Alnylam Pharmaceuticals (ALNY) 10-Q 6 Aug 19 2019 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2019	Jul. 31, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Trading Symbol	ALNY
Entity Registrant Name	ALNYLAM PHARMACEUTICALS, INC.
Entity Central Index Key	0001178670
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity Common Stock, Shares Outstanding		111,207,651
Entity File Number	001-36407
Entity Tax Identification Number	770602661
Entity Address, Address Line One	300 Third Street,
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

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 1,136,289	$ 420,146
Marketable debt securities	790,542	662,803
Accounts receivable, net	30,739	18,760
Inventory	40,587	24,068
Prepaid expenses and other current assets	65,616	74,919
Total current assets	2,063,773	1,200,696
Property, plant and equipment, net	371,769	320,658
Operating lease right-of-use assets	226,357
Restricted investments	44,825	44,825
Other assets	9,687	8,623
Total assets	2,716,411	1,574,802
Current liabilities:
Accounts payable	71,215	59,708
Accrued expenses	138,800	112,719
Operating lease liability	29,347
Deferred rent		3,571
Deferred revenue	89,242	3,496
Total current liabilities	328,604	179,494
Operating lease liability, net of current portion	277,211
Deferred rent, net of current portion		57,920
Deferred revenue, net of current portion	313,887	458
Long-term debt	30,000	30,000
Other liabilities	9,514	4,965
Total liabilities	959,216	272,837
Commitments and contingencies (Note 14)
Stockholders’ equity:
Preferred stock, $0.01 par value per share, 5,000 shares authorized and no shares issued and outstanding at June 30, 2019 and December 31, 2018
Common stock, $0.01 par value per share, 250,000 and 125,000 shares authorized at June 30, 2019 and December 31, 2018, respectively; 111,114 shares issued and outstanding at June 30, 2019; 101,177 shares issued and outstanding at December 31, 2018	1,110	1,011
Additional paid-in capital	5,034,284	4,175,139
Accumulated other comprehensive loss	(35,831)	(33,213)
Accumulated deficit	(3,242,368)	(2,840,972)
Total stockholders’ equity	1,757,195	1,301,965
Total liabilities and stockholders’ equity	$ 2,716,411	$ 1,574,802

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Jun. 30, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	250,000,000	125,000,000
Common stock, shares issued	111,114,000	101,177,000
Common stock, shares outstanding	111,114,000	101,177,000

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenues:
Revenues	$ 44,714	$ 29,907	$ 78,008	$ 51,806
Costs and expenses:
Cost of goods sold	4,326		7,673
Research and development	163,890	137,582	293,017	234,439
Selling, general and administrative	112,769	84,679	202,377	157,126
Total costs and expenses	280,985	222,261	503,067	391,565
Loss from operations	(236,271)	(192,354)	(425,059)	(339,759)
Other income (expense):
Interest income	8,781	6,101	16,306	11,895
Other (expense) income	(453)	2,208	(410)	2,543
Change in fair value of liability obligation	9,422		9,422
Gain on litigation settlement		20,564		20,564
Total other income	17,750	28,873	25,318	35,002
Loss before income taxes	(218,521)	(163,481)	(399,741)	(304,757)
Provision for income taxes	(960)	(79)	(1,655)	(17)
Net loss	$ (219,481)	$ (163,560)	$ (401,396)	$ (304,774)
Net loss per common share - basic and diluted	$ (2.02)	$ (1.63)	$ (3.75)	$ (3.04)
Weighted-average common shares used to compute basic and diluted net loss per common share	108,576	100,519	106,997	100,251
Comprehensive loss:
Net loss	$ (219,481)	$ (163,560)	$ (401,396)	$ (304,774)
Unrealized gain on marketable securities, net of tax	462	1,046	822	626
Foreign currency translation	842		842
Unrealized loss on pension obligation	(4,282)		(4,282)
Comprehensive loss	(222,459)	(162,514)	(404,014)	(304,148)
Product Revenues, Net
Revenues:
Revenues	38,231		64,522
Net Revenues from Collaborators
Revenues:
Revenues	$ 6,483	$ 29,907	$ 13,486	$ 51,806

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance at Dec. 31, 2017	$ 1,766,431	$ 997	$ 3,947,552	$ (34,433)	$ (2,147,685)
Balance (in shares) at Dec. 31, 2017		99,667
Cumulative effect adjustment from the adoption of new revenue standard	68,210				68,210
Exercise of common stock options, net of tax withholdings	44,451	$ 8	44,443
Exercise of common stock options, net of tax withholdings (in shares)		848
Issuance of common stock under equity plans	3,001		3,001
Issuance of common stock under equity plans (in shares)		43
Issuance of common stock under benefit plans	1,397		1,397
Issuance of common stock under benefit plans (in shares)		11
Stock-based compensation expense related to equity-classified awards	41,460		41,460
Other comprehensive gain (loss), net of tax	626			626
Net loss	(304,774)				(304,774)
Balance at Jun. 30, 2018	1,620,802	$ 1,005	4,037,853	(33,807)	(2,384,249)
Balance (in shares) at Jun. 30, 2018		100,569
Balance at Mar. 31, 2018	1,754,805	$ 1,005	4,009,342	(34,853)	(2,220,689)
Balance (in shares) at Mar. 31, 2018		100,468
Exercise of common stock options, net of tax withholdings	2,561		2,561
Exercise of common stock options, net of tax withholdings (in shares)		53
Issuance of common stock under equity plans	3,124		3,124
Issuance of common stock under equity plans (in shares)		41
Issuance of common stock under benefit plans	829		829
Issuance of common stock under benefit plans (in shares)		7
Stock-based compensation expense related to equity-classified awards	21,997		21,997
Other comprehensive gain (loss), net of tax	1,046			1,046
Net loss	(163,560)				(163,560)
Balance at Jun. 30, 2018	1,620,802	$ 1,005	4,037,853	(33,807)	(2,384,249)
Balance (in shares) at Jun. 30, 2018		100,569
Balance at Dec. 31, 2018	$ 1,301,965	$ 1,011	4,175,139	(33,213)	(2,840,972)
Balance (in shares) at Dec. 31, 2018	101,177	101,177
Exercise of common stock options, net of tax withholdings	$ 17,591	$ 5	17,586
Exercise of common stock options, net of tax withholdings (in shares)		410
Issuance of common stock under equity plans	3,964		3,964
Issuance of common stock under equity plans (in shares)		59
Issuance of common stock under benefit plans	1,873		1,873
Issuance of common stock under benefit plans (in shares)		24
Issuance of common stock, net of offering costs	772,477	$ 94	772,383
Issuance of common stock, net of offering costs (in shares)		9,444
Stock-based compensation expense related to equity-classified awards	63,339		63,339
Other comprehensive gain (loss), net of tax	(2,618)			(2,618)
Net loss	(401,396)				(401,396)
Balance at Jun. 30, 2019	$ 1,757,195	$ 1,110	5,034,284	(35,831)	(3,242,368)
Balance (in shares) at Jun. 30, 2019	111,114	111,114
Balance at Mar. 31, 2019	$ 1,546,986	$ 1,064	4,601,662	(32,853)	(3,022,887)
Balance (in shares) at Mar. 31, 2019		106,400
Exercise of common stock options, net of tax withholdings	6,182	$ 2	6,180
Exercise of common stock options, net of tax withholdings (in shares)		203
Issuance of common stock under equity plans	4,022		4,022
Issuance of common stock under equity plans (in shares)		55
Issuance of common stock under benefit plans	1,089		1,089
Issuance of common stock under benefit plans (in shares)		12
Issuance of common stock, net of offering costs	390,577	$ 44	390,533
Issuance of common stock, net of offering costs (in shares)		4,444
Stock-based compensation expense related to equity-classified awards	30,798		30,798
Other comprehensive gain (loss), net of tax	(2,978)			(2,978)
Net loss	(219,481)				(219,481)
Balance at Jun. 30, 2019	$ 1,757,195	$ 1,110	$ 5,034,284	$ (35,831)	$ (3,242,368)
Balance (in shares) at Jun. 30, 2019	111,114	111,114

