53,999
Loss before income taxes	(855,600)	(885,253)	(760,674)
Provision for income taxes	(2,681)	(863)	(823)
Net loss	$ (858,281)	$ (886,116)	$ (761,497)
Net loss per common share - basic and diluted (in dollars per share)	$ (7.46)	$ (8.11)	$ (7.57)
Weighted-average common shares used to compute basic and diluted net loss per common share (in shares)	114,986	109,264	100,590
Statements of Comprehensive Loss
Net loss	$ (858,281)	$ (886,116)	$ (761,497)
Other comprehensive (loss) income:
Unrealized gain on marketable securities	211	558	1,220
Foreign currency translation loss	(7,081)	(343)	0
Defined benefit pension plans, net of tax	(234)	(3,520)	0
Comprehensive loss	(865,385)	(889,421)	(760,277)
Net Revenues from Collaborators
Revenues:
Revenues	131,333	53,213	62,373
Product Revenues, Net
Revenues:
Revenues	$ 361,520	$ 166,537	$ 12,535

CONSOLIDATED STATEMENTS OF STOC

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive (Loss) Income	Accumulated Deficit	Accumulated DeficitCumulative Effect, Period of Adoption, Adjustment
Beginning Balance (in shares) at Dec. 31, 2017			99,667,000
Beginning Balance at Dec. 31, 2017	$ 1,766,431	$ 68,210	$ 997	$ 3,947,552	$ (34,433)	$ (2,147,685)	$ 68,210
Exercise of common stock options, net of tax withholdings (in shares)			1,268,000
Exercise of common stock options, net of tax withholdings	60,743		$ 12	60,731
Issuance of common stock under equity plans (in shares)			212,000
Issuance of common stock under equity plans	5,046		$ 2	5,044
Issuance of common stock under benefit plans (in shares)			30,000
Issuance of common stock under benefit plans	3,170		$ 0	3,170
Stock-based compensation expense related to equity-classified awards	158,642			158,642
Other comprehensive gain, net of tax	1,220				1,220
Net loss	(761,497)					(761,497)
Ending Balance (in shares) at Dec. 31, 2018			101,177,000
Ending Balance at Dec. 31, 2018	1,301,965		$ 1,011	4,175,139	(33,213)	(2,840,972)
Exercise of common stock options, net of tax withholdings (in shares)			1,374,000
Exercise of common stock options, net of tax withholdings	63,499		$ 15	63,484
Issuance of common stock under equity plans (in shares)			132,000
Issuance of common stock under equity plans	7,909		$ 1	7,908
Issuance of common stock under benefit plans (in shares)			61,000
Issuance of common stock under benefit plans	5,033		$ 1	5,032
Issuance of common stock, net of offering costs (in shares)			9,444,000
Issuance of common stock, net of offering costs	772,477		$ 94	772,383
Stock-based compensation expense related to equity-classified awards	177,230			177,230
Other comprehensive gain, net of tax	(3,305)				(3,305)
Net loss	$ (886,116)					(886,116)
Ending Balance (in shares) at Dec. 31, 2019	112,188,000		112,188,000
Ending Balance at Dec. 31, 2019	$ 1,438,692		$ 1,122	5,201,176	(36,518)	(3,727,088)
Exercise of common stock options, net of tax withholdings (in shares)			2,926,000
Exercise of common stock options, net of tax withholdings	189,371		$ 28	189,343
Issuance of common stock under equity plans (in shares)			350,000
Issuance of common stock under equity plans	11,083		$ 4	11,079
Issuance of common stock under benefit plans (in shares)			963,000
Issuance of common stock under benefit plans	99,498		$ 10	99,488
Stock-based compensation expense related to equity-classified awards	142,988			142,988
Other comprehensive gain, net of tax	(7,104)				(7,104)
Net loss	$ (858,281)					(858,281)
Ending Balance (in shares) at Dec. 31, 2020	116,427,000		116,427,000
Ending Balance at Dec. 31, 2020	$ 1,016,247		$ 1,164	$ 5,644,074	$ (43,622)	$ (4,585,369)

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net loss	$ (858,281)	$ (886,116)	$ (761,497)
Non-cash adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	34,772	17,175	15,248
Amortization and interest accretion related to operating leases	39,663	37,193	0
Non-cash interest expense on liability related to the sale of future royalties	84,496	0	0
Stock-based compensation	139,873	174,841	157,752
Realized and unrealized gain on marketable equity securities	(54,042)	(11,288)	(3,564)
Change in fair value of liability obligation	0	(9,422)	0
Gain on litigation settlement	0	0	(10,000)
Other	11,259	(3,191)	(5,654)
Changes in operating assets and liabilities:
Accounts receivable, net	(56,236)	(24,238)	15,242
Proceeds from landlord lease incentive for tenant improvements	5,550	30,170	25,350
Inventory	(35,426)	(32,411)	(22,645)
Prepaid expenses and other assets	15,230	(22,042)	(35,067)
Accounts payable, accrued expenses and other liabilities	143,732	92,354	74,835
Deferred revenue	(43,965)	392,251	(12,616)
Operating lease liability	(41,586)	(33,703)	0
Net cash used in operating activities	(614,961)	(278,427)	(562,616)
Cash flows from investing activities:
Purchases of property, plant and equipment	(70,361)	(140,156)	(126,887)
Purchases of marketable securities	(2,025,626)	(2,075,925)	(1,104,046)
Sales and maturities of marketable securities	1,691,669	1,775,404	1,518,703
Proceeds from maturity of restricted investments	0	30,000	0
Purchases of restricted investments	(25,900)	0	(14,825)
Other	(5,300)	(7,000)	0
Net cash (used in) provided by investing activities	(435,518)	(417,677)	272,945
Cash flows from financing activities:
Proceeds from exercise of stock options and other types of equity, net	200,484	71,284	65,470
Proceeds from the sale of future royalties	500,000	0	0
Proceeds from development derivative	8,400	0	0
Offering proceeds, net of costs	0	381,900	0
Proceeds from issuance of term loan	200,000	0	0
Repayment of term loan	0	(30,000)	0
Proceeds from issuance of common stock to strategic partners, net of closing costs	99,498	400,000	0
Payment of transaction costs related to sale of future royalties and term loan	(13,403)	0	0
Net cash provided by financing activities	994,979	823,184	65,470
Effect of exchange rate changes on cash, cash equivalents and restricted cash	4,918	(83)	0
Net (decrease) increase in cash, cash equivalents and restricted cash	(50,582)	126,997	(224,201)
Cash, cash equivalents and restricted cash, beginning of period	549,628	422,631	646,832
Cash, cash equivalents and restricted cash, end of period	499,046	549,628	422,631
Supplemental disclosure of noncash investing and financing activities:
Capital expenditures included in accounts payable and accrued expenses	14,518	14,876	33,274
Lease liabilities arising from obtaining right-of-use assets	34,435	4,530	0
Receivable and liability related to the sale of future royalties	$ 500,000	$ 0	$ 0

NATURE OF BUSINESS

NATURE OF BUSINESS	12 Months Ended
	Dec. 31, 2020
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 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., and also received marketing authorization for ONPATTRO from the European Commission, or EC. As of December 31, 2020, we have launched ONPATTRO in the U.S., Europe, Japan, Canada and several additional countries. In the fourth quarter of 2019, we received approval for GIVLAARI from the FDA and began commercializing and generating product revenues in the U.S. In 2020, we obtained additional regulatory approval from the EC for GIVLAARI and expanded into European and other markets. In the fourth quarter of 2020, we received regulatory approval from the FDA and EC for OXLUMO and began recording net product revenues subsequent to commercial launch. Regulatory filings in additional markets are pending or planned for 2021 and beyond for each of our commercial products. In 2020, we entered into a broad strategic financing collaboration with The Blackstone Group Inc. and certain of its affiliates which includes a purchase and sale agreement, a credit agreement, a funding agreement for the clinical development of vutrisiran and ALN-AGT and a stock purchase agreement, under which The Blackstone Group Inc. and certain of its affiliates will provide up to $2.00 billion to support our advancement of innovative RNAi therapeutics. Each executed agreement is a separate unit of account and was recorded at fair value. Please read Note 5, Note 7, Note 8 and Note 13, respectively, for additional information regarding each executed agreement set forth above.

SUMMARY OF SIGNIFICANT ACCOUNTI

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
	Dec. 31, 2020
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, we use estimates and assumptions related to our inventory valuation and related reserves, liability related to the sale of future royalties, development derivative liability, 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. The full extent to which the COVID-19 pandemic will directly or indirectly impact our business, results of operations and financial condition, including sales, expenses, reserves and allowances, the supply of our products and product candidates, clinical trials and research and development costs, will depend on future developments that are highly uncertain, including as a result of new information that may emerge concerning COVID-19 and variants thereof, and the actions taken to contain or treat it or vaccinate against it, as well as the economic impact on local, regional, national and international customers and markets. We have made estimates of the impact of COVID-19 within our financial statements and there may be changes to those estimates in future periods. Actual results may differ from these 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 securities as of December 31, 2020, together with the cash we expect to generate from product sales and under our current alliances, including our strategic financing collaboration with The Blackstone Group Inc. and certain of its affiliates, 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 securities. As of December 31, 2020 and 2019, substantially all of our cash, cash equivalents and marketable 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, 2020, 2019 and 2018, our revenues were generated primarily from product sales to distributors and collaborations with strategic partners. For the years ended December 31, 2020, 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: Year Ended December 31, 2020 2019 2018 Distributor A 31 % 44 % 13 % Regeneron Pharmaceuticals 12 % * * Sanofi Genzyme * * 58 % Vir Biotechnology * * 16 % __________________________________________ * Represents less than 10% The following table summarizes customers with amounts due that represent 10% or greater of our consolidated gross accounts receivable balance: As of December 31, 2020 2019 Distributor A 19 % 28 % Novartis AG 16 % * Distributor B 14 % 10 % Regeneron Pharmaceuticals 11 % * Sanofi Genzyme * 14 % __________________________________________ * 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 if an impairment loss exists, 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, 2020, 2019 or 2018. 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. 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, 2020 and 2019, based on our estimation of expected write-offs, we determined an allowance for doubtful accounts was not material. 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 and classified based on the anticipation of when it will be consumed either within our normal operating cycle (short-term) or beyond (long-term). 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, 2020, 2019 and 2018, we recorded $30.2 million, $16.6 million and $12.8 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 Leases Effective January 1, 2019, we adopted Accounting Standards Update, or ASU, 2016-02, Leases Topic 842, or ASC 842, using a modified retrospective basis and utilizing the effective date as the date of initial application. We determine if an arrangement is a lease at contract inception based on the facts and circumstances present in the arrangement. All our leases are classified as operating leases under the new leasing standard. W e record operating lease assets and lease liabilities in our consolidated balance sheets. Operating lease assets represent our right to use an underlying asset for the lease term and operating lease liabilities represent our obligation to make lease payments arising from the leasing arrangement. Operating lease assets and operating lease liabilities are recognized at commencement date based on the present value of lease payments over the lease term. As most of our leases do not provide an implicit rate, in determining the operating lease liabilities, we use an estimate of our incremental borrowing rate based on the information available at commencement. Lease expense for lease payments is recognized on a straight-line basis over the lease term. Short-term leases, or leases that have a lease term of 12 months or less at commencement date, are excluded from this treatment and are recognized on a straight-line basis over the term of the lease. Clinical 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, 2020 and 2019, we had not capitalized any costs to obtain any of our contracts. Net Product Revenues Our net product revenues 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. We record reserves, based on contractual terms, for 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. The following are the components of variable consideration related to product revenues: 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 within each customer agreement. 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. Net Revenues from Collaborations 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, Net Revenues from Collaborations. 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 (less than 12 months) or long-term (more than 12 months) deferred revenue based on our best estimate of when 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, amortization of intangible assets associated with the sale of our products and costs related to sales of product supply under our collaboration agreements. 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 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. 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 have recorded no interest and penalty expense related to uncertain tax positions for the years ended December 31, 2020, 2019 or 2018. 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, certain 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, 2020, 2019 and 2018, we charged to research and development expense costs associated with license fees of $2.8 million, $37.0 million and $8.0 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. Liability Related to the Sale of Future Royalties We account for the liability related to the sale of future royalties as a debt financing, as we have significant continuing involvement in the generation of the cash flows. Interest on the liability related to the sale of future royalties will be recognized using the effective interest rate method over the life of the related royalty stream. The liability related to the sale of future royalties and the related interest expense are based on our current estimates of future royalties and commercial milestones expected to be paid over the life of the arrangement, which we determine by using third-party forecasts of inclisiran’s global net revenue. We will periodically assess the expected payments and to the extent the amount or timing of our future estimated payments is materially different than our previous estimates, we will account for any such change by adjusting the liability related to the sale of future royalties and prospectively recognizing the related non-cash interest expense. Development Derivative Liability Development derivative liability is recorded at fair value based on the probability weighted present value of the estimated cash flows pursuant to contractual terms of the funding agreement. The liability is remeasured quarterly with any change in fair value recorded in other income (expense) on the consolidated statements of operations and 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 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 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: As of December 31, (In thousands) 2020 2019 2018 Options to purchase common stock 11,692 13,069 12,573 Unvested restricted common stock 1,160 749 36 Total 12,852 13,818 12,609 Segment Information We operate in a single reporting segment, the discovery, development and commercialization o

NET PRODUCT REVENUES

NET PRODUCT REVENUES	12 Months Ended
	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
NET PRODUCT REVENUES	NET PRODUCT REVENUES Net product revenues by geography consist of the following: Year Ended December 31, (In thousands) 2020 2019 2018 ONPATTRO United States $ 151,574 $ 116,302 $ 8,589 Europe 107,755 43,980 3,946 Rest of World (primarily Japan) 46,752 6,105 — Total $ 306,081 $ 166,387 $ 12,535 GIVLAARI United States $ 42,797 $ 150 $ — Europe 12,000 — — Rest of World 309 — — Total $ 55,106 $ 150 $ — OXLUMO Europe $ 333 $ — $ — Total net product revenues $ 361,520 $ 166,537 $ 12,535 As of December 31, 2020 and 2019, net product revenue-related receivables of $68.9 million and $28.1 million , respectively, were included in “Accounts receivable, net.” The following table summarizes balances and activity in each product revenue allowance and reserve category: As of December 31, 2020 (In thousands) Chargebacks and Rebates Trade Discounts and Allowances Returns Reserve and Other Incentives Total Beginning balance $ 32,487 $ 410 $ 1,978 $ 34,875 Provision related to current period sales 103,706 4,650 5,702 114,058 Credit or payments made during the period for current year sales (42,493) (4,388) (2,704) (49,585) Credit or payments made during the period for prior year sales (2,995) (33) (1,213) (4,241) Total $ 90,705 $ 639 $ 3,763 $ 95,107 As of December 31, 2019 (In thousands) 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 (109) (218) (220) (547) Total $ 32,487 $ 410 $ 1,978 $ 34,875