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss	$ (401,396)	$ (304,774)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation, amortization and accretion, net	21,955	5,417
Stock-based compensation	62,635	41,825
Non-cash gain on litigation settlement		(10,000)
Charge for 401(k) company stock match	2,090	2,326
Fair value adjustments on marketable equity securities	(21)	(2,044)
Change in fair value of liability obligation	(9,422)
Other	786
Changes in operating assets and liabilities:
Accounts receivable, net	(11,934)	31,041
Proceeds from landlord lease incentive for tenant improvements	18,700	4,480
Inventory	(15,040)
Prepaid expenses and other assets	4,863	(21,261)
Accounts payable	26,369	(9,735)
Accrued expenses and other	1,427	7,261
Deferred revenue	399,173	(2,887)
Net cash provided by (used in) operating activities	100,185	(258,351)
Cash flows from investing activities:
Purchases of property, plant and equipment	(65,293)	(43,965)
Purchases of restricted investments		(14,825)
Purchases of marketable debt securities	(834,563)	(642,787)
Sales and maturities of marketable securities	713,106	629,384
Net cash used in investing activities	(186,750)	(72,193)
Cash flows from financing activities:
Proceeds from exercise of stock options and other types of equity	21,725	47,735
Offering proceeds, net of costs	381,900
Proceeds from issuance of common stock to Regeneron	400,000
Payments for repurchase of common stock for employee tax withholding	(84)	(632)
Net cash provided by financing activities	803,541	47,103
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(3)
Net increase (decrease) in cash, cash equivalents and restricted cash	716,973	(283,441)
Cash, cash equivalents and restricted cash, beginning of period	422,631	646,832
Cash, cash equivalents and restricted cash, end of period	1,139,604	363,391
Supplemental disclosure of noncash investing activities:
Capital expenditures included in accounts payable and accrued expenses	$ 25,893	$ 23,288

NATURE OF BUSINESS

NATURE OF BUSINESS	6 Months Ended
	Jun. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
NATURE OF BUSINESS	1. NATURE OF BUSINESS We 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, commercial biopharmaceutical company with a sustainable pipeline of RNAi therapeutics to address the needs of patients who have limited or inadequate treatment options. Since inception, we have focused on discovering, developing and commercializing RNAi therapeutics by establishing and maintaining a strong intellectual property position in the RNAi field, establishing strategic alliances with leading pharmaceutical and life sciences companies, generating revenues through licensing agreements, and ultimately developing and commercializing RNAi therapeutics globally, either independently or with our strategic partners. We have devoted substantially all of our efforts to business planning, research, development, manufacturing and early commercial efforts, acquiring, filing and expanding intellectual property rights, recruiting management and technical staff, and raising capital. In August 2018, we received approval for ONPATTRO ® In June 2019, we received regulatory approval of ONPATTRO in Japan and Canada

BASIS OF PRESENTATION AND PRINC

BASIS OF PRESENTATION AND PRINCIPLES OF CONSOLIDATION	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
BASIS OF PRESENTATION AND PRINCIPLES OF CONSOLIDATION	2. BASIS OF PRESENTATION AND PRINCIPLES OF CONSOLIDATION The accompanying condensed consolidated financial statements of Alnylam Pharmaceuticals, Inc. are unaudited and have been prepared in accordance with accounting principles generally accepted in the United States of America, or GAAP, applicable to interim periods and, in the opinion of management, include all normal and recurring adjustments that are necessary to state fairly the results of operations for the reported periods. Our condensed consolidated financial statements have also been prepared on a basis substantially consistent with, and should be read in conjunction with, our audited consolidated financial statements for the year ended December 31, 2018, which were included in our Annual Report on Form 10-K that was filed with the Securities and Exchange Commission on February 14, 2019. The year-end condensed consolidated balance sheet data was derived from our audited financial statements but does not include all disclosures required by GAAP. The results of our operations for any interim period are not necessarily indicative of the results of our operations for any other interim period or for a full fiscal year. The accompanying condensed consolidated financial statements reflect the operations of Alnylam and our wholly-owned subsidiaries. All intercompany accounts and transactions have been eliminated. Our significant accounting policies are described in Note 2 of the Notes to Consolidated Financial Statements included in our Annual Report on Form 10-K for the year ended December 31, 2018. Updates to our significant accounting policies, including the updated lease accounting policy due to the adoption of the new leasing accounting standard, are discussed below and under “Recent Accounting Pronouncements.” Reclassification Certain prior period amounts in the condensed consolidated financial statements have been reclassified to conform to the current period presentation. Liquidity Based on our current operating plan, we believe that our cash, cash equivalents and marketable debt securities at June 30, 2019, together with the cash we expect to generate from product sales and under our alliances, will be sufficient to enable us to advance our Alnylam 2020 Leases We determine whether a contract is, or contains, a lease at inception. We classify each of our leases as operating or financing considering factors such as the length of the lease term, the present value of the lease payments, the nature of the asset being leased, and the potential for ownership of the asset to transfer during the lease term. Leases with terms greater than one-year are recognized on the condensed consolidated balance sheets as right-of-use assets and lease liabilities and are measured at the present value of the fixed payments due over the expected lease term minus the present value of any incentives, rebates or abatements we expect to receive from the lessor. Options to extend a lease are included in the expected lease term if exercise of the option is deemed reasonably certain. Costs determined to be variable and not based on an index or rate are not included in the measurement of the lease liability and are expensed as incurred. The interest rate implicit in lease contracts is typically not readily determinable. As such, we utilize the appropriate incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis an amount equal to the lease payments over a similar term and in a similar economic environment. We record expense to recognize fixed lease payments on a straight-line basis over the expected lease term. We have elected the practical expedient not to separate lease and non-lease components for real estate leases. Recent Accounting Pronouncements In February 2016, the Financial Accounting Standards Board, or FASB, issued a new leasing standard which generally requires lessees to recognize operating and financing lease liabilities and corresponding right-of-use assets on the condensed consolidated balance sheet and to provide enhanced disclosures surrounding the amount, timing and uncertainty of cash flows arising from leasing arrangements. We adopted the new standard on January 1, 2019, using a modified retrospective basis and did not restate comparative periods. In addition, we did not elect the package of practical expedients permitted under the transition guidance that permits companies to carry forward prior conclusions related to (1) whether any expired or existing contracts are, or contain, leases, (2) the lease classification for expired or existing leases, and (3) initial direct costs for existing leases. All our leases have been classified as operating leases under the new leasing standard. We elected to combine lease and non-lease components and to keep leases with an initial term of 12 months or less off the condensed consolidated balance sheets and recognize the associated lease payments in the condensed consolidated statements of comprehensive loss on a straight-line basis over the lease term. Please read Note 8 for additional disclosures related to accounting for leases under this new standard. The adoption of ASC 842 has a material impact on our condensed consolidated balance sheet as the standard requires us to measure and recognize a right of use asset and lease liability. As most leases do not provide an implicit rate, our incremental borrowing rate was determined based on the information available at the date of adoption to measure our lease liability. Costs determined to be variable and not based on an index or rate were not included in the measurement of the lease liability. We recognized approximately $290.0 million of operating lease liabilities and approximately $230.0 million of operating lease right-of-use assets on our condensed consolidated balance sheet as of January 1, 2019, which are presented as separate line items on the condensed consolidated balance sheet. Had we not adopted the new leasing standard, we would not have had operating lease right-of-use assets or operating lease liabilities on our condensed consolidated balance sheet. The adoption of the standard did not have a material impact on our condensed consolidated statement of comprehensive income. In March 2017, the FASB issued a new standard that amends the amortization period for certain purchased callable debt securities held at a premium by shortening the amortization period for the premium to the earliest call date. The new standard became effective for us on January 1, 2019. This standard did not have a significant impact on our condensed consolidated financial statements and related disclosures. In August 2018, the FASB issued amendments that eliminate, add and modify certain disclosure requirements on fair value measurements. The amendments become effective for our fiscal year, including interim periods, beginning January 1, 2020. Early adoption of the amendments in full or only the provisions that eliminate or modify the disclosure requirements for fair value measurements is permitted. We are currently evaluating the timing of our adoption and the expected impact that these amendments could have on our disclosures In November 2018, the FASB issued guidance to clarify the interaction between the accounting guidance for collaborative arrangements and revenue from contracts with customers. The amendments become effective for our fiscal year, including interim periods, beginning January 1, 2020. Early adoption, including adoption in any interim period, is permitted. This guidance is required to be applied retrospectively as of the date of our adoption of the new revenue standard on January 1, 2018. We are currently evaluating the timing of our adoption and the expected impact this guidance could have on our condensed consolidated financial statements and related disclosures

PRODUCT REVENUES, NET

PRODUCT REVENUES, NET	6 Months Ended
	Jun. 30, 2019
Revenue From Contract With Customer [Abstract]
PRODUCT REVENUES, NET	3. PRODUCT REVENUES, NET Net product revenues of ONPATTRO (in thousands) consist of the following: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 United States $ 28,192 $ — $ 46,952 $ — Rest of World 10,039 — 17,570 — Total Product Revenues, Net $ 38,231 $ — $ 64,522 $ — As of June 30, 2019 and December 31, 2018, net product revenue related receivables of $17.0 million and $13.1 million, respectively, were included in “Accounts receivable, net.”

COLLABORATION AGREEMENTS

COLLABORATION AGREEMENTS	6 Months Ended
	Jun. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
COLLABORATION AGREEMENTS	4. COLLABORATION AGREEMENTS Net revenues from collaborators (in thousands) consist of the following: Three Months Ended June 30, Six Months Ended June 30, Description 2019 2018 2019 2018 Sanofi Genzyme $ 4,383 $ 23,077 $ 8,500 $ 41,930 Vir Biotechnology (Vir) 1,091 6,113 2,019 7,356 The Medicines Company (MDCO) — 662 1,745 1,957 Regeneron 700 — 700 — Other 309 55 522 563 Total net revenues from collaborators $ 6,483 $ 29,907 $ 13,486 $ 51,806 The following table presents the balance of our receivables and contract liabilities related to our collaboration agreements at June 30, 2019 and December 31, 2018, in thousands: At June 30, 2019 At December 31, 2018 Receivables included in “Accounts receivable, net” $ 13,691 $ 5,625 Contract liabilities included in “Deferred revenue” 403,033 3,954 During the three and six months ended June 30, 2019, we recognized the following revenue as a result of the change in the contract liability balances related to our collaboration agreements, in thousands: Revenue recognized in the period from: Three Months Ended June 30, 2019 Six Months Ended June 30, 2019 Amounts included in contract liability at the beginning of the period $ 1,091 $ 2,019 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 tables provide the research and development expenses incurred by type that are directly attributable to our collaboration agreements by our collaboration partners for the periods indicated, in thousands: Three Months Ended June 30, 2019 2018 Sanofi Genzyme MDCO Vir Regeneron Sanofi Genzyme MDCO Vir Regeneron Research and development Clinical trial and manufacturing $ 2,945 $ 65 $ 248 $ 515 $ 17,572 $ — $ 5,497 $ — External services 81 — 12 51 1,834 — 6,151 — Other 34 10 211 425 241 — 292 — Total research and development expenses $ 3,060 $ 75 $ 471 $ 991 $ 19,647 $ — $ 11,940 $ — Six Months Ended June 30, 2019 2018 Sanofi Genzyme MDCO Vir Regeneron Sanofi Genzyme MDCO Vir Regeneron Research and development Clinical trial and manufacturing $ 7,771 $ 1,677 $ 542 $ 515 $ 28,095 $ 641 $ 6,051 $ — External services 216 10 248 51 4,507 — 6,351 — Other 93 60 340 425 750 — 980 — Total research and development expenses $ 8,080 $ 1,747 $ 1,130 $ 991 $ 33,352 $ 641 $ 13,382 $ — The research and development expenses incurred for each agreement listed in the tables above consist of costs incurred for external development and manufacturing services for which we are reimbursed, licensing payments made to the counterparty to such agreement and costs directly attributable to Sanofi Genzyme transition services. In addition, these expenses include a reasonable estimate of compensation and related costs as billed to our counterparties. As part of our revenue recognition policy adopted on January 1, 2018, the costs in the above table are considered as an input in our determination of transaction price when they relate to consideration received for the delivery of goods or services. For the three and six months ended June 30, 2019 and 2018, we did not incur material selling, general and administrative expenses related to our collaboration agreements. Regeneron Collaboration On April 8, 2019, we entered into a global, strategic collaboration with Regeneron Pharmaceuticals, Inc., or Regeneron, to discover, develop and commercialize RNAi therapeutics for a broad range of diseases by addressing therapeutic targets expressed in the eye and central nervous system, or CNS, in addition to a select number of targets expressed in the liver, which we refer to as the Regeneron Collaboration. The Regeneron Collaboration is governed by a Master Agreement, referred to as the Regeneron Master Agreement, which became effective on May 21, 2019, or the Effective Date, upon closing of the Stock Purchase Agreement dated April 8, 2019, referred to as the Regeneron SPA, and issuance of 4,444,445 shares of our newly issued common stock to Regeneron for aggregate cash consideration of $400.0 million, or $90.00 per share. Please read Footnote 10 for a full discussion regarding the Regeneron SPA. Under the terms of the Regeneron Collaboration, we will work 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 preclinical 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 c o- c o c ollaboration a greement with respect to a given CNS or liver program, we and Regeneron will enter into a l icense a greement with respect to such program and the lead party will be the “Licensee” for the purposes of the l icense a greement. If the lead party for a CNS or liver program elects to not enter into the c o - c o c ollaboration a greement , then we and Regeneron will enter into a l icense a greement 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 l icense a greement. In addition, we and Regeneron have agreed to evaluate anti-C5 antibody-siRNA combinations for C5 complement-mediated diseases including evaluating the combination of Regeneron’s pozelimab (REGN3918), currently in Phase 1 development, and cemdisiran, currently in Phase 2 development. We retain control of cemdisiran monotherapy development, and Regeneron will lead combination product development. We and Regeneron will negotiate and enter into a co-co collaboration agreement to equally share costs and potential future profits on the monotherapy program, and a license agreement for cemdisiran to be used in a combination product pursuant to which Regeneron will be 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, in each case in accordance with the terms set forth in the agreed-upon binding term sheet attached to the Regeneron Master Agreement, 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 Regeneron has the right to terminate the Regeneron Master Agreement for convenience upon ninety days’ notice. The termination of the Regeneron Master Agreement does not affect the term of any license agreement or co-co collaboration agreement then in effect. In addition, either party may terminate the Regeneron Master Agreement for a material breach by, or insolvency of, the other party. Unless earlier terminated pursuant to its terms, the Regeneron Master Agreement will remain in effect with respect to each program until (a) such program becomes a terminated program or (b) the parties enter into a license agreement or co-co collaboration agreement with respect to such program. The Regeneron Master Agreement includes various representations, warranties, covenants, dispute escalation and resolution mechanisms, indemnities and other provisions customary for transactions of this nature. For any license agreement subsequently entered into, the licensee will generally be responsible for its own costs and expenses incurred in connection with the development and commercialization of the collaboration products. The licensee will pay to the licensor certain development and/or commercialization milestone payments totaling up to $150.0 million for each collaboration product. In addition, following the first commercial sale of the applicable collaboration product under a license agreement, the licensee is required to make certain tiered royalty payments, ranging from low double-digits up to 20%, to the licensor based on the aggregate annual net sales of the collaboration product, subject to customary reductions. For any co-co collaboration agreement subsequently entered into, we and Regeneron will share equally all costs of, and profits from, development and commercialization activities. In the event that a party exercises its opt-out right, the lead party will be responsible for all costs and expenses incurred in connection with the development and commercialization of the collaboration products under the applicable co-co collaboration agreement, subject to continued sharing of costs through defined points. If a party exercises its opt-out right, following the first commercial sale of the applicable collaboration product under a co-co collaboration agreement, the lead party is required to make certain tiered royalty payments, ranging from low double-digits up to 20%, to the other party based on the aggregate annual net sales of the collaboration product and the timing of the exercise of the opt-out right, subject to customary reductions and a reduction for opt-out transition costs. Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with drug development, we may not receive any milestone or royalty payments from Regeneron under the Regeneron Master Agreement 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 through completion of Phase 2 clinical trials, 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, t he 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, manufacturing and commercialization activities and are exposed to significant risks and rewards that are dependent on commercial success of the activities of the arrangement. The development, manufacturing and commercialization activities are a combined unit of account under the scope of ASC 808 and are not deliverables under ASC 606. As of the Effective Date, the total transaction price was determined to be $521.6 million, comprised of the $400.0 million upfront payment and $121.6 million of variable consideration related to research, development, manufacturing and supply activities. 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 ocular, CNS or liver targets, if any; however, these amounts are excluded from variable consideration under the combined Regeneron agreements 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 will re-evaluate the transaction price at the end of each reporting period. There was no change in the transaction price from the Effective Date to June 30, 2019. 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 also considered applicable market conditions and relevant entity-specific factors, including those factors contemplated in negotiating the agreement, the probability of success and the time needed to commercialize a product candidate pursuant to the associated license. 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. We allocated the transaction price to each unit of account based on the applicable accounting guidance as follows: (i) $178.5 million to the Research Services Obligation; (ii) $93.5 million to the C5 License Obligation; and (iii) $249.6 million to the C5 Co-Co Obligation. 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. As of June 30, 2019, the aggregate amount of the transaction price allocated to the remaining Research Services Obligation and C5 License Obligation that was unsatisfied is $271.3 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 remaining C5 Co-Co Obligation accounted for under ASC 808. We had deferred revenue of $401.8 million as of June 30, 2019, which is classified as current or non-current in the condensed consolidated balance sheets based on the period the services are expected to be performed. Sanofi Genzyme Collaboration Collaboration Amendment On April 8, 2019, we and Sanofi Genzyme entered into an amendment to our 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 ALN-AT3 Global License Terms entered into by us and Sanofi Genzyme in January 2018 to modify certain of the business terms. The material collaboration terms for fitusiran, as previously announced, will continue unchanged. In connection with entering into the Collaboration Amendment and 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 IND-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. No changes were made to our Exclusive License Agreement with Sanofi Genzyme, referred to as the Exclusive TTR License, pursuant to which we have global rights for the development and commercialization of ONPATTRO, together with vutrisiran and all back-up products, which remains in full force and effect. Amended and Restated Investor Agreement In connection with the Collaboration Amendment, we and Sanofi Genzyme also entered into an Amended and Restated Investor Agreement, referred to as the A&R Investor Agreement, which amended and restated the Investor Agreement entered into by us and Sanofi Genzyme in February 2014, referred to as the Original Investor Agreement. Pursuant to the A&R Investor Agreement, Sanofi Genzyme was released from the lock-up restrictions under the Original Investor Agreement and was permitted to sell shares of our common stock in transactions approved by us or in fully bought block sale transactions satisfying the conditions set forth in the A&R Investor Agreement. As of January 17, 2019, Sanofi Genzyme owned 10,554,134 shares of our common stock and as of May 3, 2019, Sanofi Genzyme reported owning no shares of our common stock. Under the A&R Investor Agreement, until the earlier of (i) the fifth anniversary of the expiration of the last to expire royalty term or the earlier termination of the collaboration agreement, as amended by the Collaboration Amendment, and (ii) the date after December 31, 2021 on which the beneficial ownership of Sanofi Genzyme and its affiliates no longer represents at least 5% of the outstanding shares of common stock, Sanofi Genzyme and its affiliates will be bound by certain “standstill” provisions, including an agreement not to propose or support a proposal to acquire us.