NET REVENUES FROM COLLABORATION

NET REVENUES FROM COLLABORATIONS	12 Months Ended
	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
NET REVENUES FROM COLLABORATIONS	NET REVENUES FROM COLLABORATIONS The following table summarizes our total consolidated net revenues from collaborations: Year Ended December 31, (In thousands) 2020 2019 2018 Regeneron Pharmaceuticals $ 74,072 $ 26,075 $ — Vir Biotechnology 31,396 12,809 12,778 Novartis AG 22,208 2,315 2,789 Sanofi Genzyme 995 10,976 46,000 Other 2,662 1,038 806 Total $ 131,333 $ 53,213 $ 62,373 The following table presents the balance of our receivables and contract liabilities related to our collaboration agreements: As of December 31, (In thousands) 2020 2019 Receivables included in "Accounts receivable, net" $ 33,542 $ 14,929 Contract liabilities included in "Deferred revenue" 120,021 153,117 We recognized revenue of $54.4 million and $4.0 million in the years ended December 31, 2020 and 2019, respectively, that was included in the contract liability balance at the beginning of the period. 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: Year Ended December 31, 2020 2019 2018 (In thousands) Clinical Trial and Manufacturing External Services Other Clinical Trial and Manufacturing External Services Other Clinical Trial and Manufacturing External Services Other Regeneron $ 13,302 $ 171 $ 44,360 $ 2,793 $ 344 $ 21,779 $ — $ — $ — Sanofi Genzyme 644 181 2,132 11,505 334 2,017 36,600 5,340 1,279 Vir 18,470 584 11,590 10,353 381 4,745 7,272 8,251 548 Novartis 999 — 700 2,025 — 696 1,664 2 203 Other — — — — — — — 2,150 1,097 Total $ 33,415 $ 936 $ 58,782 $ 26,676 $ 1,059 $ 29,237 $ 45,536 $ 15,743 $ 3,127 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 revenues from collaborations. For the years ended December 31, 2020, 2019 and 2018, we did not incur material selling, general and administrative expenses related to our collaboration agreements. Product Alliances Regeneron Pharmaceuticals, Inc. 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. 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 2 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. Reimbursement of our share of costs will be recognized as a reduction to research and development expense in the consolidated statements of operations and comprehensive loss. During the year ended December 31, 2020 and 2019, we recognized $5.4 million and $0.0 million, respectively, as a reduction to research and development expense. 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 as of December 31, 2020, the total transaction price was determined to be $531.8 million, a decrease of $23.3 million from December 31, 2019. As of December 31, 2020, the transaction price is comprised of the upfront payment and variable consideration related to development, manufacture and supply activities. 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, is accounted for as collaboration revenue. The following tables provide 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: Transaction Price Allocated Deferred Revenue Performance Obligations As of December 31, 2020 As of December 31, 2020 As of December 31, 2019 Accounting Guidance Research Services Obligation $ 200,600 $ 54,900 $ 84,800 ASC 606 C5 License Obligation 85,200 58,700 65,800 ASC 606 C5 Co-Co Obligation 246,000 231,400 243,000 ASC 808 Total $ 531,800 $ 345,000 $ 393,600 Revenue Recognized During Performance Obligations Year Ended December 31, 2020 Year Ended December 31, 2019 Accounting Guidance Research Services Obligation $ 44,800 $ 21,000 ASC 606 C5 License Obligation 7,100 — ASC 606 C5 Co-Co Obligation 11,700 2,900 ASC 808 $ 63,600 $ 23,900 As of December 31, 2020, the aggregate amount of the transaction price allocated to the remaining Research Services Obligation and C5 License Obligation that was unsatisfied was $212.9 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 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 were set at 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 ASC 606 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 ASC 606 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 ASC 606 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 accounted for as a contract, including that it is probable that we will collect the consideration to which we are entitled in exchange for the goods or services that will be delivered to Sanofi Genzyme. We identified contract promises or deliverables for licenses to our intellectual property and know-how rights, associated development activities, joint steering committee participation and information exchange. We determined that, pursuant to ASC 606 (and consistent with our accounting prior to the adoption of the new revenue standard), the pe

LIABILITY RELATED TO THE SALE O

LIABILITY RELATED TO THE SALE OF FUTURE ROYALTIES	12 Months Ended
	Dec. 31, 2020
Related Party Transactions [Abstract]
LIABILITY RELATED TO SALE OF FUTURE ROYALTIES	LIABILITY RELATED TO THE SALE OF FUTURE ROYALTIES On April 10, 2020, we entered into a purchase and sale agreement, or Purchase Agreement, with BX Bodyguard Royalties L.P. (an affiliate of The Blackstone Group Inc.), or Blackstone Royalties, under which Blackstone Royalties acquired 50% of royalties payable, or Royalty Interest, with respect to net sales by MDCO, its affiliates or sublicensees of inclisiran and any other licensed products under the MDCO License Agreement, and 75% of the commercial milestone payments payable under the MDCO License Agreement, together with the Royalty Interest, referred to as the Purchased Interest. If Blackstone Royalties does not receive payments in respect of the Royalty Interest by December 31, 2029, equaling at least $1.00 billion, Blackstone Royalties will receive 55% of the Royalty Interest beginning on January 1, 2030. In consideration for the sale of the Purchased Interest, Blackstone Royalties paid us $500.0 million in April 2020 and has an unconditional obligation to pay us an additional $500.0 million on September 30, 2021, which was recorded as a receivable upon execution of the Purchase Agreement. We continue to own or control all inclisiran intellectual property rights and are responsible for certain ongoing manufacturing and supply obligations related to the generation of the Purchased Interest. Due to our continuing involvement, we will continue to account for any royalties and commercial milestones due to us under the MDCO License Agreement as revenue on our consolidated statement of operations and comprehensive loss and record the proceeds from this transaction as a liability, net of closing costs, on our consolidated balance sheet. In order to determine the amortization of the liability related to the sale of future royalties, we are required to estimate the total amount of future payments to Blackstone Royalties over the life of the Purchase Agreement. The $1.00 billion liability, recorded at execution of the agreement, will be accreted to the total of these royalty and commercial milestone payments as interest expense over the life of the Purchase Agreement. At execution and as of December 31, 2020, our estimate of this total interest expense resulted in an effective annual interest rate of 11%. This estimate contains assumptions that impact both the amount recorded at execution and the interest expense that will be recognized in future periods. As payments are made to Blackstone Royalties, the balance of the liability will be effectively repaid over the life of the Purchase Agreement. The exact timing and amount of repayment is likely to change each reporting period. A significant increase or decrease in net sales of inclisiran will materially impact the liability related to the sale of future royalties, interest expense and the time period for repayment. We will periodically assess the expected payments to Blackstone Royalties and to the extent the amount or timing of such payments is materially different than our initial estimates, we will prospectively adjust the amortization of the liability related to the sale of future royalties and the related interest expense. As of December 31, 2020, the carrying value of the liability related to the sale of future royalties was $1.07 billion, net of closing costs of $13.0 million. The carrying value of the liability related to the sale of future royalties approximates fair value as of December 31, 2020 and is based on our current estimates of future royalties and commercial milestones expected to be paid to Blackstone Royalties over the life of the arrangement, which are considered Level 3 inputs. For the year ended December 31, 2020, we recognized interest expense of $84.5 million. The following table shows the activity with respect to the liability related to the sale of future royalties, in thousands: Liability related to the sale of future royalties as of April 10, 2020 $ 1,000,000 Capitalized closing costs (12,955) Interest expense recognized 84,496 Carrying value of liability related to sale of future royalties as of December 31, 2020 $ 1,071,541

OTHER BALANCE SHEET DETAILS

OTHER BALANCE SHEET DETAILS	12 Months Ended
	Dec. 31, 2020
Balance Sheet Related Disclosures [Abstract]
OTHER BALANCE SHEET DETAILS	OTHER BALANCE SHEET DETAILS Inventory The components of inventory are summarized as follows: As of December 31, (In thousands) 2020 2019 Raw materials $ 63,460 $ 15,418 Work in process 16,149 38,275 Finished goods 12,693 2,655 Total inventory $ 92,302 $ 56,348 As of December 31, 2020, we had long-term inventory of $17.1 million in other assets in our consolidated balance sheet as we anticipate such inventory being consumed beyond our normal operating cycle. As of December 31, 2019, we had no long-term inventory. As of December 31, 2020 and 2019, there was no capitalized inventory for products awaiting regulatory approval. Property, Plant and Equipment, Net Property, plant and equipment, net consist of the following: As of December 31, (In thousands) 2020 2019 Buildings $ 262,637 $ 250,380 Leasehold improvements 149,505 132,632 Laboratory equipment 48,930 29,755 Manufacturing equipment 41,089 — Construction in progress 28,005 54,195 Computer equipment and software 19,064 14,956 Furniture and fixtures 11,066 10,339 Land 9,080 9,080 569,376 501,337 Less: accumulated depreciation (104,347) (76,158) Total $ 465,029 $ 425,179 Accrued Expenses Accrued expenses consist of the following: As of December 31, (In thousands) 2020 2019 Compensation and related $ 97,433 $ 68,304 Product rebates and discounts 94,242 32,670 Pre-clinical, clinical trial and manufacturing 46,506 34,269 Contingent liabilities 41,216 — Licensing and collaboration agreements 15,424 20,622 Consulting and professional services 11,501 14,251 Other 49,587 27,085 Total $ 355,909 $ 197,201 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: As of December 31, (In thousands) 2020 2019 2018 Cash and cash equivalents $ 496,580 $ 547,178 $ 420,146 Total restricted cash included in prepaid expenses, other current assets and long-term other assets 2,466 2,450 2,485 Total cash, cash equivalents, and restricted cash shown in the consolidated $ 499,046 $ 549,628 $ 422,631 Accumulated Other Comprehensive (Loss) Income The following table summarizes the changes in accumulated other comprehensive (loss) income, by component: (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, 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) Other comprehensive (loss) income before reclassifications — (531) (14) (7,081) (7,626) Amounts reclassified from other comprehensive income — 297 225 — 522 Net other comprehensive (loss) income — (234) 211 (7,081) (7,104) Balance as of December 31, 2020 $ (32,792) $ (3,754) $ 348 $ (7,424) $ (43,622)

CREDIT AGREEMENT

CREDIT AGREEMENT	12 Months Ended
	Dec. 31, 2020
Line of Credit Facility [Abstract]
CREDIT AGREEMENT	CREDIT AGREEMENT On April 10, 2020, we entered into a credit agreement, or Credit Agreement, among us, certain of our subsidiaries (such subsidiaries, together with us, the Loan Parties), funds or accounts managed or advised by GSO Capital Partners LP (now Blackstone Alternative Credit Advisors LP) and certain other affiliates of T he Blackstone Group Inc. , and the other lenders from time to time parties thereto, collectively, the Lenders, and Wilmington Trust, National Association, as the administrative agent for the Lenders. The Credit Agreement provides for a senior secured delayed draw term loan facility, referred to as the Term Loans, which consists of three tranches providing funding up to $700.0 million. The Tranche 1 Loan of $200.0 million was drawn as of December 31, 2020 and is included in long-term debt in the consolidated balance sheets. The remaining two tranches will provide funds as follows: Tranche Requested No Later Than Aggregate Principal Amount, up to (in thousands) Tranche 2 Loan June 30, 2021 $ 250,000 Tranche 3 Loan December 31, 2021 $ 250,000 In addition, we may (a) at any time following April 10, 2021, request an increase in respect of the unfunded commitments in an amount not to exceed $50.0 million on terms to be agreed and subject to the consent of the Lenders providing such increase and/or (b) at any time prior to April 10, 2021, cancel the unfunded commitments or reallocate the unfunded commitments in respect of the Tranche 2 Loan or Tranche 3 Loan to the Tranche 1 Loan and/or the Tranche 2 Loan in an amount not to exceed $100.0 million in the aggregate for all such cancellations or reallocations. The Tranche 2 Loan will be requested no later than June 30, 2021 and the Tranche 3 Loan will be requested no later than December 31, 2021, in each case, subject to customary terms and conditions, including, in the case of the Tranche 2 Loan and Tranche 3 Loan, either (a) the first sale of inclisiran in the U.S. for end use or consumption after FDA regulatory approval thereof or (b) revenue attributable to ONPATTRO and GIVLAARI equal to or greater than $300.0 million as of the last day of the most recently ended twelve month period, referred to as the Subsequent Borrowing Conditions. As of December 31, 2020, the Subsequent Borrowing Conditions have been satisfied. The Term Loans mature in December 2027. We can elect an interest rate of either LIBOR plus 7%, subject to a floor of 1%, referred to as the LIBOR Rate, or a base rate plus 6%, subject to a floor of 2%. We may, at our option, pay interest in kind on interest due through 2023 at a rate that is 1% higher than the interest rate otherwise applicable to such Term Loan . We drew the Tranche 1 Loan in December 2020, elected a LIBOR Rate plus 7%, and paid a $5.0 million funding fee. On the date the Tranche 2 Loan or Tranche 3 Loan is funded, we will pay a funding fee equal to 2.5% of the principal amount of the Term Loans funded on such date. In addition, we will pay an exit fee equal to 1% of the commitments in respect of the Term Loans, payable upon any repayment of the Term Loans or termination of the unfunded Term Loan commitments. As of December 31, 2020, our interest rate was 8%. We are obligated to pay interest due on the Term Loans from 2021 to 2022 which will be calculated without regard to the Term Loans being prepaid or an unfunded tranche being terminated during this period (in whole or in part). Any prepayments of Term Loans or terminations of unfunded tranches that occur between 2023 and 2025 are subject to a fee of up to 5% of the loan principal that is prepaid or the amount of the unfunded tranche that is terminated. All obligations under the Credit Agreement are secured, subject to certain exceptions, by security interests in the following assets: (1) intellectual property owned by us relating to ONPATTRO, GIVLAARI and vutrisiran, (2) the equity interests held by the Loan Parties in their subsidiaries, (3) all of our ownership of the inclisiran royalty remaining after the royalty purchase under the Purchase Agreement, and (4) material real property, and certain personal property, including, without limitation, cash held in certain deposit accounts of the Loan Parties and equipment. The Credit Agreement contains negative covenants that, among other things and subject to certain exceptions, could restrict our ability to, incur additional liens, incur additional indebtedness, make investments, including acquisitions, engage in fundamental changes, sell or dispose of assets that constitute collateral, including certain intellectual property, pay dividends or make any distribution or payment on or redeem, retire or purchase any equity interests, amend, modify or waive certain material agreements or organizational documents and make payments of certain subordinated indebtedness. Additionally, the Credit Agreement contains certain customary representations and warranties, affirmative covenants and provisions relating to events of default, including nonpayment of principal, interest and other amounts; failure to comply with covenants; the rendering of judgments or orders or default by us in respect of other material indebtedness; and certain insolvency and ERISA events.