INVENTORY

INVENTORY	6 Months Ended
	Jun. 30, 2019
Inventory Disclosure [Abstract]
INVENTORY	5. INVENTORY The following table presents our inventory at June 30, 2019 and December 31, 2018, in thousands: At June 30, 2019 At December 31, 2018 Raw materials $ 8,446 $ 8,709 Work in progress 31,696 15,262 Finished goods 445 97 Total inventory $ 40,587 $ 24,068

FAIR VALUE MEASUREMENTS

FAIR VALUE MEASUREMENTS	6 Months Ended
	Jun. 30, 2019
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	6. FAIR VALUE MEASUREMENTS The following tables present information about our assets that are measured at fair value on a recurring basis at June 30, 2019 and December 31, 2018, and indicate the fair value hierarchy of the valuation techniques we utilized to determine such fair value, in thousands: Description At June 30, 2019 Quoted Prices in Active Markets (Level 1) Significant Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash equivalents: Commercial paper $ 2,989 $ — $ 2,989 $ — U.S. treasury securities 692,310 — 692,310 — U.S. government-sponsored enterprise securities 103,485 — 103,485 — Money market funds 94,496 94,496 — — Marketable debt securities: Certificates of deposit 6,500 — 6,500 — Commercial paper 19,169 — 19,169 — Corporate notes 86,157 — 86,157 — U.S. government-sponsored enterprise securities 70,607 — 70,607 — U.S. treasury securities 608,109 — 608,109 — Restricted cash (money market funds) 1,480 1,480 — — Total $ 1,685,302 $ 95,976 $ 1,589,326 $ — Description At December 31, 2018 Quoted Prices in Active Markets (Level 1) Significant Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash equivalents: U.S. treasury securities $ 221,281 $ — $ 221,281 $ — Money market funds 102,445 102,445 — — Marketable debt securities: Certificates of deposit 8,951 — 8,951 — Commercial paper 57,197 — 57,197 — Corporate notes 232,410 — 232,410 — U.S. government-sponsored enterprise securities 39,018 — 39,018 — U.S. treasury securities 325,227 — 325,227 — Marketable equity securities 1,206 1,206 — — Restricted cash (money market funds) 1,477 1,477 — — Total $ 989,212 $ 105,128 $ 884,084 $ — During the six months ended June 30, 2019 and 2018, there were no transfers between Level 1 and Level 2 financial assets. The carrying amounts reflected in our condensed The fair value of our long-term debt at , computed pursuant to a discounted cash flow technique using a market interest rate, was $30.1 million and is considered a Level 3 fair value measurement. The effective interest rate reflects the current market rate.

MARKETABLE DEBT SECURITIES

MARKETABLE DEBT SECURITIES	6 Months Ended
	Jun. 30, 2019
Investments Debt And Equity Securities [Abstract]
MARKETABLE DEBT SECURITIES	7. MARKETABLE DEBT SECURITIES 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. Additionally, we perform our own review of prices received from the independent pricing services by comparing these prices to other sources and confirming those securities are trading in active markets. We did not record any impairment charges related to our marketable debt securities during the six months ended June 30, 2019 or 2018. The following tables summarize our marketable debt securities at June 30, 2019 and December 31, 2018, in thousands: At June 30, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Certificates of deposit $ 6,500 $ — $ — $ 6,500 Commercial paper 22,158 — — 22,158 Corporate notes 86,122 45 (10 ) 86,157 U.S. government-sponsored enterprise securities 174,061 34 (3 ) 174,092 U.S. treasury securities 1,300,084 340 (5 ) 1,300,419 Total $ 1,588,925 $ 419 $ (18 ) $ 1,589,326 At December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Certificates of deposit $ 8,951 $ — $ — $ 8,951 Commercial paper 57,197 — — 57,197 Corporate notes 232,695 — (285 ) 232,410 U.S. government-sponsored enterprise securities 39,031 — (13 ) 39,018 U.S. treasury securities 546,631 1 (124 ) 546,508 Total $ 884,505 $ 1 $ (422 ) $ 884,084 The fair values of our marketable debt securities by classification in the condensed consolidated balance sheets were as follows, in thousands: At June 30, 2019 At December 31, 2018 Cash and cash equivalents $ 798,784 $ 221,281 Marketable debt securities 790,542 662,803 Total $ 1,589,326 $ 884,084 We classify our debt security investments based on their contractual maturity dates. At June 30, 2019, the contract maturity for all of our available-for-sale debt securities was less than one year.

LEASES

LEASES	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
LEASES	8. 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 In April 2015, we entered into a non-cancelable real property lease for approximately 295,000 square feet of laboratory and office space located at 675 West Kendall Street, Cambridge, Massachusetts. We intend to move our corporate headquarters and research facility to this location in 2019. 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 terms of five years each. Exercise of these options was not determined to be reasonably certain and thus was not included in the operating lease liability on the condensed consolidated balance sheet as of June 30, 2019. 300 Third Street We lease approximately 129,000 square feet of office and laboratory space located at 300 Third Street, Cambridge, Massachusetts, which to date has served as our corporate headquarters under a non-cancelable real property lease agreement dated as of September 26, 2003, as amended. The term of the lease expires on January 31, 2034, with an option to extend for two additional five-year terms. Exercise of these options was not determined to be reasonably certain and thus was not included in the operating lease liability on the condensed consolidated balance sheet as of June 30, 2019. 101 Main Street In March and May 2015, we entered into non-cancelable real property lease agreements for approximately 72,000 square feet of office space located on several floors at 101 Main Street, Cambridge, Massachusetts that will expire in March 2024 and June 2021, respectively, each with an option to renew for one five-year term. Exercise of these options was not determined to be reasonably certain and thus was not included in the operating lease liability on the condensed consolidated balance sheet as of June 30, 2019. Other Lease Disclosures Our facility leases described above generally contain customary provisions allowing the landlords to terminate the leases if we fail to remedy a breach of any of our obligations under any such lease within specified time periods, or upon our bankruptcy or insolvency. The below table summarizes our costs included in operating expenses related to leases we have entered into through June 30, 2019: Lease Cost Three Months Ended June 30, 2019 Six Months Ended June 30, 2019 Operating lease cost $ 9,291 $ 18,581 Variable lease cost 4,486 9,533 Total $ 13,777 $ 28,114 Short-term lease costs were not material for the three and six months ended June 3 0 , 2019. Net cash paid for the amounts included in the measurement of the operating lease liability on our condensed consolidated balance sheet and included in accrued expenses and other within operating activities in our condensed consolidated statement of cash flow was $16.0 million for the six months ended June 30, 2019. The weighted-average remaining lease term and weighted-average discount rate for all leases as of June 30, 2019 was 13.6 years and 8.2%, respectively. Future lease payments for non-cancellable operating leases as of June 30, 2019 and a reconciliation to the carrying amount of the operating lease liability presented in the condensed consolidated balance sheet as of June 30, 2019 is as follows: Year Ending December 31, 2019 (excluding the six months ended June 30, 2019) $ 16,378 2020 28,686 2021 36,357 2022 36,469 2023 35,585 2024 34,676 2025 and thereafter 356,302 Total undiscounted payments due under non-cancellable operating leases 544,453 Less imputed interest (237,895 ) Total $ 306,558 Current operating lease liability $ 29,347 Non-current operating lease liability 277,211 Total $ 306,558 Under the prior lease guidance, minimum payments under our non-cancelable facility leases, as of December 31, 2018, were approximately as follows, in thousands: Year Ending December 31, 2019 $ 32,228 2020 34,826 2021 34,410 2022 34,826 2023 35,270 Thereafter 390,455 Total $ 562,015

CREDIT AGREEMENT

CREDIT AGREEMENT	6 Months Ended
	Jun. 30, 2019
Line Of Credit Facility [Abstract]
CREDIT AGREEMENT	9. CREDIT AGREEMENT On April 29, 2016, we entered into a Credit Agreement by and among Alnylam U.S., Inc., as the borrower, us, as a guarantor, and Wells Fargo Bank, National Association, as the lender. The Credit Agreement was entered into in connection with the planned build out of our drug substance manufacturing facility. The Credit Agreement provides for a $30.0 million term loan facility and matures on April 29, 2021. The proceeds of the borrowing under the Credit Agreement are to be used for working capital and general corporate purposes. Interest on borrowings under the Credit Agreement is calculated based on LIBOR plus 0.45 percent, except in the event of default. The borrower may prepay loans under the Credit Agreement at any time, without premium or penalty, subject to certain notice requirements and LIBOR breakage costs. The obligations of the borrower and us under the Credit Agreement are secured by cash collateral in an amount equal to, at any given time, at least 100 percent of the principal amount of all term loans outstanding under such Credit Agreement at such time. At each of June 30, 2019 and December 31, 2018, we recorded $30.0 million of cash collateral in connection with the Credit Agreement as restricted investments on our condensed consolidated balance sheets. The Credit Agreement contains limited representations and warranties and limited affirmative and negative covenants, including quarterly reporting obligations, as well as certain customary events of default.

EQUITY

EQUITY	6 Months Ended
	Jun. 30, 2019
Equity [Abstract]
EQUITY	10. EQUITY 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 offering expenses of approximately $5.6 million. Regeneron Equity Placement On April 8, 2019, we executed the Regeneron SPA with Regeneron to sell 4,444,445 shares of our common stock, par value $0.01 per share 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. The Regeneron SPA contains customary representations, warranties and covenants of each of the parties thereto. As a condition to consummating the transactions contemplated by the Regeneron SPA, we and Regeneron entered into an Investor Agreement dated April 8, 2019, or the Investor Agreement. Under the Investor Agreement, until the expiration or termination of the Research Term under the Regeneron Master Agreement subject to extension by one year if the Research Term or Regeneron Master Agreement is terminated by Regeneron at will, or by up to two years if as of the expiration or termination of the Research Term Regeneron owns more than 19.99% of our outstanding shares, Regeneron and its affiliates will be bound by certain “standstill” provisions. The standstill provisions include agreements not to acquire more than 30% of our outstanding shares of common stock, call stockholder meetings, nominate directors other than those approved by our board of directors, subject to certain limited exceptions, or propose or support a proposal to acquire us. Further, under the Investor Agreement, Regeneron agreed to vote, and cause its affiliates to vote, all shares of our voting securities Regeneron is entitled to vote in a manner as recommended by our board of directors, except with respect to certain change of control transactions, liquidation or dissolution of our company, or, after the standstill term, any contested election of directors. Under the Investor Agreement, Regeneron agreed not to dispose of any of the purchased shares or any shares of common stock beneficially owned by it immediately after the closing of the Regeneron Master Agreement, until the earlier of (i) the four-year anniversary of the closing of the Equity Transaction and (ii) the termination of the Regeneron Collaboration, subject to limited exceptions, which we refer to as the Lock-Up Period. Following the expiration of the Lock-Up Period, if at any time Regeneron beneficially owns at least 9.9% of our outstanding shares, then until such time as Regeneron beneficially owns less than 5% of our outstanding shares, Regeneron will not dispose of any shares except (a) pursuant to a registered underwritten public offering pursuant to the Investor Agreement, (b) in a manner consistent with the volume limitations set forth in Rule 144 under the Securities Act, or (c) as otherwise approved by us. Under the Investor Agreement, following the Lock-Up Period, Regeneron will have three demand rights to require us to conduct a registered underwritten public offering with respect to the shares of common stock beneficially owned by Regeneron immediately after the closing of the Equity Transaction. In addition, following the Lock-Up Period, subject to certain conditions, Regeneron will be entitled to participate in registered underwritten public offerings by us if other selling stockholders are included in the registration. The rights and restrictions under the Investor Agreement are subject to termination upon the occurrence of certain events. 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 with the offsetting adjustment to equity netting against the $ 400.0 million proceeds that were received upon closing.