DEVELOPMENT DERIVATIVE LIABILIT

DEVELOPMENT DERIVATIVE LIABILITY	12 Months Ended
	Dec. 31, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
DEVELOPMENT DERIVATIVE LIABILITY	DEVELOPMENT DERIVATIVE LIABILITY On August 15, 2020, we entered into a co-development agreement, referred to as the Funding Agreement, with BXLS V Bodyguard – PCP L.P. and BXLS Family Investment Partnership V – ESC L.P., collectively referred to as Blackstone Life Sciences, pursuant to which Blackstone Life Sciences will provide up to $150.0 million in funding for the clinical development of vutrisiran and ALN-AGT, two of our cardiometabolic programs. With respect to vutrisiran, Blackstone Life Sciences has committed to provide up to $70.0 million to fund development costs related to the HELIOS-B Phase 3 clinical trial. In addition, Blackstone Life Sciences has the right, but is not obligated, to fund up to $26.0 million for development costs related to a Phase 2 clinical trial of ALN-AGT and up to $54.0 million for development costs related to a Phase 3 clinical trial of ALN-AGT. The amount of funding ultimately provided by Blackstone Life Sciences is dependent on us achieving specified development milestones with respect to each clinical trial. We retain sole responsibility for the development and commercialization of both vutrisiran and ALN-AGT. As consideration for Blackstone Life Sciences’ funding for vutrisiran clinical development costs, we have agreed to pay Blackstone Life Sciences a 1% royalty on net sales of vutrisiran for a 10-year term beginning upon the first commercial sale following regulatory approval of vutrisiran for ATTR-cardiomyopathy, as well as fixed payments of up to 2.5 times their investment over a two-year period upon regulatory approval of vutrisiran for ATTR-cardiomyopathy in specified countries, unless it is later withdrawn from the market following a mandatory recall. As consideration for Blackstone Life Sciences’ funding for Phase 2 clinical development costs of ALN-AGT, we have agreed to pay Blackstone Life Sciences fixed payments of up to 3.25 times their Phase 2 investment over a four-year period upon the successful completion of the ALN-AGT Phase 2 clinical trial, unless certain regulatory events affecting the continued development of ALN-AGT occur. As consideration for Blackstone Life Sciences’ funding for Phase 3 clinical development costs of ALN-AGT, we have agreed to pay Blackstone Life Sciences fixed payments of up to 4.5 times their Phase 3 investment over a four-year period upon regulatory approval of ALN-AGT in specified countries, unless it is later withdrawn from the market following a mandatory recall. Our payment obligations under the Funding Agreement will be secured, subject to certain exceptions, by security interests in intellectual property owned by us relating to vutrisiran and ALN-AGT, as well as in our bank account in which the funding deposits will be made. We and Blackstone Life Sciences each have the right to terminate the Funding Agreement in its entirety in the event of the other party’s bankruptcy or similar proceedings. We and Blackstone Life Sciences may each terminate the Funding Agreement in its entirety or with respect to either product in the event of an uncured material breach by the other party, or with respect to a product for certain patient health and safety reasons, or if regulatory approval in specified major market countries is not obtained for the product following the completion of clinical trials for the product. In addition, Blackstone Life Sciences has the right to terminate the Funding Agreement in its entirety upon the occurrence of certain events affecting our ability to make payments under the agreement or to develop or commercialize the products, or upon a change of control of us. Blackstone Life Sciences may also terminate the Funding Agreement with respect to a product if the joint steering committee elects to terminate the development program for that product in its entirety, if certain clinical endpoints are not achieved for that product or, with respect to vutrisiran only, if our right to develop or commercialize vutrisiran is enjoined in a specified major market as a result of an alleged patent infringement. In certain termination circumstances, we will be obligated to pay Blackstone Life Sciences an amount that is equal to, or a multiplier of, the development funding received from Blackstone Life Sciences, and we may remain obligated under certain circumstances to make the payments to Blackstone Life Sciences described above, or the royalty described above in the case of vutrisiran, should we obtain regulatory approval for vutrisiran or ALN-AGT following termination. We account for the Funding Agreement under ASC 815 as a derivative liability, measured at fair value, within other liabilities on our consolidated balance sheets. The liability was initially recorded at $4.2 million upon receipt of funding in the third quarter of 2020, pursuant to the contractual terms, and then subsequently increased in the fourth quarter of 2020 upon receipt of additional funding. The change in fair value due to the remeasurement of the development derivative liability resulted in a $17.2 million loss for the year ended December 31, 2020, recorded within other income, net on our consolidated statements of operations and comprehensive loss. As of December 31, 2020, the derivative liability is classified as a Level 3 financial liability in the fair value hierarchy. The valuation method incorporates certain unobservable Level 3 key inputs including (i) the probability and timing of achieving stated development milestones to receive payments from Blackstone Life Sciences, (ii) the probability and timing of achieving regulatory approval and payments to Blackstone Life Sciences, (iii) an estimate of the amount and timing of the royalty payable on net sales of vutrisiran, assuming regulatory approval, (iv) our cost of borrowing (15%), and (v) Blackstone Life Sciences' cost of borrowing (4%). The following table presents the activity with respect to the development derivative liability, in thousands: Development derivative liability as of August 15, 2020 $ — Amount received under the Funding Agreement 8,400 Loss recorded from remeasurement of development derivative liability 17,185 Development derivative liability as of December 31, 2020 $ 25,585

FAIR VALUE MEASUREMENTS

FAIR VALUE MEASUREMENTS	12 Months Ended
	Dec. 31, 2020
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 and indicate the fair value hierarchy of the valuation techniques we utilized to determine such fair value: (In thousands) As of December 31, Quoted Prices in Active Markets Significant Observable Inputs Significant Unobservable Inputs Financial assets Cash equivalents: U.S. treasury securities $ 20,000 $ — $ 20,000 $ — Money market funds 75,726 75,726 — — Marketable debt securities: U.S. government-sponsored enterprise securities 245,214 — 245,214 — U.S. treasury securities 1,087,968 — 1,087,968 — Marketable equity securities 44,633 44,633 — — Restricted cash (money market funds) 1,483 1,483 — — Total financial assets $ 1,475,024 $ 121,842 $ 1,353,182 $ — Financial liabilities Development derivative liability $ 25,585 $ — $ — $ 25,585 (In thousands) As of December 31, Quoted Prices in Active Markets Significant Observable Inputs Significant Unobservable Inputs Financial assets 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 financial assets $ 1,450,480 $ 135,331 $ 1,315,149 $ — For the year ended December 31, 2019, there were no transfers between Level 1 and Level 2 financial assets. During the year ended December 31, 2020, we transferred one financial asset from Level 2 to Level 1 as a result of the expiration of a securities' holding restriction on a marketable equity security. There were no other transfers between Level 1 and Level 2 financial assets or liabilities during the year ended December 31, 2020. 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 carrying amount of our debt as of December 31, 2020 approximates fair value as the debt was drawn on December 31, 2020 and has a variable interest rate.

MARKETABLE DEBT SECURITIES

MARKETABLE DEBT SECURITIES	12 Months Ended
	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
MARKETABLE DEBT SECURITIES	MARKETABLE DEBT SECURITIES The following tables summarize our marketable debt securities: As of December 31, 2020 (In thousands) Amortized Gross Gross Fair Value U.S. government-sponsored enterprise securities $ 245,113 $ 135 $ (34) $ 245,214 U.S. treasury securities 1,107,721 328 (81) 1,107,968 Total $ 1,352,834 $ 463 $ (115) $ 1,353,182 As of December 31, 2019 (In thousands) 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 The fair values of our marketable debt securities by classification in the consolidated balance sheets were as follows: (In thousands) December 31, 2020 December 31, 2019 Cash and cash equivalents $ 20,000 $ 340,132 Marketable debt securities 1,333,182 975,017 Total $ 1,353,182 $ 1,315,149

LEASES

LEASES	12 Months Ended
	Dec. 31, 2020
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 for our corporate headquarters from BMR-675 West Kendall Street, LLC, or BMR, under a non-cancelable real property lease. 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, 2020. 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, 2020. 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, 2020. 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, as amended in September 2020, that will expire in March 2024 and June 2026, 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, 2020. 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 all of our real property leases was $50.7 million, $52.4 million and $40.6 million for the years ended December 31, 2020, 2019 and 2018, respectively. The following table summarizes our costs included in operating expenses related to right of use lease assets we have entered into through December 31, 2020: (In thousands) Year Ended December 31, 2020 Year Ended December 31, 2019 Operating lease cost $ 42,271 $ 38,613 Variable lease cost 11,049 15,209 Total $ 53,320 $ 53,822 Short-term lease costs were not material for the years ended December 31, 2020 and 2019. Net cash paid for the amounts included in the measurement of the operating lease liability in our consolidated balance sheet and included in change in operating lease liability within operating activities in our consolidated statement of cash flow was $38.0 million and $33.7 million for the years ended December 31, 2020 and 2019, respectively. The weighted-average remaining lease term and weighted-average discount rate for all leases as of December 31, 2020 was 12 years and 8%, respectively, and as of December 31, 2019 was 13 years and 8%, 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, 2020 were as follows, in thousands: Year Ending December 31 2021 $ 38,593 2022 46,392 2023 43,764 2024 42,918 2025 41,190 2026 and thereafter 323,473 Total undiscounted lease liability 536,330 Less imputed interest (206,419) Total discounted lease liability $ 329,911 Current operating lease liability $ 36,872 Non-current operating lease liability 293,039 Total $ 329,911

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	12 Months Ended
	Dec. 31, 2020
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, 2020, 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 Pharmaceuticals, Inc., or 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 are immaterial in connection with our various collaboration 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, 2020, such contingencies have not been recorded in our consolidated financial statements. Litigation From time to time, we may be a party to litigation, arbitration or other 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 products or 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 or breach our license or other agreements with such third parties. 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. Ionis Arbitration In June 2018, Ionis claimed it was owed payments under our second amended and restated strategic collaboration and license agreement as a result of the January 2018 restructuring of our Sanofi Genzyme collaboration and the related Exclusive TTR License and AT3 License Terms described above. We disputed this and in November 2018, Ionis filed a Demand for Arbitration with the American Arbitration Association against us. The hearing portion of the arbitration process was completed in June 2020, and in October 2020, a partial award was issued by the arbitration panel seeking additional information from us. The arbitration panel issued its final award in December 2020 and required us to pay $41.2 million to Ionis. For the year ended December 31, 2020, we increased our contingent liability related to our arbitration with Ionis by $38.2 million due to the issuance of this final award, and in January 2021 we paid $41.2 million to Ionis. 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 named as defendants certain of our other executive officers, and purported to be brought on behalf of a class of persons who acquired our securities between September 20, 2017 and September 12, 2018 and sought 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 alleged, among other things, that the defendants made materially false and misleading statements related to the efficacy and safety of our product, ONPATTRO. The plaintiff sought, 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. On March 23, 2020, the Court granted our motion and dismissed the Complaint without prejudice. Pursuant to a prior Order of the Court, on June 1, 2020, plaintiff filed a motion seeking leave to file a further amended complaint. We opposed the motion which was fully briefed on June 22, 2020, and remains pending with the Court. 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. On November 2, 2020, the Court entered a Decision and Order denying defendants’ motion to dismiss. Defendants filed a notice of appeal of that decision on November 12, 2020, and filed their opening appellate brief on January 4, 2021. Plaintiff’s responsive appellate brief is due to be filed on or before March 3, 2021. 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. 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, 2020 or 2019.

STOCKHOLDERS' EQUITY

STOCKHOLDERS' EQUITY	12 Months Ended
	Dec. 31, 2020
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, 2020 and 2019, there were no shares of preferred stock outstanding. Blackstone Equity Placement In April 2020, we entered into a stock purchase agreement, or Investors SPA, with certain affiliates of The Blackstone Group Inc., or Investors, pursuant to which we sold 963,486 shares of our common stock to the Investors for aggregate cash consideration of $100.0 million, or $103.79 per share, as part of the broad strategic financing collaboration with The Blackstone Group Inc. The Investors SPA contains customary representations, warranties, and covenants of each of the parties thereto. Regeneron Equity Placement In April 2019, we executed a stock purchase agreement, or 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. 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 $400.0 million of equity under the Regeneron SPA would instead have been issued in the form of 1,481,482 shares of our Series A redeemable convertible preferred stock, par value $0.01 per share, at a purchase price of $270.00 per share, that would have converted automatically into common stock on a 1-for-3 basis upon stockholder approval of additional authorized shares of common 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 comprehensive loss during the year ended December 31, 2019, with the offsetting adjustment to equity netting against the $400.0 million proceeds that were received upon closing. Public Offering 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.

STOCK-BASED COMPENSATION

STOCK-BASED COMPENSATION	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
STOCK-BASED COMPENSATION	STOCK-BASED COMPENSATION Stock Plans In May 2017, our stockholders approved a second amendment and restatement of the 2009 Stock Incentive Plan, or the Amended 2009 Plan, pursuant to which 15,480,000 shares of common stock were authorized for issuance. In May 2020, our stockholders approved a second amendment to the 2018 Stock Incentive Plan, as amended, or the Amended 2018 Plan, to increase the number of shares authorized for issuance thereunder by 7,000,000 shares. The Amended 2018 Plan 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, 2020, an aggregate of 23,035,819 shares of common stock were reserved for issuance under our stock plans, including outstanding stock options to purchase 11,692,209 shares of common stock, 1,160,427 outstanding restricted stock units, 9,223,025 of common stock available for additional equity awards and 960,158 shares available for future grant under our Amended and Restated 2004 Employee Stock Purchase Plan, as amended, 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. Stock-Based Compensation The following table summarizes stock-based compensation expenses included in operating costs and expenses: Year Ended December 31, (In thousands) 2020 2019 2018 Research and development $ 60,464 $ 88,930 $ 80,509 Selling, general and administrative 79,409 85,911 77,243 Total $ 139,873 $ 174,841 $ 157,752 The following table summarizes stock-based compensation expense: Year Ended December 31, (In thousands) 2020 2019 2018 Stock-based compensation expense by type of award: Time-based stock options $ 112,971 $ 99,097 $ 83,403 Performance-based stock options 6,340 48,207 56,419 Time-based restricted stock units 6,909 2,351 538 Performance-based restricted stock units 11,162 22,123 13,144 Other equity programs 3,062 6,466 5,672 Less: Stock-based compensation expense capitalized to inventory (571) (3,403) (1,424) Total $ 139,873 $ 174,841 $ 157,752 The following table summarizes our unrecognized stock-based compensation expense, net of estimated forfeitures, by type of awards, and the weighted-average period over which that expense is expected to be recognized: As of December 31, 2020 Unrecognized Weighted- Type of award: Time-based stock options $ 183,700 2.44 Time-based restricted stock units 1,593 0.2 Performance-based restricted stock units 1,286 * Other equity programs 8,159 2.57 __________________________________________ * 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 employee stock options granted: Year Ended December 31, 2020 2019 2018 Risk-free interest rate 0.3 - 1.7% 1.4 - 2.6% 2.7 - 2.9% Expected dividend yield — — — Expected option life 5.4 - 7.2 years 5.6 - 7.3 years 5.7 - 7.2 years Expected volatility 61 - 63% 63 - 66% 64 - 67% Stock Option Activity The following table summarizes the activity of our stock option plans, excluding performance-based stock options: Number of Weighted- Weighted- Aggregate Outstanding as of December 31, 2019 10,878 $ 76.92 Granted 2,086 119.96 Exercised (2,390) 63.34 Cancelled (506) 94.53 Outstanding as of December 31, 2020 10,068 $ 88.18 6.56 $ 423,864 Exercisable as of December 31, 2020 5,835 $ 76.79 5.19 $ 310,933 Vested or expected to vest as of December 31, 2020 9,692 $ 87.53 6.48 $ 414,208 The weighted-average fair value of stock options granted was $66.28, $49.27 and $66.49 per share for the years ended December 31, 2020, 2019 and 2018, respectively. The intrinsic value of stock options exercised was $177.8 million, $55.4 million and $87.1 million for the years ended December 31, 2020, 2019 and 2018, 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 people, culture and compensation committee. The following table summarizes the activity of our performance-based stock options granted under our equity plans: Number of Weighted- Weighted- Aggregate Outstanding as of December 31, 2019 2,191 $ 86.77 Granted — — Exercised (538) 71.28 Cancelled (28) 118.62 Outstanding as of December 31, 2020 1,625 $ 91.35 5.14 $ 62,740 Exercisable as of December 31, 2020 1,414 $ 87.22 4.88 $ 60,458 During the years ended December 31, 2020, 2019 and 2018, there were 0, 889,896 and 763,982 performance-based stock options that vested, respectively. The intrinsic value of performance-based stock options exercised was $34.1 million, $11.0 million and $8.0 million for the years ended December 31, 2020, 2019 and 2018, 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 restricted stock units: Number of Weighted- Outstanding as of December 31, 2019 127 $ 77.23 Awarded 17 134.34 Released (16) 92.97 Cancelled (11) 76.98 Outstanding as of December 31, 2020 117 $ 83.66 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 as of December 31, 2019 623 $ 85.36 Awarded 715 119.11 Released (206) 85.67 Cancelled (89) 99.08 Outstanding as of December 31, 2020 1,043 $ 107.26 The performance-based restricted stock units granted in 2020 and 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 people, culture and compensation committee. 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. In May 2020 our stockholders approved an amendment to the Amended and Restated ESPP, to further increase the number of shares authorized for issuance thereunder from 1,215,789 shares to 1,965,789 shares. Under the Amended and Restated ESPP, as amended, 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 129,394, 109,590 and 78,085 shares during the years ended December 31, 2020, 2019 and 2018, respectively, and as of December 31, 2020, we had 960,158 shares available for issuance under the Amended and Restated ESPP, as amended. We estimate the fair value of shares to be issued under the Amended and Restated ESPP, as amended, using the Black-Scholes option-pricing model on the date of grant, or first day of the offering period, using the same methodology approach as the employee stock option grants. The following table summarizes the Black-Scholes valuation assumption inputs for stock purchase rights granted under the employee stock purchase plan: Year Ended December 31, 2020 2019 2018 Risk-free interest rate 0.1% - 0.1% 1.6% - 2.4% 2.0% - 2.5% Expected dividend yield — — — Expected option life 6 months 6 months 6 months Expected volatility 40% - 50% 37% - 56% 47% - 49%

INCOME TAXES

INCOME TAXES	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES The domestic and foreign components of loss before income taxes are as follows: (In thousands) 2020 2019 2018 Domestic $ (682,859) $ (597,602) $ (573,245) Foreign (172,741) (287,651) (187,429) Loss before income taxes $ (855,600) $ (885,253) $ (760,674) The provision for income taxes consisted of the following: Year Ended December 31, (In thousands) 2020 2019 2018 Current provision: Domestic $ 61 $ (394) $ — Foreign 5,837 3,232 1,611 Total current provision 5,898 2,838 1,611 Deferred benefit: Domestic 393 394 (788) Foreign (3,610) (2,369) — Total deferred benefit (3,217) (1,975) (788) Total provision for income taxes $ 2,681 $ 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 are as follows: As of December 31, (In thousands) 2020 2019 Deferred tax assets: Net operating loss carryforwards $ 537,382 $ 627,466 Research and development and ODC credits 301,792 261,616 Sale of future royalties 259,014 — Lease liability 70,402 69,334 Deferred revenue 84,946 — Deferred compensation 67,530 75,058 Intangible assets 148,168 66,615 Other 32,725 13,660 Total deferred tax assets 1,501,959 1,113,749 Deferred tax liabilities: Property, plant and equipment, net (10,812) (10,077) Unrealized gain on marketable securities (12,766) (3,932) Right of use assets (50,323) (50,294) Deferred revenue tax accounting method change (71,812) — Deferred tax asset valuation allowance (1,349,729) (1,046,013) Net deferred tax asset $ 6,517 $ 3,433 Our effective income tax rate differs from the statutory federal income tax rate, as follows: Year Ended December 31, (In thousands) 2020 2019 2018 At U.S. federal statutory rate 21.0 % 21.0 % 21.0 % State taxes, net of federal effect 4.5 3.6 2.1 Stock-based compensation 2.2 — 0.8 Tax credits 3.3 3.7 4.2 Other permanent items (1.5) (0.3) (0.3) Foreign rate differential (3.5) (6.9) (5.3) Internal reorganization of certain intellectual property rights 12.3 — — Revaluation of deferred credits due to rate change — — (3.5) Other (2.7) (0.1) — Valuation allowance (35.9) (21.0) (19.0) Effective income tax rate (0.3) % — % — % 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. 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 $303.7 million, $185.9 million and $171.8 million for the years ended December 31, 2020, 2019 and 2018, respectively, primarily due to the liability related to the sale of future royalties for the year ended December 31, 2020 and additional net operating losses for the years ended December 31, 2019 and 2018. During the year ended December 31, 2020, we recorded a net provision for income taxes of $2.7 million. This is primarily comprised of $5.8 million of foreign current provision offset by $3.6 million of deferred provision, primarily related to foreign jurisdictions. As of December 31, 2020, we had federal and state net operating loss carryforwards of $2.0 billion and $1.9 billion, respectively, to reduce future taxable income. As of December 31, 2020, approximately $0.9 billion of our federal net operating loss carryforward can be carried forward indefinitely while the remaining federal net operating loss of $1.1 billion expires at various dates through 2037. As of December 31, 2020, we had federal and state research and development, including Orphan Drug, and state investment tax credit carryforwards of $272.5 million and $43.1 million, respectively, available to reduce future tax liabilities that expire at various dates through 2040. We have a valuation allowance against the net operating loss and credit carryforwards 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, 2020. 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 income 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. We recognize potential interest and penalties related to unrecognized tax benefits in our provision for income taxes. Our reserve related to income taxes, including potential interest and penalties, was not material as of December 31, 2020 and 2019. Our uncertain income tax positions do not impact our effective tax rate due to our full valuation allowance in the U.S. As of December 31, 2020, the unremitted earnings of our foreign subsidiaries are not material. We have not provided for U.S. income taxes or foreign withholding taxes on these earnings as it is our current intention to permanently reinvest these earnings outside the U.S. The tax liability on these earnings is also not material. Events that could trigger a tax liability include, but are not limited to, distributions, reorganizations or restructurings and/or tax law changes. The tax years 2017 through 2020 remain open to examination by major taxing jurisdictions, which are primarily in the U.S., although net operating loss and credit carryforwards generated prior to 2017 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, 2020
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 $5.2 million and $4.3 million as of December 31, 2020 and 2019, respectively, and is recorded in other liabilities on the consolidated balance sheet. The total net periodic benefit cost for the years ended December 31, 2020, 2019 and 2018 were not material.

QUARTERLY FINANCIAL DATA (UNAUD

QUARTERLY FINANCIAL DATA (UNAUDITED)	12 Months Ended
	Dec. 31, 2020
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, 2020 June 30, 2020 September 30, 2020 December 31, (In thousands, except per share data) Total revenues $ 99,476 $ 103,962 $ 125,853 $ 163,562 Operating costs and expenses 309,634 302,821 351,052 357,784 Net loss $ (182,221) $ (179,229) $ (253,291) $ (243,540) Net loss per common share — basic and diluted $ (1.62) $ (1.56) $ (2.18) $ (2.09) Weighted-average common shares — basic and diluted 112,748 114,911 115,986 116,274 Three Months Ended March 31, 2019 June 30, 2019 September 30, 2019 December 31, (In thousands, except per share data) Total 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

SUMMARY OF SIGNIFICANT ACCOUN_2

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
	Dec. 31, 2020
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, we use estimates and assumptions related to our inventory valuation and related reserves, liability related to the sale of future royalties, development derivative liability, 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. The full extent to which the COVID-19 pandemic will directly or indirectly impact our business, results of operations and financial condition, including sales, expenses, reserves and allowances, the supply of our products and product candidates, clinical trials and research and development costs, will depend on future developments that are highly uncertain, including as a result of new information that may emerge concerning COVID-19 and variants thereof, and the actions taken to contain or treat it or vaccinate against it, as well as the economic impact on local, regional, national and international customers and markets. We have made estimates of the impact of COVID-19 within our financial statements and there may be changes to those estimates in future periods. Actual results may differ from these 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 securities as of December 31, 2020, together with the cash we expect to generate from product sales and under our current alliances, including our strategic financing collaboration with The Blackstone Group Inc. and certain of its affiliates, 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 securities. As of December 31, 2020 and 2019, substantially all of our cash, cash equivalents and marketable 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
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, 2020 and 2019, based on our estimation of expected write-offs, we determined an allowance for doubtful accounts was not material. 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 and classified based on the anticipation of when it will be consumed either within our normal operating cycle (short-term) or beyond (long-term). 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, 2020, 2019 and 2018, we recorded $30.2 million, $16.6 million and $12.8 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
Leases	Leases Effective January 1, 2019, we adopted Accounting Standards Update, or ASU, 2016-02, Leases Topic 842, or ASC 842, using a modified retrospective basis and utilizing the effective date as the date of initial application. We determine if an arrangement is a lease at contract inception based on the facts and circumstances present in the arrangement. All our leases are classified as operating leases under the new leasing standard. W e record operating lease assets and lease liabilities in our consolidated balance sheets. Operating lease assets represent our right to use an underlying asset for the lease term and operating lease liabilities represent our obligation to make lease payments arising from the leasing arrangement. Operating lease assets and operating lease liabilities are recognized at commencement date based on the present value of lease payments over the lease term. As most of our leases do not provide an implicit rate, in determining the operating lease liabilities, we use an estimate of our incremental borrowing rate based on the information available at commencement. Lease expense for lease payments is recognized on a straight-line basis over the lease term. Short-term leases, or leases that have a lease term of 12 months or less at commencement date, are excluded from this treatment and are recognized on a straight-line basis over the term of the lease.
Clinical Accruals	Clinical 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	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, 2020 and 2019, we had not capitalized any costs to obtain any of our contracts. Net Product Revenues Our net product revenues 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. We record reserves, based on contractual terms, for 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. The following are the components of variable consideration related to product revenues: 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 within each customer agreement. 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. Net Revenues from Collaborations 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, Net Revenues from Collaborations. 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 (less than 12 months) or long-term (more than 12 months) deferred revenue based on our best estimate of when 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, amortization of intangible assets associated with the sale of our products and costs related to sales of product supply under our collaboration agreements. 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 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. 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 have recorded no interest and penalty expense related to uncertain tax positions for the years ended December 31, 2020, 2019 or 2018.
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, certain 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.
Liability Related to the Sale of Future Royalties	Liability Related to the Sale of Future Royalties We account for the liability related to the sale of future royalties as a debt financing, as we have significant continuing involvement in the generation of the cash flows. Interest on the liability related to the sale of future royalties will be recognized using the effective interest rate method over the life of the related royalty stream. The liability related to the sale of future royalties and the related interest expense are based on our current estimates of future royalties and commercial milestones expected to be paid over the life of the arrangement, which we determine by using third-party forecasts of inclisiran’s global net revenue. We will periodically assess the expected payments and to the extent the amount or timing of our future estimated payments is materially different than our previous estimates, we will account for any such change by adjusting the liability related to the sale of future royalties and prospectively recognizing the related non-cash interest expense.
Development Derivative Liability	Development Derivative Liability Development derivative liability is recorded at fair value based on the probability weighted present value of the estimated cash flows pursuant to contractual terms of the funding agreement. The liability is remeasured quarterly with any change in fair value recorded in other income (expense) on the consolidated statements of operations and comprehensive loss.
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, 2020 and 2019, substantially all of our consolidated property, plant and equipment, net was from U.S. operations. For the years ended December 31, 2020, 2019 and 2018, net revenues from collaborations 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 June 2016, the Financial Accounting Standards Board, or FASB, issued ASU 2016-13 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 and did not have a significant impact on our consolidated financial statements and related disclosures. In August 2018, the FASB issued ASU 2018-13 amending 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 and did not have a significant impact on our consolidated financial statements and related disclosures. In August 2018, the FASB issued ASU 2018-15 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 and did not have a significant impact on our consolidated financial statements and related disclosures. In November 2018, the FASB issued ASU 2018-18 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. This standard did not have a significant impact on our consolidated financial statements and related disclosures. In December 2019, the FASB issued ASU 2019-12 amending accounting guidance that simplify 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. We early adopted the amendments as of January 1, 2020, on a prospective basis. The amendments did not have a significant impact on our consolidated financial statements and related disclosures. 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. We adopted ASC 606 in the first quarter of 2018 and recognized the cumulative effect of initially applying ASC 606 as an adjustment to opening retained earnings for the year ended December 31, 2018.

SUMMARY OF SIGNIFICANT ACCOUN_3

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	12 Months Ended
	Dec. 31, 2020
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: Year Ended December 31, 2020 2019 2018 Distributor A 31 % 44 % 13 % Regeneron Pharmaceuticals 12 % * * Sanofi Genzyme * * 58 % Vir Biotechnology * * 16 % __________________________________________ * Represents less than 10% The following table summarizes customers with amounts due that represent 10% or greater of our consolidated gross accounts receivable balance: As of December 31, 2020 2019 Distributor A 19 % 28 % Novartis AG 16 % * Distributor B 14 % 10 % Regeneron Pharmaceuticals 11 % * Sanofi Genzyme * 14 % __________________________________________ * 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
Common Shares Excluded from the Calculation of Net Loss Per Common Share	The following table sets forth 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: As of December 31, (In thousands) 2020 2019 2018 Options to purchase common stock 11,692 13,069 12,573 Unvested restricted common stock 1,160 749 36 Total 12,852 13,818 12,609

NET PRODUCT REVENUES (Tables)

NET PRODUCT REVENUES (Tables)	12 Months Ended
	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Net Product Revenues by Geography	Net product revenues by geography consist of the following: Year Ended December 31, (In thousands) 2020 2019 2018 ONPATTRO United States $ 151,574 $ 116,302 $ 8,589 Europe 107,755 43,980 3,946 Rest of World (primarily Japan) 46,752 6,105 — Total $ 306,081 $ 166,387 $ 12,535 GIVLAARI United States $ 42,797 $ 150 $ — Europe 12,000 — — Rest of World 309 — — Total $ 55,106 $ 150 $ — OXLUMO Europe $ 333 $ — $ — Total net product revenues $ 361,520 $ 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: As of December 31, 2020 (In thousands) Chargebacks and Rebates Trade Discounts and Allowances Returns Reserve and Other Incentives Total Beginning balance $ 32,487 $ 410 $ 1,978 $ 34,875 Provision related to current period sales 103,706 4,650 5,702 114,058 Credit or payments made during the period for current year sales (42,493) (4,388) (2,704) (49,585) Credit or payments made during the period for prior year sales (2,995) (33) (1,213) (4,241) Total $ 90,705 $ 639 $ 3,763 $ 95,107 As of December 31, 2019 (In thousands) 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 (109) (218) (220) (547) Total $ 32,487 $ 410 $ 1,978 $ 34,875