STOCK-BASED COMPENSATION

STOCK-BASED COMPENSATION	6 Months Ended
	Jun. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
STOCK-BASED COMPENSATION	11. STOCK-BASED COMPENSATION The following table summarizes stock-based compensation expenses included in operating costs and expenses, for the periods indicated, in thousands: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Research and development $ 15,282 $ 11,616 $ 31,407 $ 21,753 Selling, general and administrative 15,321 10,625 31,228 20,072 Total stock-based compensation $ 30,603 $ 22,241 $ 62,635 $ 41,825

NET LOSS PER COMMON SHARE

NET LOSS PER COMMON SHARE	6 Months Ended
	Jun. 30, 2019
Earnings Per Share [Abstract]
NET LOSS PER COMMON SHARE	12. 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 common share equivalents (in thousands) were excluded from the calculation of net loss per common share because their inclusion would be anti-dilutive: At June 30, 2019 2018 Options to purchase common stock 13,771 12,794 Unvested restricted common stock 696 150 14,467 12,944

RECONCILIATION OF CASH, CASH EQ

RECONCILIATION OF CASH, CASH EQUIVALENTS AND RESTRICTED CASH	6 Months Ended
	Jun. 30, 2019
Cash Cash Equivalents And Restricted Cash [Abstract]
RECONCILIATION OF CASH, CASH EQUIVALENTS AND RESTRICTED CASH	13. RECONCILIATION OF CASH, CASH EQUIVALENTS AND RESTRICTED CASH The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within our condensed consolidated balance sheets that sum to the total of these amounts shown in the condensed consolidated statements of cash flows, in thousands: At June 30, 2019 2018 Cash and cash equivalents $ 1,136,289 $ 361,457 Restricted cash included in prepaid expenses and other current assets 332 — Restricted cash included in long-term other assets 2,983 1,934 Total cash, cash equivalents, and restricted cash shown in the condensed consolidated statements of cash flows $ 1,139,604 $ 363,391

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	6 Months Ended
	Jun. 30, 2019
Commitments And Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	14. COMMITMENTS AND CONTINGENCIES Manufacturing Facility In April 2016, we purchased 12 acres of undeveloped land in Norton, Massachusetts. We are constructing a manufacturing facility at this site for drug substance for clinical and commercial use. At June 30, 2019 and December 31, 2018, property, plant and equipment, net, on our condensed consolidated balance sheets reflect $259.4 million and $227.7 million, respectively, of land and associated costs related to the construction of our drug substance manufacturing facility. Litigation From time to time, we are a party to legal proceedings in the course of our business, including the matters described below. The claims and legal proceedings in which we could be involved include challenges to the scope, validity or enforceability of patents relating to our 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. 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 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 Chief Financial Officer in the United States District Court for the Southern District of New York. By stipulation of the parties and Order of the Court dated November 20, 2018, the action was transferred to the United States District Court for the District of Massachusetts. On May 8, 2019, the Court entered an order appointing a lead plaintiff, and on July 3, 2019, lead plaintiff filed a consolidated class action complaint, or the Complaint. In addition to the originally named defendants, the Complaint also names as defendants certain of our other executive officers, and purports to be brought on behalf of a class of persons who acquired our securities between September 20, 2017 and September 12, 2018 and seeks to recover damages caused by defendants’ alleged violations of the federal securities laws and to pursue remedies under Sections 10(b) and 20(a) of the Securities Exchange Act of 1934, as amended, and Rule 10b-5 promulgated thereunder. The Complaint alleges, among other things, that the defendants made materially false and misleading statements related to the efficacy and safety of our product, ONPATTRO. The plaintiff seeks, among other things, the designation of this action as a class action, an award of unspecified compensatory damages, interest, costs and expenses, including counsel fees and expert fees, and other relief as the court deems appropriate. All defendants filed a motion to dismiss the Complaint in its entirety on July 31, 2019. We believe that the allegations contained in the Complaint are without merit and intend to defend the case vigorously. We cannot predict at this point the length of time that this action will be ongoing or the liability, if any, which may arise therefrom. Dicerna Litigation On June 10, 2015, we filed a trade secret misappropriation lawsuit against Dicerna Pharmaceuticals, Inc., or Dicerna, in the Superior Court of Middlesex County, Massachusetts seeking to stop misappropriation by Dicerna of our confidential, proprietary and trade secret information related to the RNAi assets we purchased from Merck Sharp & Dohme Corp., including certain N-acetylgalactosamine, or GalNAc, conjugate technology. In addition to permanent injunctive relief, we were also seeking monetary damages from Dicerna. On April 18, 2018, we and Dicerna entered into a settlement agreement resolving all ongoing litigation between the companies. Under the terms of the settlement agreement, Dicerna was required to pay us an aggregate of $25.0 million, including an upfront cash payment of $2.0 million and 983,208 shares of Dicerna common stock, valued at $10.0 million, that were received in the second quarter of 2018, and an additional $13.0 million over the next four years, the timing of which was dependent upon revenue Dicerna received pursuant to future partnerships and collaborations related to GalNAc-conjugated RNAi research and development. As a result of Dicerna collaborations entered into subsequent to the settlement agreement, we were due the remaining $13.0 million as of December 31, 2018, of which $2.5 million was received in December 2018 and $10.5 million was received in January 2019.

DEFINED BENEFIT PLANS

DEFINED BENEFIT PLANS	6 Months Ended
	Jun. 30, 2019
Compensation And Retirement Disclosure [Abstract]
DEFINED BENEFIT PLANS	15. DEFINED BENEFIT PLANS We 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 plan. The unfunded benefit obligation was approximately $4.0 million as of June 30, 2019 and is recorded in other liabilities on the condensed consolidated balance sheet. The unfunded benefit obligation as of December 31, 2018 and the total net periodic benefit cost for the three and six months ended June 30, 2019 and 2018 were not material.

OTHER COMPREHENSIVE INCOME AND

OTHER COMPREHENSIVE INCOME AND ACCUMULATED OTHER COMPREHENSIVE INCOME	6 Months Ended
	Jun. 30, 2019
Accumulated Other Comprehensive Income Loss Net Of Tax [Abstract]
OTHER COMPREHENSIVE INCOME AND ACCUMULATED OTHER COMPREHENSIVE INCOME	16. OTHER COMPREHENSIVE INCOME AND ACCUMULATED OTHER COMPREHENSIVE INCOME The following tables summarize the changes in accumulated other comprehensive income (loss), by component, for the six months ended June 30, 2019 and 2018, in thousands: Loss on Investment in Joint Venture Defined Benefit Pension Plans Unrealized Gains (Losses) from Debt Securities Foreign Currency Translation Adjustment Total Accumulated Other Comprehensive Income (Loss) Balance at December 31, 2018 $ (32,792 ) $ — $ (421 ) $ — $ (33,213 ) Other comprehensive income (loss) before reclassifications — (4,282 ) 478 842 (2,962 ) Amounts reclassified from other comprehensive income — — 344 — 344 Net other comprehensive income (loss) — (4,282 ) 822 842 (2,618 ) Balance at June 30, 2019 $ (32,792 ) $ (4,282 ) $ 401 $ 842 $ (35,831 ) Loss on Investment in Joint Venture Defined Benefit Pension Plans Unrealized Gains (Losses) from Debt Securities Foreign Currency Translation Adjustment Total Accumulated Other Comprehensive Income (Loss) Balance at December 31, 2017 $ (32,792 ) $ — $ (1,641 ) $ — $ (34,433 ) Other comprehensive income before reclassifications — — 69 — 69 Amounts reclassified from other comprehensive income — — 557 — 557 Net other comprehensive income — — 626 — 626 Balance at June 30, 2018 $ (32,792 ) $ — $ (1,015 ) $ — $ (33,807 ) Amounts reclassified out of accumulated other comprehensive income (loss) relate to settlements of debt securities and are recorded as interest income in the condensed consolidated statements of comprehensive loss.