NET REVENUES FROM COLLABORATI_2

NET REVENUES FROM COLLABORATIONS (Tables)	12 Months Ended
	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Revenue from Collaborators	The following table summarizes our total consolidated net revenues from collaborations: Year Ended December 31, (In thousands) 2020 2019 2018 Regeneron Pharmaceuticals $ 74,072 $ 26,075 $ — Vir Biotechnology 31,396 12,809 12,778 Novartis AG 22,208 2,315 2,789 Sanofi Genzyme 995 10,976 46,000 Other 2,662 1,038 806 Total $ 131,333 $ 53,213 $ 62,373
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: As of December 31, (In thousands) 2020 2019 Receivables included in "Accounts receivable, net" $ 33,542 $ 14,929 Contract liabilities included in "Deferred revenue" 120,021 153,117
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: Year Ended December 31, 2020 2019 2018 (In thousands) Clinical Trial and Manufacturing External Services Other Clinical Trial and Manufacturing External Services Other Clinical Trial and Manufacturing External Services Other Regeneron $ 13,302 $ 171 $ 44,360 $ 2,793 $ 344 $ 21,779 $ — $ — $ — Sanofi Genzyme 644 181 2,132 11,505 334 2,017 36,600 5,340 1,279 Vir 18,470 584 11,590 10,353 381 4,745 7,272 8,251 548 Novartis 999 — 700 2,025 — 696 1,664 2 203 Other — — — — — — — 2,150 1,097 Total $ 33,415 $ 936 $ 58,782 $ 26,676 $ 1,059 $ 29,237 $ 45,536 $ 15,743 $ 3,127
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 The following tables provide 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: Transaction Price Allocated Deferred Revenue Performance Obligations As of December 31, 2020 As of December 31, 2020 As of December 31, 2019 Accounting Guidance Research Services Obligation $ 200,600 $ 54,900 $ 84,800 ASC 606 C5 License Obligation 85,200 58,700 65,800 ASC 606 C5 Co-Co Obligation 246,000 231,400 243,000 ASC 808 Total $ 531,800 $ 345,000 $ 393,600
Schedule of Revenue Recognized Based on Accounting Guidance	Revenue Recognized During Performance Obligations Year Ended December 31, 2020 Year Ended December 31, 2019 Accounting Guidance Research Services Obligation $ 44,800 $ 21,000 ASC 606 C5 License Obligation 7,100 — ASC 606 C5 Co-Co Obligation 11,700 2,900 ASC 808 $ 63,600 $ 23,900

LIABILITY RELATED TO THE SALE_2

LIABILITY RELATED TO THE SALE OF FUTURE ROYALTIES (Tables)	12 Months Ended
	Dec. 31, 2020
Related Party Transactions [Abstract]
Schedule of Royalty Liability	The following table shows the activity with respect to the liability related to the sale of future royalties, in thousands: Liability related to the sale of future royalties as of April 10, 2020 $ 1,000,000 Capitalized closing costs (12,955) Interest expense recognized 84,496 Carrying value of liability related to sale of future royalties as of December 31, 2020 $ 1,071,541

OTHER BALANCE SHEET DETAILS (Ta

OTHER BALANCE SHEET DETAILS (Tables)	12 Months Ended
	Dec. 31, 2020
Balance Sheet Related Disclosures [Abstract]
Schedule of Inventory	The components of inventory are summarized as follows: As of December 31, (In thousands) 2020 2019 Raw materials $ 63,460 $ 15,418 Work in process 16,149 38,275 Finished goods 12,693 2,655 Total inventory $ 92,302 $ 56,348
Property, Plant and Equipment, Net	Property, plant and equipment, net consist of the following: As of December 31, (In thousands) 2020 2019 Buildings $ 262,637 $ 250,380 Leasehold improvements 149,505 132,632 Laboratory equipment 48,930 29,755 Manufacturing equipment 41,089 — Construction in progress 28,005 54,195 Computer equipment and software 19,064 14,956 Furniture and fixtures 11,066 10,339 Land 9,080 9,080 569,376 501,337 Less: accumulated depreciation (104,347) (76,158) Total $ 465,029 $ 425,179 Accrued Expenses
Accrued Expenses	Accrued expenses consist of the following: As of December 31, (In thousands) 2020 2019 Compensation and related $ 97,433 $ 68,304 Product rebates and discounts 94,242 32,670 Pre-clinical, clinical trial and manufacturing 46,506 34,269 Contingent liabilities 41,216 — Licensing and collaboration agreements 15,424 20,622 Consulting and professional services 11,501 14,251 Other 49,587 27,085 Total $ 355,909 $ 197,201 Cash, Cash Equivalents and Restricted Cash
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: As of December 31, (In thousands) 2020 2019 2018 Cash and cash equivalents $ 496,580 $ 547,178 $ 420,146 Total restricted cash included in prepaid expenses, other current assets and long-term other assets 2,466 2,450 2,485 Total cash, cash equivalents, and restricted cash shown in the consolidated $ 499,046 $ 549,628 $ 422,631
Summary of Changes in Accumulated Other Comprehensive Income (Loss)	The following table summarizes the changes in accumulated other comprehensive (loss) income, by component: (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, 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) Other comprehensive (loss) income before reclassifications — (531) (14) (7,081) (7,626) Amounts reclassified from other comprehensive income — 297 225 — 522 Net other comprehensive (loss) income — (234) 211 (7,081) (7,104) Balance as of December 31, 2020 $ (32,792) $ (3,754) $ 348 $ (7,424) $ (43,622)

CREDIT AGREEMENT (Tables)

CREDIT AGREEMENT (Tables)	12 Months Ended
	Dec. 31, 2020
Line of Credit Facility [Abstract]
Schedule of Term Loan	The remaining two tranches will provide funds as follows: Tranche Requested No Later Than Aggregate Principal Amount, up to (in thousands) Tranche 2 Loan June 30, 2021 $ 250,000 Tranche 3 Loan December 31, 2021 $ 250,000

DEVELOPMENT DERIVATIVE LIABIL_2

DEVELOPMENT DERIVATIVE LIABILITY (Tables)	12 Months Ended
	Dec. 31, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Development Derivative Liability Activity	The following table presents the activity with respect to the development derivative liability, in thousands: Development derivative liability as of August 15, 2020 $ — Amount received under the Funding Agreement 8,400 Loss recorded from remeasurement of development derivative liability 17,185 Development derivative liability as of December 31, 2020 $ 25,585

FAIR VALUE MEASUREMENTS (Tables

FAIR VALUE MEASUREMENTS (Tables)	12 Months Ended
	Dec. 31, 2020
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 and indicate the fair value hierarchy of the valuation techniques we utilized to determine such fair value: (In thousands) As of December 31, Quoted Prices in Active Markets Significant Observable Inputs Significant Unobservable Inputs Financial assets Cash equivalents: U.S. treasury securities $ 20,000 $ — $ 20,000 $ — Money market funds 75,726 75,726 — — Marketable debt securities: U.S. government-sponsored enterprise securities 245,214 — 245,214 — U.S. treasury securities 1,087,968 — 1,087,968 — Marketable equity securities 44,633 44,633 — — Restricted cash (money market funds) 1,483 1,483 — — Total financial assets $ 1,475,024 $ 121,842 $ 1,353,182 $ — Financial liabilities Development derivative liability $ 25,585 $ — $ — $ 25,585 (In thousands) As of December 31, Quoted Prices in Active Markets Significant Observable Inputs Significant Unobservable Inputs Financial assets 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 financial assets $ 1,450,480 $ 135,331 $ 1,315,149 $ —

MARKETABLE DEBT SECURITIES (Tab

MARKETABLE DEBT SECURITIES (Tables)	12 Months Ended
	Dec. 31, 2020
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, 2020 (In thousands) Amortized Gross Gross Fair Value U.S. government-sponsored enterprise securities $ 245,113 $ 135 $ (34) $ 245,214 U.S. treasury securities 1,107,721 328 (81) 1,107,968 Total $ 1,352,834 $ 463 $ (115) $ 1,353,182 As of December 31, 2019 (In thousands) 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
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, 2020 December 31, 2019 Cash and cash equivalents $ 20,000 $ 340,132 Marketable debt securities 1,333,182 975,017 Total $ 1,353,182 $ 1,315,149

LEASES (Tables)

LEASES (Tables)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Summary of Costs Included in Operating Expenses Related to Leases	The following table summarizes our costs included in operating expenses related to right of use lease assets we have entered into through December 31, 2020: (In thousands) Year Ended December 31, 2020 Year Ended December 31, 2019 Operating lease cost $ 42,271 $ 38,613 Variable lease cost 11,049 15,209 Total $ 53,320 $ 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, 2020 were as follows, in thousands: Year Ending December 31 2021 $ 38,593 2022 46,392 2023 43,764 2024 42,918 2025 41,190 2026 and thereafter 323,473 Total undiscounted lease liability 536,330 Less imputed interest (206,419) Total discounted lease liability $ 329,911 Current operating lease liability $ 36,872 Non-current operating lease liability 293,039 Total $ 329,911

STOCK-BASED COMPENSATION (Table

STOCK-BASED COMPENSATION (Tables)	12 Months Ended
	Dec. 31, 2020
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: Year Ended December 31, (In thousands) 2020 2019 2018 Research and development $ 60,464 $ 88,930 $ 80,509 Selling, general and administrative 79,409 85,911 77,243 Total $ 139,873 $ 174,841 $ 157,752
Summary of Stock-Based Compensation Expense	The following table summarizes stock-based compensation expense: Year Ended December 31, (In thousands) 2020 2019 2018 Stock-based compensation expense by type of award: Time-based stock options $ 112,971 $ 99,097 $ 83,403 Performance-based stock options 6,340 48,207 56,419 Time-based restricted stock units 6,909 2,351 538 Performance-based restricted stock units 11,162 22,123 13,144 Other equity programs 3,062 6,466 5,672 Less: Stock-based compensation expense capitalized to inventory (571) (3,403) (1,424) Total $ 139,873 $ 174,841 $ 157,752
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, by type of awards, and the weighted-average period over which that expense is expected to be recognized: As of December 31, 2020 Unrecognized Weighted- Type of award: Time-based stock options $ 183,700 2.44 Time-based restricted stock units 1,593 0.2 Performance-based restricted stock units 1,286 * Other equity programs 8,159 2.57 __________________________________________ * 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 following table summarizes the Black-Scholes valuation assumption inputs for employee stock options granted: Year Ended December 31, 2020 2019 2018 Risk-free interest rate 0.3 - 1.7% 1.4 - 2.6% 2.7 - 2.9% Expected dividend yield — — — Expected option life 5.4 - 7.2 years 5.6 - 7.3 years 5.7 - 7.2 years Expected volatility 61 - 63% 63 - 66% 64 - 67%
Stock Option Activity	The following table summarizes the activity of our stock option plans, excluding performance-based stock options: Number of Weighted- Weighted- Aggregate Outstanding as of December 31, 2019 10,878 $ 76.92 Granted 2,086 119.96 Exercised (2,390) 63.34 Cancelled (506) 94.53 Outstanding as of December 31, 2020 10,068 $ 88.18 6.56 $ 423,864 Exercisable as of December 31, 2020 5,835 $ 76.79 5.19 $ 310,933 Vested or expected to vest as of December 31, 2020 9,692 $ 87.53 6.48 $ 414,208 The following table summarizes the activity of our performance-based stock options granted under our equity plans: Number of Weighted- Weighted- Aggregate Outstanding as of December 31, 2019 2,191 $ 86.77 Granted — — Exercised (538) 71.28 Cancelled (28) 118.62 Outstanding as of December 31, 2020 1,625 $ 91.35 5.14 $ 62,740 Exercisable as of December 31, 2020 1,414 $ 87.22 4.88 $ 60,458
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 restricted stock units: Number of Weighted- Outstanding as of December 31, 2019 127 $ 77.23 Awarded 17 134.34 Released (16) 92.97 Cancelled (11) 76.98 Outstanding as of December 31, 2020 117 $ 83.66 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 as of December 31, 2019 623 $ 85.36 Awarded 715 119.11 Released (206) 85.67 Cancelled (89) 99.08 Outstanding as of December 31, 2020 1,043 $ 107.26
Stock Purchase Rights Granted Under the Employee Stock Purchase Plan	The following table summarizes the Black-Scholes valuation assumption inputs for stock purchase rights granted under the employee stock purchase plan: Year Ended December 31, 2020 2019 2018 Risk-free interest rate 0.1% - 0.1% 1.6% - 2.4% 2.0% - 2.5% Expected dividend yield — — — Expected option life 6 months 6 months 6 months Expected volatility 40% - 50% 37% - 56% 47% - 49%

INCOME TAXES (Tables)

INCOME TAXES (Tables)	12 Months Ended
	Dec. 31, 2020
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) 2020 2019 2018 Domestic $ (682,859) $ (597,602) $ (573,245) Foreign (172,741) (287,651) (187,429) Loss before income taxes $ (855,600) $ (885,253) $ (760,674)
Schedule of Provision for Income Taxes	The provision for income taxes consisted of the following: Year Ended December 31, (In thousands) 2020 2019 2018 Current provision: Domestic $ 61 $ (394) $ — Foreign 5,837 3,232 1,611 Total current provision 5,898 2,838 1,611 Deferred benefit: Domestic 393 394 (788) Foreign (3,610) (2,369) — Total deferred benefit (3,217) (1,975) (788) Total provision for income taxes $ 2,681 $ 863 $ 823
Schedule of Components of Net Deferred Tax (Liability) Asset	Components of the net deferred tax (liability) asset are as follows: As of December 31, (In thousands) 2020 2019 Deferred tax assets: Net operating loss carryforwards $ 537,382 $ 627,466 Research and development and ODC credits 301,792 261,616 Sale of future royalties 259,014 — Lease liability 70,402 69,334 Deferred revenue 84,946 — Deferred compensation 67,530 75,058 Intangible assets 148,168 66,615 Other 32,725 13,660 Total deferred tax assets 1,501,959 1,113,749 Deferred tax liabilities: Property, plant and equipment, net (10,812) (10,077) Unrealized gain on marketable securities (12,766) (3,932) Right of use assets (50,323) (50,294) Deferred revenue tax accounting method change (71,812) — Deferred tax asset valuation allowance (1,349,729) (1,046,013) Net deferred tax asset $ 6,517 $ 3,433
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: Year Ended December 31, (In thousands) 2020 2019 2018 At U.S. federal statutory rate 21.0 % 21.0 % 21.0 % State taxes, net of federal effect 4.5 3.6 2.1 Stock-based compensation 2.2 — 0.8 Tax credits 3.3 3.7 4.2 Other permanent items (1.5) (0.3) (0.3) Foreign rate differential (3.5) (6.9) (5.3) Internal reorganization of certain intellectual property rights 12.3 — — Revaluation of deferred credits due to rate change — — (3.5) Other (2.7) (0.1) — Valuation allowance (35.9) (21.0) (19.0) Effective income tax rate (0.3) % — % — %

QUARTERLY FINANCIAL DATA (UNA_2

QUARTERLY FINANCIAL DATA (UNAUDITED) (Tables)	12 Months Ended
	Dec. 31, 2020
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, 2020 June 30, 2020 September 30, 2020 December 31, (In thousands, except per share data) Total revenues $ 99,476 $ 103,962 $ 125,853 $ 163,562 Operating costs and expenses 309,634 302,821 351,052 357,784 Net loss $ (182,221) $ (179,229) $ (253,291) $ (243,540) Net loss per common share — basic and diluted $ (1.62) $ (1.56) $ (2.18) $ (2.09) Weighted-average common shares — basic and diluted 112,748 114,911 115,986 116,274 Three Months Ended March 31, 2019 June 30, 2019 September 30, 2019 December 31, (In thousands, except per share data) Total 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