BASIS OF PRESENTATION AND PRI_2

BASIS OF PRESENTATION AND PRINCIPLES OF CONSOLIDATION (Policies)	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	BASIS OF PRESENTATION AND PRINCIPLES OF CONSOLIDATION The accompanying condensed consolidated financial statements of Alnylam Pharmaceuticals, Inc. are unaudited and have been prepared in accordance with accounting principles generally accepted in the United States of America, or GAAP, applicable to interim periods and, in the opinion of management, include all normal and recurring adjustments that are necessary to state fairly the results of operations for the reported periods. Our condensed consolidated financial statements have also been prepared on a basis substantially consistent with, and should be read in conjunction with, our audited consolidated financial statements for the year ended December 31, 2018, which were included in our Annual Report on Form 10-K that was filed with the Securities and Exchange Commission on February 14, 2019. The year-end condensed consolidated balance sheet data was derived from our audited financial statements but does not include all disclosures required by GAAP. The results of our operations for any interim period are not necessarily indicative of the results of our operations for any other interim period or for a full fiscal year. The accompanying condensed consolidated financial statements reflect the operations of Alnylam and our wholly-owned subsidiaries. All intercompany accounts and transactions have been eliminated. Our significant accounting policies are described in Note 2 of the Notes to Consolidated Financial Statements included in our Annual Report on Form 10-K for the year ended December 31, 2018. Updates to our significant accounting policies, including the updated lease accounting policy due to the adoption of the new leasing accounting standard, are discussed below and under “Recent Accounting Pronouncements.”
Reclassification	Reclassification Certain prior period amounts in the condensed consolidated financial statements have been reclassified to conform to the current period presentation.
Liquidity	Liquidity Based on our current operating plan, we believe that our cash, cash equivalents and marketable debt securities at June 30, 2019, together with the cash we expect to generate from product sales and under our alliances, will be sufficient to enable us to advance our Alnylam 2020
Leases	Leases We determine whether a contract is, or contains, a lease at inception. We classify each of our leases as operating or financing considering factors such as the length of the lease term, the present value of the lease payments, the nature of the asset being leased, and the potential for ownership of the asset to transfer during the lease term. Leases with terms greater than one-year are recognized on the condensed consolidated balance sheets as right-of-use assets and lease liabilities and are measured at the present value of the fixed payments due over the expected lease term minus the present value of any incentives, rebates or abatements we expect to receive from the lessor. Options to extend a lease are included in the expected lease term if exercise of the option is deemed reasonably certain. Costs determined to be variable and not based on an index or rate are not included in the measurement of the lease liability and are expensed as incurred. The interest rate implicit in lease contracts is typically not readily determinable. As such, we utilize the appropriate incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis an amount equal to the lease payments over a similar term and in a similar economic environment. We record expense to recognize fixed lease payments on a straight-line basis over the expected lease term. We have elected the practical expedient not to separate lease and non-lease components for real estate leases.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In February 2016, the Financial Accounting Standards Board, or FASB, issued a new leasing standard which generally requires lessees to recognize operating and financing lease liabilities and corresponding right-of-use assets on the condensed consolidated balance sheet and to provide enhanced disclosures surrounding the amount, timing and uncertainty of cash flows arising from leasing arrangements. We adopted the new standard on January 1, 2019, using a modified retrospective basis and did not restate comparative periods. In addition, we did not elect the package of practical expedients permitted under the transition guidance that permits companies to carry forward prior conclusions related to (1) whether any expired or existing contracts are, or contain, leases, (2) the lease classification for expired or existing leases, and (3) initial direct costs for existing leases. All our leases have been classified as operating leases under the new leasing standard. We elected to combine lease and non-lease components and to keep leases with an initial term of 12 months or less off the condensed consolidated balance sheets and recognize the associated lease payments in the condensed consolidated statements of comprehensive loss on a straight-line basis over the lease term. Please read Note 8 for additional disclosures related to accounting for leases under this new standard. The adoption of ASC 842 has a material impact on our condensed consolidated balance sheet as the standard requires us to measure and recognize a right of use asset and lease liability. As most leases do not provide an implicit rate, our incremental borrowing rate was determined based on the information available at the date of adoption to measure our lease liability. Costs determined to be variable and not based on an index or rate were not included in the measurement of the lease liability. We recognized approximately $290.0 million of operating lease liabilities and approximately $230.0 million of operating lease right-of-use assets on our condensed consolidated balance sheet as of January 1, 2019, which are presented as separate line items on the condensed consolidated balance sheet. Had we not adopted the new leasing standard, we would not have had operating lease right-of-use assets or operating lease liabilities on our condensed consolidated balance sheet. The adoption of the standard did not have a material impact on our condensed consolidated statement of comprehensive income. In March 2017, the FASB issued a new standard that amends the amortization period for certain purchased callable debt securities held at a premium by shortening the amortization period for the premium to the earliest call date. The new standard became effective for us on January 1, 2019. This standard did not have a significant impact on our condensed consolidated financial statements and related disclosures. In August 2018, the FASB issued amendments that eliminate, add and modify certain disclosure requirements on fair value measurements. The amendments become effective for our fiscal year, including interim periods, beginning January 1, 2020. Early adoption of the amendments in full or only the provisions that eliminate or modify the disclosure requirements for fair value measurements is permitted. We are currently evaluating the timing of our adoption and the expected impact that these amendments could have on our disclosures In November 2018, the FASB issued guidance to clarify the interaction between the accounting guidance for collaborative arrangements and revenue from contracts with customers. The amendments become effective for our fiscal year, including interim periods, beginning January 1, 2020. Early adoption, including adoption in any interim period, is permitted. This guidance is required to be applied retrospectively as of the date of our adoption of the new revenue standard on January 1, 2018. We are currently evaluating the timing of our adoption and the expected impact this guidance could have on our condensed consolidated financial statements and related disclosures

PRODUCT REVENUES, NET (Tables)

PRODUCT REVENUES, NET (Tables)	6 Months Ended
	Jun. 30, 2019
Revenue From Contract With Customer [Abstract]
Summary of Net Product Revenues Of ONPATTRO	Net product revenues of ONPATTRO (in thousands) consist of the following: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 United States $ 28,192 $ — $ 46,952 $ — Rest of World 10,039 — 17,570 — Total Product Revenues, Net $ 38,231 $ — $ 64,522 $ —

COLLABORATION AGREEMENTS (Table

COLLABORATION AGREEMENTS (Tables)	6 Months Ended
	Jun. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Revenue from Collaborators	Net revenues from collaborators (in thousands) consist of the following: Three Months Ended June 30, Six Months Ended June 30, Description 2019 2018 2019 2018 Sanofi Genzyme $ 4,383 $ 23,077 $ 8,500 $ 41,930 Vir Biotechnology (Vir) 1,091 6,113 2,019 7,356 The Medicines Company (MDCO) — 662 1,745 1,957 Regeneron 700 — 700 — Other 309 55 522 563 Total net revenues from collaborators $ 6,483 $ 29,907 $ 13,486 $ 51,806
Balance and Change in Receivables and Contract Liabilities Related to Collaboration Agreements	The following table presents the balance of our receivables and contract liabilities related to our collaboration agreements at June 30, 2019 and December 31, 2018, in thousands: At June 30, 2019 At December 31, 2018 Receivables included in “Accounts receivable, net” $ 13,691 $ 5,625 Contract liabilities included in “Deferred revenue” 403,033 3,954 During the three and six months ended June 30, 2019, we recognized the following revenue as a result of the change in the contract liability balances related to our collaboration agreements, in thousands: Revenue recognized in the period from: Three Months Ended June 30, 2019 Six Months Ended June 30, 2019 Amounts included in contract liability at the beginning of the period $ 1,091 $ 2,019
Schedule of Research and Development Expenses Incurred by Type that are Directly Attributable to Collaboration Agreements	The following tables provide the research and development expenses incurred by type that are directly attributable to our collaboration agreements by our collaboration partners for the periods indicated, in thousands: Three Months Ended June 30, 2019 2018 Sanofi Genzyme MDCO Vir Regeneron Sanofi Genzyme MDCO Vir Regeneron Research and development Clinical trial and manufacturing $ 2,945 $ 65 $ 248 $ 515 $ 17,572 $ — $ 5,497 $ — External services 81 — 12 51 1,834 — 6,151 — Other 34 10 211 425 241 — 292 — Total research and development expenses $ 3,060 $ 75 $ 471 $ 991 $ 19,647 $ — $ 11,940 $ — Six Months Ended June 30, 2019 2018 Sanofi Genzyme MDCO Vir Regeneron Sanofi Genzyme MDCO Vir Regeneron Research and development Clinical trial and manufacturing $ 7,771 $ 1,677 $ 542 $ 515 $ 28,095 $ 641 $ 6,051 $ — External services 216 10 248 51 4,507 — 6,351 — Other 93 60 340 425 750 — 980 — Total research and development expenses $ 8,080 $ 1,747 $ 1,130 $ 991 $ 33,352 $ 641 $ 13,382 $ —