NATURE OF BUSINESS (Details)

NATURE OF BUSINESS (Details)	12 Months Ended
	Dec. 31, 2020USD ($)
Blackstone Group Inc. | Net Revenues from Collaborators | RNAi Therapeutics
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum proceeds from collaborators	$ 2,000,000,000

SUMMARY OF SIGNIFICANT ACCOUN_4

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Customers that Represent Greater than Ten Percent of Gross Revenues (Detail) - Revenue from Rights Concentration Risk - Gross Revenues	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Distributor A
Concentration Risk [Line Items]
Concentration risk percentage	31.00%	44.00%	13.00%
Regeneron Pharmaceuticals
Concentration Risk [Line Items]
Concentration risk percentage	12.00%
Sanofi Genzyme
Concentration Risk [Line Items]
Concentration risk percentage			58.00%
Vir Biotechnology
Concentration Risk [Line Items]
Concentration risk percentage			16.00%

SUMMARY OF SIGNIFICANT ACCOUN_5

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Customers that Represent Greater than Ten Percent of Gross Accounts Receivable (Detail) - Accounts Receivable - Credit Concentration Risk	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Distributor A
Concentration Risk [Line Items]
Concentration risk percentage	19.00%	28.00%
Novartis AG
Concentration Risk [Line Items]
Concentration risk percentage	16.00%
Distributor B
Concentration Risk [Line Items]
Concentration risk percentage	14.00%	10.00%
Regeneron Pharmaceuticals
Concentration Risk [Line Items]
Concentration risk percentage	11.00%
Sanofi Genzyme
Concentration Risk [Line Items]
Concentration risk percentage		14.00%

SUMMARY OF SIGNIFICANT ACCOUN_6

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Investments in Marketable Securities and Cash Equivalents - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Marketable securities classified as cash equivalents, maximum original maturity	90 days
Policy for marketable securities	90 days

SUMMARY OF SIGNIFICANT ACCOUN_7

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020
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_8

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Estimated Useful Lives of Property, Plant and Equipment (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 30.2	$ 16.6	$ 12.8
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_9

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Capitalized cost	$ 0	$ 0

SUMMARY OF SIGNIFICANT ACCOU_10

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Interest and penalty expense related to uncertain tax positions	$ 0	$ 0	$ 0

SUMMARY OF SIGNIFICANT ACCOU_11

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Research and development
Significant Accounting Policies [Line Items]
Research and development expense costs associated with license fees	$ 2.8	$ 37	$ 8

SUMMARY OF SIGNIFICANT ACCOU_12

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Potential Common Shares Excluded from Calculation of Net Loss Per Common Share (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share	12,852	13,818	12,609
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	11,692	13,069	12,573
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	1,160	749	36

SUMMARY OF SIGNIFICANT ACCOU_13

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Segment Information - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020segment
Accounting Policies [Abstract]
Number of reporting segments	1

NET PRODUCT REVENUES - Summary

NET PRODUCT REVENUES - Summary of Net Product Revenues (Detail) - USD ($) $ in Thousands	Jan. 01, 2018	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 2,900	$ 163,562	$ 125,853	$ 103,962	$ 99,476	$ 71,681	$ 70,061	$ 44,714	$ 33,294	$ 492,853	$ 219,750	$ 74,908
Product Revenues, Net
Disaggregation of Revenue [Line Items]
Revenues										361,520	166,537	12,535
ONPATTRO
Disaggregation of Revenue [Line Items]
Revenues										306,081	166,387	12,535
GIVLAARI
Disaggregation of Revenue [Line Items]
Revenues										55,106	150	0
United States | ONPATTRO
Disaggregation of Revenue [Line Items]
Revenues										151,574	116,302	8,589
United States | GIVLAARI
Disaggregation of Revenue [Line Items]
Revenues										42,797	150	0
Europe | ONPATTRO
Disaggregation of Revenue [Line Items]
Revenues										107,755	43,980	3,946
Europe | GIVLAARI
Disaggregation of Revenue [Line Items]
Revenues										12,000	0	0
Europe | OXLUMO
Disaggregation of Revenue [Line Items]
Revenues										333	0	0
Rest of World (primarily Japan) | ONPATTRO
Disaggregation of Revenue [Line Items]
Revenues										46,752	6,105	0
Rest of World (primarily Japan) | GIVLAARI
Disaggregation of Revenue [Line Items]
Revenues										$ 309	$ 0	$ 0

NET PRODUCT REVENUES - Addition

NET PRODUCT REVENUES - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Accounts receivable, net	$ 102,413	$ 43,011
Product Revenues, Net
Disaggregation of Revenue [Line Items]
Accounts receivable, net	$ 68,900	$ 28,100

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, 2020	Dec. 31, 2019
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balance	$ 34,875	$ 3,980
Provision related to current period sales	114,058	52,706
Credit or payments made during the period for current year sales	(49,585)	(21,264)
Credit or payments made during the period for prior year sales	(4,241)	(547)
Total	95,107	34,875
Chargebacks and Rebates
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balance	32,487	3,441
Provision related to current period sales	103,706	44,371
Credit or payments made during the period for current year sales	(42,493)	(15,216)
Credit or payments made during the period for prior year sales	(2,995)	(109)
Total	90,705	32,487
Trade Discounts and Allowances
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balance	410	218
Provision related to current period sales	4,650	3,227
Credit or payments made during the period for current year sales	(4,388)	(2,817)
Credit or payments made during the period for prior year sales	(33)	(218)
Total	639	410
Returns Reserve and Other Incentives
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balance	1,978	321
Provision related to current period sales	5,702	5,108
Credit or payments made during the period for current year sales	(2,704)	(3,231)
Credit or payments made during the period for prior year sales	(1,213)	(220)
Total	$ 3,763	$ 1,978

NET REVENUES FROM COLLABORATI_3

NET REVENUES FROM COLLABORATIONS - Net Revenue from Collaborators (Detail) - USD ($) $ in Thousands	Jan. 01, 2018	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	$ 2,900	$ 163,562	$ 125,853	$ 103,962	$ 99,476	$ 71,681	$ 70,061	$ 44,714	$ 33,294	$ 492,853	$ 219,750	$ 74,908
Net Revenues from Collaborators
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										131,333	53,213	62,373
Net Revenues from Collaborators | Regeneron Pharmaceuticals
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										74,072	26,075	0
Net Revenues from Collaborators | Vir Biotechnology
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										31,396	12,809	12,778
Net Revenues from Collaborators | Novartis AG
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										22,208	2,315	2,789
Net Revenues from Collaborators | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										995	10,976	46,000
Net Revenues from Collaborators | Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										$ 2,662	$ 1,038	$ 806

NET REVENUES FROM COLLABORATI_4

NET REVENUES FROM COLLABORATIONS - Balance of Receivables and Contract Liabilities Related to Collaboration Agreements (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Receivables included in "Accounts receivable, net"	$ 33,542	$ 14,929
Contract liabilities included in "Deferred revenue"	120,021	153,117
Revenue recognized on contract liability	$ 54,400	$ 4,000

NET REVENUES FROM COLLABORATI_5

NET REVENUES FROM COLLABORATIONS - 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, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	$ 654,819	$ 655,114	$ 505,420
Clinical Trial and Manufacturing
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	33,415	26,676	45,536
Clinical Trial and Manufacturing | Regeneron Pharmaceuticals
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	13,302	2,793	0
Clinical Trial and Manufacturing | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	644	11,505	36,600
Clinical Trial and Manufacturing | Vir Biotechnology
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	18,470	10,353	7,272
Clinical Trial and Manufacturing | Novartis
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	999	2,025	1,664
Clinical Trial and Manufacturing | Other
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	936	1,059	15,743
External Services | Regeneron Pharmaceuticals
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	171	344	0
External Services | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	181	334	5,340
External Services | Vir Biotechnology
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	584	381	8,251
External Services | Novartis
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	0	0	2
External Services | Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	0	0	2,150
Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	58,782	29,237	3,127
Other | Regeneron Pharmaceuticals
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	44,360	21,779	0
Other | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	2,132	2,017	1,279
Other | Vir Biotechnology
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	11,590	4,745	548
Other | Novartis
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	700	696	203
Other | Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	$ 0	$ 0	$ 1,097

NET REVENUES FROM COLLABORATI_6

NET REVENUES FROM COLLABORATIONS - Regeneron Pharmaceuticals (Detail) - Global Strategic Collaboration - Regeneron Pharmaceuticals	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Apr. 08, 2019USD ($)program	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
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
Collaborative agreement termination notice period			90 days
Increase (decrease) in research and development expense				$ 5,400,000	$ 0
Percentage of maximum royalty payments			20.00%
Transaction Price Allocated	$ 531,800,000	$ 531,800,000	$ 521,600,000
Increase (decrease) in transaction price				(23,300,000)
Transactional price remaining performance obligation	$ 212,900,000			$ 212,900,000
Funding At Program Initiation
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential proceeds from collaboration arrangement			2,500,000
Funding At Lead Candidate Identification
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential proceeds from collaboration arrangement			2,500,000
Funding An Annual Discovery
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential proceeds from collaboration arrangement			30,000,000
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%

NET REVENUES FROM COLLABORATI_7

NET REVENUES FROM COLLABORATIONS - Schedule of Transaction Price Allocated (Details) - Global Strategic Collaboration - Regeneron Pharmaceuticals - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Apr. 08, 2019
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction Price Allocated	$ 531,800	$ 531,800	$ 521,600
Research Services Obligation
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Standalone Selling Price	130,700
Transaction Price Allocated		200,600	183,100
C5 License Obligation
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Standalone Selling Price	97,600
Transaction Price Allocated		85,200	92,500
C5 Co-Co Obligation
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Standalone Selling Price	$ 364,600
Transaction Price Allocated		$ 246,000	$ 246,000

NET REVENUES FROM COLLABORATI_8

NET REVENUES FROM COLLABORATIONS - Schedule of Deferred Revenue (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Apr. 08, 2019
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 127,207	$ 77,821
Global Strategic Collaboration | Regeneron Pharmaceuticals
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction Price Allocated	531,800	531,800	$ 521,600
Deferred revenue	345,000	393,600
Research Services Obligation | Global Strategic Collaboration | Regeneron Pharmaceuticals
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction Price Allocated		200,600	183,100
Deferred revenue	54,900	84,800
C5 License Obligation | Global Strategic Collaboration | Regeneron Pharmaceuticals
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction Price Allocated		85,200	92,500
Deferred revenue	58,700	65,800
C5 Co-Co Obligation | Global Strategic Collaboration | Regeneron Pharmaceuticals
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction Price Allocated		246,000	$ 246,000
Deferred revenue	$ 231,400	$ 243,000

NET REVENUES FROM COLLABORATI_9

NET REVENUES FROM COLLABORATIONS - Schedule of Revenue Recognized by Accounting Guidance (Details) - USD ($) $ in Thousands	Jan. 01, 2018	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues recognized	$ 2,900	$ 163,562	$ 125,853	$ 103,962	$ 99,476	$ 71,681	$ 70,061	$ 44,714	$ 33,294	$ 492,853	$ 219,750	$ 74,908
Regeneron Pharmaceuticals | Global Strategic Collaboration
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues										63,600	23,900
Regeneron Pharmaceuticals | Research Services Obligation | Global Strategic Collaboration
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues recognized										44,800	21,000
Regeneron Pharmaceuticals | C5 License Obligation | Global Strategic Collaboration
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues recognized										7,100	0
Regeneron Pharmaceuticals | C5 Co-Co Obligation | Global Strategic Collaboration
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue recognized under ASC 808										$ 11,700	$ 2,900

NET REVENUES FROM COLLABORAT_10

NET REVENUES FROM COLLABORATIONS - Additional Information (Detail) $ in Thousands

Dec. 31, 2020USD ($)candidate

Jan. 01, 2018USD ($)shares

Jun. 30, 2020USD ($)

Mar. 31, 2020USD ($)shares

Jan. 31, 2019shares

Jan. 31, 2018USD ($)milestonePayment

Oct. 31, 2017USD ($)shares

Jan. 31, 2017USD ($)

Nov. 30, 2016

Dec. 31, 2014USD ($)

Feb. 28, 2013USD ($)

Dec. 31, 2020USD ($)candidate

Sep. 30, 2020USD ($)

Jun. 30, 2020USD ($)

Mar. 31, 2020USD ($)

Dec. 31, 2019USD ($)

Sep. 30, 2019USD ($)

Jun. 30, 2019USD ($)

Mar. 31, 2019USD ($)

Dec. 31, 2020USD ($)candidate

Dec. 31, 2019USD ($)

Dec. 31, 2018USD ($)

Dec. 31, 2020USD ($)candidate

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

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

5,000,000

Proceeds from issuance of common stock to Sanofi Genzyme

$ 99,498

$ 400,000

$ 0

Revenues recognized

$ 2,900

$ 163,562

$ 125,853

$ 103,962

$ 99,476

$ 71,681

$ 70,061

$ 44,714

$ 33,294

492,853

219,750

$ 74,908

Deferred revenue

$ 120,021

$ 120,021

$ 153,117

$ 120,021

153,117

$ 120,021

Vir Biotechnology

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

Upfront fee received

$ 10,000

Shares of common stock (in shares) | shares

1,111,111

Product Alliances

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

Upfront fee received

25,000

Product Alliances | Novartis AG

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

Amount earned upon achievement of milestone

$ 45,000

Maximum number of potential future milestones

$ 135,000

Potential future payment for the achievement of specified regulatory milestones

25,000

Potential future payment for the achievement of other regulatory milestones

10,000

Potential future payment for the achievement of specified commercialization milestones

$ 100,000

Product Alliances | Maximum | Novartis AG

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

Royalty rate

20.00%

20.00%

20.00%

20.00%

Product Alliances | Minimum | Novartis AG

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

Royalty rate

10.00%

10.00%

10.00%

10.00%

2018 Restructured Agreement | Fitusiran

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

Total Cost Incurred in Transition

38,000

Sanofi Genzyme

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

Amount earned upon achievement of milestone

11,000

Upfront fee received

22,500

Excess of fair value over cash received for stock issuance

$ 51,500

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

8,766,338

Proceeds from issuance of common stock to Sanofi Genzyme

$ 700,000

Sanofi Genzyme | Revusiran

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

Amount earned upon achievement of milestone

$ 25,000

Sanofi Genzyme | Fitusiran

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

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

147,300

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

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

Sanofi Genzyme | Global Collaboration Product

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

Royalty rate

20.00%

Sanofi Genzyme | TTR License

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

Transaction Price Allocated

127,600

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

Sanofi Genzyme | TTR License | Fitusiran

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

Amount earned upon achievement of milestone

50,000

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

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 | 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

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 | 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

Sanofi Genzyme | 2018 Restructured Agreement | Fitusiran

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

Amount earned upon achievement of milestone

$ 50,000

Revenues recognized

$ 37,600

Deferred revenue

$ 600

Vir Biotechnology

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

Amount earned upon achievement of milestone

$ 10,000

$ 15,000

Transaction Price Allocated

$ 110,100

Milestone shares earned (in shares) | shares

1,111,111

Collaborative agreement termination notice period

90 days

Development candidates to be delivered | candidate

4

4

4

4

Increase (decrease) in transaction price

$ 72,400

Transactional price remaining performance obligation

$ 51,700

$ 51,700

$ 51,700

$ 51,700

LIABILITY RELATED TO THE SALE_3

LIABILITY RELATED TO THE SALE OF FUTURE ROYALTIES (Details) - USD ($)	1 Months Ended	12 Months Ended
	Apr. 30, 2020	Dec. 31, 2020	Jan. 01, 2030	Apr. 10, 2020	Dec. 31, 2019
Liability Related To The Sale Of Future Royalties Line Items [Line Items]
Receivable related to the sale of future royalties		$ 500,000,000			$ 0
Liability related to the sale of future royalties, net of current portion		1,058,225,000			$ 0
Blackstone Group Inc. | Net Revenues from Collaborators
Liability Related To The Sale Of Future Royalties Line Items [Line Items]
Collaborative arrangement, royalties and commercial milestones acquired by collaborator, percent				50.00%
Commercial milestones acquired by collaborator, percent				75.00%
Expected royalty interest payments		1,000,000,000
Consideration received	$ 500,000,000
Receivable related to the sale of future royalties				$ 500,000,000
Liability related to the sale of future royalties, net of current portion		1,071,541,000		$ 1,000,000,000
Interest rate				11.00%
Closing costs		12,955,000
Interest expense including amortization of closing costs		$ 84,496,000
Forecast | Blackstone Group Inc. | Net Revenues from Collaborators
Liability Related To The Sale Of Future Royalties Line Items [Line Items]
Collaborative arrangement, royalties and commercial milestones acquired by collaborator, percent			55.00%

OTHER BALANCE SHEET DETAILS - S

OTHER BALANCE SHEET DETAILS - Schedule of Inventory (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Inventory [Line Items]
Raw materials	$ 63,460,000	$ 15,418,000
Work in process	16,149,000	38,275,000
Finished goods	12,693,000	2,655,000
Total inventory	92,302,000	56,348,000
Capitalized inventory	0	0
Other Assets
Inventory [Line Items]
Long-term inventory	$ 17,100,000	$ 0

OTHER BALANCE SHEET DETAILS -_2

OTHER BALANCE SHEET DETAILS - Schedule of Property, Plant, and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 569,376	$ 501,337
Less: accumulated depreciation	(104,347)	(76,158)
Total	465,029	425,179
Buildings
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	262,637	250,380
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	149,505	132,632
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	48,930	29,755
Manufacturing equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	41,089	0
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	28,005	54,195
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	19,064	14,956
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	11,066	10,339
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, 2020	Dec. 31, 2019
Balance Sheet Related Disclosures [Abstract]
Compensation and related	$ 97,433	$ 68,304
Product rebates and discounts	94,242	32,670
Pre-clinical, clinical trial and manufacturing	46,506	34,269
Contingent liabilities	41,216	0
Licensing and collaboration agreements	15,424	20,622
Consulting and professional services	11,501	14,251
Other	49,587	27,085
Total	$ 355,909	$ 197,201

OTHER BALANCE SHEET DETAILS -_4

OTHER BALANCE SHEET DETAILS - Schedule of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Balance Sheet Related Disclosures [Abstract]
Cash and cash equivalents	$ 496,580	$ 547,178	$ 420,146
Total restricted cash included in prepaid expenses, other current assets and long-term other assets	2,466	2,450	2,485
Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows	$ 499,046	$ 549,628	$ 422,631	$ 646,832

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, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 1,438,692	$ 1,301,965	$ 1,766,431
Other comprehensive (loss) income before reclassifications	(7,626)	(3,982)
Amounts reclassified from other comprehensive income	522	677
Net other comprehensive (loss) income	(7,104)	(3,305)	1,220
Ending Balance	1,016,247	1,438,692	1,301,965
Loss on Investment in Joint Venture
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(32,792)	(32,792)
Other comprehensive (loss) income before reclassifications	0	0
Amounts reclassified from other comprehensive income	0	0
Net other comprehensive (loss) 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	(3,520)	0
Other comprehensive (loss) income before reclassifications	(531)	(3,661)
Amounts reclassified from other comprehensive income	297	141
Net other comprehensive (loss) income	(234)	(3,520)
Ending Balance	(3,754)	(3,520)	0
Unrealized (Losses) Gains from Debt Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	137	(421)
Other comprehensive (loss) income before reclassifications	(14)	22
Amounts reclassified from other comprehensive income	225	536
Net other comprehensive (loss) income	211	558
Ending Balance	348	137	(421)
Foreign Currency Translation Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(343)	0
Other comprehensive (loss) income before reclassifications	(7,081)	(343)
Amounts reclassified from other comprehensive income	0	0
Net other comprehensive (loss) income	(7,081)	(343)
Ending Balance	(7,424)	(343)	0
Accumulated Other Comprehensive (Loss) Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(36,518)	(33,213)	(34,433)
Net other comprehensive (loss) income	(7,104)	(3,305)	1,220
Ending Balance	$ (43,622)	$ (36,518)	$ (33,213)

CREDIT AGREEMENT - Additional I

CREDIT AGREEMENT - Additional Information (Details) - Line of Credit - Secured Debt	Apr. 10, 2020USD ($)tranche	Dec. 31, 2020USD ($)	Dec. 31, 2020USD ($)
Financing Receivable, Impaired [Line Items]
Loan facility	$ 700,000,000
Number of tranches | tranche	3
Proceeds from lines of credit			$ 200,000,000
Maximum borrowing capacity, accordion feature	$ 50,000,000
Cancellation and reallocation feature	$ 100,000,000
Interest rate floor		8.00%	8.00%
Interest in kind, interest rate increase	1.00%
Funding fee		$ 5,000,000
Funding fee percentage	2.50%
Exit fee percent	1.00%
Prepayment and termination fee percentage	5.00%
Minimum consolidated liquidity	$ 100,000,000
LIBOR
Financing Receivable, Impaired [Line Items]
Debt instrument, basis spread on variable rate	7.00%	7.00%
LIBOR | Minimum
Financing Receivable, Impaired [Line Items]
Interest rate floor	1.00%
Base Rate
Financing Receivable, Impaired [Line Items]
Debt instrument, basis spread on variable rate	6.00%
Base Rate | Minimum
Financing Receivable, Impaired [Line Items]
Interest rate floor	2.00%
Tranche 2 Loan
Financing Receivable, Impaired [Line Items]
Term loan facility principal		$ 250,000,000	$ 250,000,000
Tranche 3 Loan
Financing Receivable, Impaired [Line Items]
Term loan facility principal		$ 250,000,000	$ 250,000,000
Tranche 2 Loan And Tranche 3 Loan | ONPATTRO And GIVLAARI
Financing Receivable, Impaired [Line Items]
Minimum product revenue	$ 300,000,000

DEVELOPMENT DERIVATIVE LIABIL_3

DEVELOPMENT DERIVATIVE LIABILITY - Narrative (Details) - USD ($)	Aug. 15, 2020	Dec. 31, 2020	Sep. 30, 2020
Derivative [Line Items]
Cost of borrowing	15.00%
Other Expense
Derivative [Line Items]
Fair value adjustment		$ 17,200,000
Not Designated as Hedging Instrument | Other Contract
Derivative [Line Items]
Derivative liability			$ 4,200,000
Blackstone Life Sciences
Derivative [Line Items]
Cost of borrowing	4.00%
Net Revenues from Collaborators | Blackstone Life Sciences | Vutrisiran and ALN-AGT
Derivative [Line Items]
Maximum funding	$ 150,000,000
Net Revenues from Collaborators | Blackstone Life Sciences | HELIOS-B Phase 3 Clinical Trial
Derivative [Line Items]
Maximum funding	70,000,000
Net Revenues from Collaborators | Blackstone Life Sciences | ALN-AGT Phase 2 Clinical Trial
Derivative [Line Items]
Maximum funding	$ 26,000,000
Fixed payment multiplier	3.25
Fixed payment, term	4 years
Net Revenues from Collaborators | Blackstone Life Sciences | ALN-AGT Phase 3 Clinical Trial
Derivative [Line Items]
Maximum funding	$ 54,000,000
Fixed payment multiplier	4.5
Fixed payment, term	4 years
Net Revenues from Collaborators | Blackstone Life Sciences | Vutrisiran
Derivative [Line Items]
Royalties payable, percent	1000.00%
Royalties payable, term	10 years
Fixed payment multiplier	2.5
Fixed payment, term	2 years

DEVELOPMENT DERIVATIVE LIABIL_4

DEVELOPMENT DERIVATIVE LIABILITY - Development Derivative Liability Activity (Details) - Derivative $ in Thousands	5 Months Ended
	Dec. 31, 2020USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Development derivative liability beginning balance	$ 0
Amount received under the Funding Agreement	8,400
Loss recorded from remeasurement of development derivative liability	17,185
Development derivative liability as of ending balance	$ 25,585

FAIR VALUE MEASUREMENTS - Fair

FAIR VALUE MEASUREMENTS - Fair Value of Assets Measured on a Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financial assets
Cash equivalents	$ 20,000	$ 340,132
Marketable debt securities	1,353,182	1,315,149
Marketable equity securities	44,633	13,967
Recurring
Financial assets
Marketable equity securities	44,633	13,967
Total financial assets	1,475,024	1,450,480
Financial liabilities
Development derivative liability	25,585
Recurring | Commercial paper
Financial assets
Cash equivalents		3,439
Marketable debt securities		36,474
Recurring | U.S. treasury securities
Financial assets
Cash equivalents	20,000	336,693
Marketable debt securities	1,087,968	755,714
Recurring | Money market funds
Financial assets
Cash equivalents	75,726	119,882
Restricted cash (money market funds)	1,483	1,482
Recurring | Certificates of deposit
Financial assets
Marketable debt securities		4,301
Recurring | Corporate notes
Financial assets
Marketable debt securities		146,040
Recurring | U.S. government-sponsored enterprise securities
Financial assets
Marketable debt securities	245,214	32,488
Recurring | Quoted Prices in Active Markets (Level 1)
Financial assets
Marketable equity securities	44,633	13,967
Total financial assets	121,842	135,331
Financial liabilities
Development derivative liability	0
Recurring | Quoted Prices in Active Markets (Level 1) | Commercial paper
Financial assets
Cash equivalents		0
Marketable debt securities		0
Recurring | Quoted Prices in Active Markets (Level 1) | U.S. treasury securities
Financial assets
Cash equivalents	0	0
Marketable debt securities	0	0
Recurring | Quoted Prices in Active Markets (Level 1) | Money market funds
Financial assets
Cash equivalents	75,726	119,882
Restricted cash (money market funds)	1,483	1,482
Recurring | Quoted Prices in Active Markets (Level 1) | Certificates of deposit
Financial assets
Marketable debt securities		0
Recurring | Quoted Prices in Active Markets (Level 1) | Corporate notes
Financial assets
Marketable debt securities		0
Recurring | Quoted Prices in Active Markets (Level 1) | U.S. government-sponsored enterprise securities
Financial assets
Marketable debt securities	0	0
Recurring | Significant Observable Inputs (Level 2)
Financial assets
Marketable equity securities	0	0
Total financial assets	1,353,182	1,315,149
Financial liabilities
Development derivative liability	0
Recurring | Significant Observable Inputs (Level 2) | Commercial paper
Financial assets
Cash equivalents		3,439
Marketable debt securities		36,474
Recurring | Significant Observable Inputs (Level 2) | U.S. treasury securities
Financial assets
Cash equivalents	20,000	336,693
Marketable debt securities	1,087,968	755,714
Recurring | Significant Observable Inputs (Level 2) | Money market funds
Financial assets
Cash equivalents	0	0
Restricted cash (money market funds)	0	0
Recurring | Significant Observable Inputs (Level 2) | Certificates of deposit
Financial assets
Marketable debt securities		4,301
Recurring | Significant Observable Inputs (Level 2) | Corporate notes
Financial assets
Marketable debt securities		146,040
Recurring | Significant Observable Inputs (Level 2) | U.S. government-sponsored enterprise securities
Financial assets
Marketable debt securities	245,214	32,488
Recurring | Significant Unobservable Inputs (Level 3)
Financial assets
Marketable equity securities	0	0
Total financial assets	0	0
Financial liabilities
Development derivative liability	25,585
Recurring | Significant Unobservable Inputs (Level 3) | Commercial paper
Financial assets
Cash equivalents		0
Marketable debt securities		0
Recurring | Significant Unobservable Inputs (Level 3) | U.S. treasury securities
Financial assets
Cash equivalents	0	0
Marketable debt securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Money market funds
Financial assets
Cash equivalents	0	0
Restricted cash (money market funds)	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Certificates of deposit
Financial assets
Marketable debt securities		0
Recurring | Significant Unobservable Inputs (Level 3) | Corporate notes
Financial assets
Marketable debt securities		0
Recurring | Significant Unobservable Inputs (Level 3) | U.S. government-sponsored enterprise securities
Financial assets
Marketable debt securities	$ 0	$ 0

FAIR VALUE MEASUREMENTS - Addit

FAIR VALUE MEASUREMENTS - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020asset
Fair Value Disclosures [Abstract]
Transfers from Level 2 to Level 1 financial assets	1

MARKETABLE DEBT SECURITIES - Su

MARKETABLE DEBT SECURITIES - Summary of Company's Marketable Debt Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,352,834	$ 1,315,012
Gross Unrealized Gains	463	246
Gross Unrealized Losses	(115)	(109)
Fair Value	1,353,182	1,315,149
Corporate notes
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		146,016
Gross Unrealized Gains		58
Gross Unrealized Losses		(34)
Fair Value		146,040
U.S. government-sponsored enterprise securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	245,113	32,487
Gross Unrealized Gains	135	3
Gross Unrealized Losses	(34)	(2)
Fair Value	245,214	32,488
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,107,721	1,092,293
Gross Unrealized Gains	328	185
Gross Unrealized Losses	(81)	(71)
Fair Value	$ 1,107,968	1,092,407
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		4,303
Gross Unrealized Gains		0
Gross Unrealized Losses		(2)
Fair Value		4,301
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		39,913
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		$ 39,913

MARKETABLE DEBT SECURITIES - _2

MARKETABLE DEBT SECURITIES - Summary of Fair Value of Marketable Debt Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Cash and cash equivalents	$ 20,000	$ 340,132
Marketable debt securities	1,333,182	975,017
Total	$ 1,353,182	$ 1,315,149

LEASES - Additional Information

LEASES - Additional Information (Detail) $ in Thousands	May 01, 2018	Dec. 31, 2020USD ($)renewal_option	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Operating Leased Assets [Line Items]
Security deposit		$ 40,725	$ 14,825
Lease expense		50,700	52,400	$ 40,600
Net cash paid included in operating activities in cash flow		$ 38,000	$ 33,700
Operating lease, weighted average remaining lease term		12 years	13 years
Operating lease, weighted-average discount rate		8.00%	8.00%
BMR-675 West Kendall Lease
Operating Leased Assets [Line Items]
Lease term	15 years
Number of lease extension options | renewal_option		2
Operating lease renewal options period		5 years
Security deposit		$ 14,800
Third Street Lease
Operating Leased Assets [Line Items]
Number of lease extension options | renewal_option		2
Operating lease renewal options period		5 years
101 Main Street Leases
Operating Leased Assets [Line Items]
Number of lease extension options | renewal_option		1
Operating lease renewal options period		5 years

LEASES - Summary of Costs Inclu

LEASES - Summary of Costs Included in Operating Expenses Related to Leases (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease cost	$ 42,271	$ 38,613
Variable lease cost	11,049	15,209
Total	$ 53,320	$ 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) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
2021	$ 38,593
2022	46,392
2023	43,764
2024	42,918
2025	41,190
2026 and thereafter	323,473
Total undiscounted lease liability	536,330
Less imputed interest	(206,419)
Total discounted lease liability	329,911
Current operating lease liability	36,872	$ 27,688
Non-current operating lease liability	$ 293,039	$ 276,135

COMMITMENTS AND CONTINGENCIES -

COMMITMENTS AND CONTINGENCIES - Additional Information (Detail) - Ionis - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	Jan. 31, 2021	Dec. 31, 2020	Dec. 31, 2020
Commitments And Contingencies [Line Items]
Amount awarded to other party		$ 41.2
Increase in contingent liability			$ 38.2
Subsequent Event
Commitments And Contingencies [Line Items]
Litigation settlement expense	$ 41.2

STOCKHOLDERS' EQUITY - Addition

STOCKHOLDERS' EQUITY - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	May 21, 2019	Apr. 25, 2019	Apr. 30, 2020	Apr. 30, 2019	Jan. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
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
Offering proceeds, net of costs						$ 0	$ 381,900	$ 0
Underwritten public offering amount per share (in dollars per share)					$ 77.50
Common stock, shares authorized (in shares)						250,000,000	250,000,000
Issuance of common stock, net of offering costs					$ 381,900		$ 772,477
Underwriting discounts and commissions and other offering expenses					$ 5,600
Common Stock
Equity [Line Items]
Issuance of common stock, net of offering costs (in shares)							9,444,000
Issuance of common stock, net of offering costs							$ 94
Blackstone Group Inc. | Common Stock | Private Placement
Equity [Line Items]
Number of shares issued in transaction (in shares)			963,486
Consideration received on transaction			$ 100,000
Price per share (in dollars per share)			$ 103.79
Global Strategic Collaboration | Regeneron Pharmaceuticals
Equity [Line Items]
Issuance of common stock, net of offering costs (in shares)	4,444,445			4,444,445
Offering proceeds, net of costs	$ 400,000			$ 400,000
Underwritten public offering amount per share (in dollars per share)				$ 90
Mark-to-market adjustment gain		$ 9,400
Global Strategic Collaboration | Regeneron Pharmaceuticals | Series A Redeemable Convertible Preferred Stock
Equity [Line Items]
Preferred stock, par value (in dollars per share)				$ 0.01
Issuance of common stock, net of offering costs (in shares)				1,481,482
Underwritten public offering amount per share (in dollars per share)				$ 270
Global Strategic Collaboration | Regeneron Pharmaceuticals | Common Stock | Series A Redeemable Convertible Preferred Stock
Equity [Line Items]
Convertible preferred stock, shares issued upon conversion (in shares)				3
Global Strategic Collaboration | Minimum | Regeneron Pharmaceuticals
Equity [Line Items]
Common stock, shares authorized (in shares)		125,000,000
Global Strategic Collaboration | Maximum | Regeneron Pharmaceuticals
Equity [Line Items]
Common stock, shares authorized (in shares)		250,000,000

STOCK-BASED COMPENSATION - Addi

STOCK-BASED COMPENSATION - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		12 Months Ended
	May 31, 2020	May 31, 2017	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock reserved for future issuance under stock options (in shares)			23,035,819
Employee stock purchase plan, offering period		6 months
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)			11,692,209
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)			1,160,427
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)			9,223,025
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock incentive plan, shares authorized (in shares)	1,965,789	1,215,789
Shares of common stock reserved for future issuance under stock options (in shares)			960,158
Employee stock purchase plan, purchase price as a percentage of common stock closing price		85.00%
Employee stock purchase plan, shares issued			129,394	109,590	78,085
Performance-based stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value of stock option exercised			$ 34.1	$ 11	$ 8
Stock options vested (in shares)			0	889,896	763,982
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)			$ 66.28	$ 49.27	$ 66.49
Aggregate intrinsic value of stock option exercised			$ 177.8	$ 55.4	$ 87.1
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
Stock Incentive Plan 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock incentive plan, additional shares authorized (in shares)	7,000,000

STOCK-BASED COMPENSATION - Expe

STOCK-BASED COMPENSATION - Expenses included in operating costs and expenses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 139,873	$ 174,841	$ 157,752
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	60,464	88,930	80,509
Selling, general and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 79,409	$ 85,911	$ 77,243

STOCK-BASED COMPENSATION - Summ

STOCK-BASED COMPENSATION - Summary of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Less: Stock-based compensation expense capitalized to inventory	$ (571)	$ (3,403)	$ (1,424)
Total stock-based compensation expense	139,873	174,841	157,752
Time-based stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expensed and capitalized	112,971	99,097	83,403
Performance-based stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expensed and capitalized	6,340	48,207	56,419
Time-based restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expensed and capitalized	6,909	2,351	538
Performance-based restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expensed and capitalized	11,162	22,123	13,144
Other equity programs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, expensed and capitalized	$ 3,062	$ 6,466	$ 5,672

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, 2020USD ($)
Time-based stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized Expense, Net of Estimated Forfeitures (in thousands)	$ 183,700
Weighted- average Recognition Period (in years)	2 years 5 months 8 days
Time-based restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized Expense, Net of Estimated Forfeitures (in thousands)	$ 1,593
Weighted- average Recognition Period (in years)	2 months 12 days
Performance-based restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized Expense, Net of Estimated Forfeitures (in thousands)	$ 1,286
Other equity programs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized Expense, Net of Estimated Forfeitures (in thousands)	$ 8,159
Weighted- average Recognition Period (in years)	2 years 6 months 25 days

STOCK-BASED COMPENSATION - Valu

STOCK-BASED COMPENSATION - Valuation Assumptions for Stock Options (Detail) - Stock Options	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum	0.30%	1.40%	2.70%
Risk-free interest rate, maximum	1.70%	2.60%	2.90%
Expected dividend yield	0.00%	0.00%	0.00%
Expected volatility, minimum	61.00%	63.00%	64.00%
Expected volatility, maximum	63.00%	66.00%	67.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected option life	5 years 4 months 24 days	5 years 7 months 6 days	5 years 8 months 12 days
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected option life	7 years 2 months 12 days	7 years 3 months 18 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, 2020USD ($)$ / sharesshares
Time-based stock options
Number of Options
Outstanding, beginning balance (in shares) | shares	10,878
Granted (in shares) | shares	2,086
Exercised (in shares) | shares	(2,390)
Cancelled (in shares) | shares	(506)
Outstanding, ending balance (in shares) | shares	10,068
Exercisable at December 31, 2020 (in shares) | shares	5,835
Vested or expected to vest at December 31, 2020 (in shares) | shares	9,692
Weighted-average Exercise Price
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 76.92
Granted (in dollars per share) | $ / shares	119.96
Exercised (in dollars per share) | $ / shares	63.34
Cancelled (in dollars per share) | $ / shares	94.53
Outstanding, ending balance (in dollars per share) | $ / shares	88.18
Exercisable at December 31, 2020 (in dollars per share) | $ / shares	76.79
Vested or expected to vest at December 31, 2020 (in dollars per share) | $ / shares	$ 87.53
Weighted-average Remaining Contractual Term
Outstanding	6 years 6 months 21 days
Exercisable	5 years 2 months 8 days
Vested or expected to vest	6 years 5 months 23 days
Aggregate Intrinsic Value
Outstanding | $	$ 423,864
Exercisable | $	310,933
Vested or expected to vest | $	$ 414,208
Performance-based stock options
Number of Options
Outstanding, beginning balance (in shares) | shares	2,191
Granted (in shares) | shares	0
Exercised (in shares) | shares	(538)
Cancelled (in shares) | shares	(28)
Outstanding, ending balance (in shares) | shares	1,625
Exercisable at December 31, 2020 (in shares) | shares	1,414
Weighted-average Exercise Price
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 86.77
Granted (in dollars per share) | $ / shares	0
Exercised (in dollars per share) | $ / shares	71.28
Cancelled (in dollars per share) | $ / shares	118.62
Outstanding, ending balance (in dollars per share) | $ / shares	91.35
Exercisable at December 31, 2020 (in dollars per share) | $ / shares	$ 87.22
Weighted-average Remaining Contractual Term
Outstanding	5 years 1 month 20 days
Exercisable	4 years 10 months 17 days
Aggregate Intrinsic Value
Outstanding | $	$ 62,740
Exercisable | $	$ 60,458

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, 2020$ / sharesshares
Restricted stock units
Number of Units
Outstanding, beginning balance (in shares) | shares	127
Granted (in shares) | shares	17
Vested (in shares) | shares	(16)
Cancelled (in shares) | shares	(11)
Outstanding, ending balance (in shares) | shares	117
Weighted-average Grant Date Fair Value
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 77.23
Granted (in dollars per share) | $ / shares	134.34
Vested (in dollars per share) | $ / shares	92.97
Cancelled (in dollars per share) | $ / shares	76.98
Outstanding, ending balance (in dollars per share) | $ / shares	$ 83.66
Performance-based restricted stock units
Number of Units
Outstanding, beginning balance (in shares) | shares	623
Granted (in shares) | shares	715
Vested (in shares) | shares	(206)
Cancelled (in shares) | shares	(89)
Outstanding, ending balance (in shares) | shares	1,043
Weighted-average Grant Date Fair Value
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 85.36
Granted (in dollars per share) | $ / shares	119.11
Vested (in dollars per share) | $ / shares	85.67
Cancelled (in dollars per share) | $ / shares	99.08
Outstanding, ending balance (in dollars per share) | $ / shares	$ 107.26

STOCK-BASED COMPENSATION - St_2

STOCK-BASED COMPENSATION - Stock Purchase Rights Granted Under ESPP (Details) - Employee Stock Purchase Plan	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum	0.10%	1.60%	2.00%
Risk-free interest rate, maximum	0.10%	2.40%	2.50%
Expected dividend yield	0.00%	0.00%	0.00%
Expected option life	6 months	6 months	6 months
Expected volatility, minimum	40.00%	37.00%	47.00%
Expected volatility, maximum	50.00%	56.00%	49.00%

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, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Domestic	$ (682,859)	$ (597,602)	$ (573,245)
Foreign	(172,741)	(287,651)	(187,429)
Loss before income taxes	$ (855,600)	$ (885,253)	$ (760,674)

INCOME TAXES - Schedule of Prov

INCOME TAXES - Schedule of Provision for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current provision:
Domestic	$ 61	$ (394)	$ 0
Foreign	5,837	3,232	1,611
Total current provision	5,898	2,838	1,611
Deferred benefit:
Domestic	393	394	(788)
Foreign	(3,610)	(2,369)	0
Total deferred benefit	(3,217)	(1,975)	(788)
Total provision for income taxes	$ 2,681	$ 863	$ 823

INCOME TAXES - Schedule of Comp

INCOME TAXES - Schedule of Components of Net Deferred Tax (Liability) Asset (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating loss carryforwards	$ 537,382	$ 627,466
Research and development and ODC credits	301,792	261,616
Sale of future royalties	259,014	0
Lease liability	70,402	69,334
Deferred revenue	84,946	0
Deferred compensation	67,530	75,058
Intangible assets	148,168	66,615
Other	32,725	13,660
Total deferred tax assets	1,501,959	1,113,749
Deferred tax liabilities:
Property, plant and equipment, net	(10,812)	(10,077)
Unrealized gain on marketable securities	(12,766)	(3,932)
Right of use assets	(50,323)	(50,294)
Deferred revenue tax accounting method change	(71,812)	0
Deferred tax asset valuation allowance	(1,349,729)	(1,046,013)
Net deferred tax asset	$ 6,517	$ 3,433

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, 2020	Dec. 31, 2019	Dec. 31, 2018
Effective Income Tax Rate Reconciliation, Percent
At U.S. federal statutory rate	21.00%	21.00%	21.00%
State taxes, net of federal effect	4.50%	3.60%	2.10%
Stock-based compensation	2.20%	0.00%	0.80%
Tax credits	3.30%	3.70%	4.20%
Other permanent items	(1.50%)	(0.30%)	(0.30%)
Foreign rate differential	(3.50%)	(6.90%)	(5.30%)
Internal reorganization of certain intellectual property rights	12.30%	0.00%	0.00%
Revaluation of deferred credits due to rate change	0.00%	0.00%	(3.50%)
Other	(2.70%)	(0.10%)	0.00%
Valuation allowance	(35.90%)	(21.00%)	(19.00%)
Effective income tax rate	(0.30%)	0.00%	0.00%

INCOME TAXES - Additional Infor

INCOME TAXES - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes [Line Items]
Increase (decrease) in valuation allowance	$ 303,700	$ 185,900	$ 171,800
Provision for income taxes	2,681	863	823
Foreign	5,837	3,232	1,611
Deferred tax benefit in foreign jurisdictions	3,610	$ 2,369	$ 0
Federal
Income Taxes [Line Items]
Net operating loss carryforward	2,000,000
Net operating loss carryforwards, not subject to expiration	900,000
Net operating loss carryforwards, subject to expiration	1,100,000
State and local jurisdiction
Income Taxes [Line Items]
Net operating loss carryforward	1,900,000
Research and Development, Including Orphan Drug, and State Investments Tax Credit | Federal
Income Taxes [Line Items]
Tax credit carryforwards	272,500
Research and Development, Including Orphan Drug, and State Investments Tax Credit | State and local jurisdiction
Income Taxes [Line Items]
Tax credit carryforwards	$ 43,100

DEFINED BENEFIT PLANS - Additio

DEFINED BENEFIT PLANS - Additional Information (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Other Liabilities | Unfunded Plan
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	$ (5.2)	$ (4.3)

QUARTERLY FINANCIAL DATA (UNA_3

QUARTERLY FINANCIAL DATA (UNAUDITED) - Schedule of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	Jan. 01, 2018	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Selected Quarterly Financial Information [Abstract]
Revenues	$ 2,900	$ 163,562	$ 125,853	$ 103,962	$ 99,476	$ 71,681	$ 70,061	$ 44,714	$ 33,294	$ 492,853	$ 219,750	$ 74,908
Operating costs and expenses		357,784	351,052	302,821	309,634	369,754	286,360	280,985	222,082	1,321,291	1,159,181	889,581
Net loss		$ (243,540)	$ (253,291)	$ (179,229)	$ (182,221)	$ (276,185)	$ (208,535)	$ (219,481)	$ (181,915)	$ (858,281)	$ (886,116)	$ (761,497)
Net loss per common share - basic and diluted (in dollars per share)		$ (2.09)	$ (2.18)	$ (1.56)	$ (1.62)	$ (2.47)	$ (1.92)	$ (2.02)	$ (1.73)	$ (7.46)	$ (8.11)	$ (7.57)
Weighted-average common shares — basic and diluted (in shares)		116,274	115,986	114,911	112,748	111,750	108,701	108,576	105,400	114,986	109,264	100,590

Uncategorized Items - alny-2020

Label	Element	Value
Accounting Standards Update [Extensible List]	us-gaap_AccountingStandardsUpdateExtensibleList	us-gaap:AccountingStandardsUpdate201409Member