INVENTORY (Tables)

INVENTORY (Tables)	6 Months Ended
	Jun. 30, 2019
Inventory Disclosure [Abstract]
Schedule of Inventory	The following table presents our inventory at June 30, 2019 and December 31, 2018, in thousands: At June 30, 2019 At December 31, 2018 Raw materials $ 8,446 $ 8,709 Work in progress 31,696 15,262 Finished goods 445 97 Total inventory $ 40,587 $ 24,068

FAIR VALUE MEASUREMENTS (Tables

FAIR VALUE MEASUREMENTS (Tables)	6 Months Ended
	Jun. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value of Assets Measured on a Recurring Basis	The following tables present information about our assets that are measured at fair value on a recurring basis at June 30, 2019 and December 31, 2018, and indicate the fair value hierarchy of the valuation techniques we utilized to determine such fair value, in thousands: Description At June 30, 2019 Quoted Prices in Active Markets (Level 1) Significant Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash equivalents: Commercial paper $ 2,989 $ — $ 2,989 $ — U.S. treasury securities 692,310 — 692,310 — U.S. government-sponsored enterprise securities 103,485 — 103,485 — Money market funds 94,496 94,496 — — Marketable debt securities: Certificates of deposit 6,500 — 6,500 — Commercial paper 19,169 — 19,169 — Corporate notes 86,157 — 86,157 — U.S. government-sponsored enterprise securities 70,607 — 70,607 — U.S. treasury securities 608,109 — 608,109 — Restricted cash (money market funds) 1,480 1,480 — — Total $ 1,685,302 $ 95,976 $ 1,589,326 $ — Description At December 31, 2018 Quoted Prices in Active Markets (Level 1) Significant Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash equivalents: U.S. treasury securities $ 221,281 $ — $ 221,281 $ — Money market funds 102,445 102,445 — — Marketable debt securities: Certificates of deposit 8,951 — 8,951 — Commercial paper 57,197 — 57,197 — Corporate notes 232,410 — 232,410 — U.S. government-sponsored enterprise securities 39,018 — 39,018 — U.S. treasury securities 325,227 — 325,227 — Marketable equity securities 1,206 1,206 — — Restricted cash (money market funds) 1,477 1,477 — — Total $ 989,212 $ 105,128 $ 884,084 $ —

MARKETABLE DEBT SECURITIES (Tab

MARKETABLE DEBT SECURITIES (Tables)	6 Months Ended
	Jun. 30, 2019
Investments Debt And Equity Securities [Abstract]
Summary of Company's Marketable Debt Securities	The following tables summarize our marketable debt securities at June 30, 2019 and December 31, 2018, in thousands: At June 30, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Certificates of deposit $ 6,500 $ — $ — $ 6,500 Commercial paper 22,158 — — 22,158 Corporate notes 86,122 45 (10 ) 86,157 U.S. government-sponsored enterprise securities 174,061 34 (3 ) 174,092 U.S. treasury securities 1,300,084 340 (5 ) 1,300,419 Total $ 1,588,925 $ 419 $ (18 ) $ 1,589,326 At December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Certificates of deposit $ 8,951 $ — $ — $ 8,951 Commercial paper 57,197 — — 57,197 Corporate notes 232,695 — (285 ) 232,410 U.S. government-sponsored enterprise securities 39,031 — (13 ) 39,018 U.S. treasury securities 546,631 1 (124 ) 546,508 Total $ 884,505 $ 1 $ (422 ) $ 884,084
Summary of Fair Value of Marketable Debt Securities	The fair values of our marketable debt securities by classification in the condensed consolidated balance sheets were as follows, in thousands: At June 30, 2019 At December 31, 2018 Cash and cash equivalents $ 798,784 $ 221,281 Marketable debt securities 790,542 662,803 Total $ 1,589,326 $ 884,084

LEASES (Tables)

LEASES (Tables)	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
Summary of Costs Included in Operating Expenses Related to Leases	The below table summarizes our costs included in operating expenses related to leases we have entered into through June 30, 2019: Lease Cost Three Months Ended June 30, 2019 Six Months Ended June 30, 2019 Operating lease cost $ 9,291 $ 18,581 Variable lease cost 4,486 9,533 Total $ 13,777 $ 28,114
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 as of June 30, 2019 and a reconciliation to the carrying amount of the operating lease liability presented in the condensed consolidated balance sheet as of June 30, 2019 is as follows: Year Ending December 31, 2019 (excluding the six months ended June 30, 2019) $ 16,378 2020 28,686 2021 36,357 2022 36,469 2023 35,585 2024 34,676 2025 and thereafter 356,302 Total undiscounted payments due under non-cancellable operating leases 544,453 Less imputed interest (237,895 ) Total $ 306,558 Current operating lease liability $ 29,347 Non-current operating lease liability 277,211 Total $ 306,558
Future Minimum Payments Under Non-cancelable Leases	Under the prior lease guidance, minimum payments under our non-cancelable facility leases, as of December 31, 2018, were approximately as follows, in thousands: Year Ending December 31, 2019 $ 32,228 2020 34,826 2021 34,410 2022 34,826 2023 35,270 Thereafter 390,455 Total $ 562,015

STOCK-BASED COMPENSATION (Table

STOCK-BASED COMPENSATION (Tables)	6 Months Ended
	Jun. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Stock Based Compensation	The following table summarizes stock-based compensation expenses included in operating costs and expenses, for the periods indicated, in thousands: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Research and development $ 15,282 $ 11,616 $ 31,407 $ 21,753 Selling, general and administrative 15,321 10,625 31,228 20,072 Total stock-based compensation $ 30,603 $ 22,241 $ 62,635 $ 41,825

NET LOSS PER COMMON SHARE (Tabl

NET LOSS PER COMMON SHARE (Tables)	6 Months Ended
	Jun. 30, 2019
Earnings Per Share [Abstract]
Common Share Equivalents Excluded from the Calculation of Net Loss Per Common Share	The following common share equivalents (in thousands) were excluded from the calculation of net loss per common share because their inclusion would be anti-dilutive: At June 30, 2019 2018 Options to purchase common stock 13,771 12,794 Unvested restricted common stock 696 150 14,467 12,944

RECONCILIATION OF CASH, CASH _2

RECONCILIATION OF CASH, CASH EQUIVALENTS AND RESTRICTED CASH (Tables)	6 Months Ended
	Jun. 30, 2019
Cash Cash Equivalents And Restricted Cash [Abstract]
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 condensed consolidated balance sheets that sum to the total of these amounts shown in the condensed consolidated statements of cash flows, in thousands: At June 30, 2019 2018 Cash and cash equivalents $ 1,136,289 $ 361,457 Restricted cash included in prepaid expenses and other current assets 332 — Restricted cash included in long-term other assets 2,983 1,934 Total cash, cash equivalents, and restricted cash shown in the condensed consolidated statements of cash flows $ 1,139,604 $ 363,391

OTHER COMPREHENSIVE INCOME AN_2

OTHER COMPREHENSIVE INCOME AND ACCUMULATED OTHER COMPREHENSIVE INCOME (Tables)	6 Months Ended
	Jun. 30, 2019
Accumulated Other Comprehensive Income Loss Net Of Tax [Abstract]
Summary of Changes in Accumulated Other Comprehensive Income (Loss)	The following tables summarize the changes in accumulated other comprehensive income (loss), by component, for the six months ended June 30, 2019 and 2018, in thousands: Loss on Investment in Joint Venture Defined Benefit Pension Plans Unrealized Gains (Losses) from Debt Securities Foreign Currency Translation Adjustment Total Accumulated Other Comprehensive Income (Loss) Balance at December 31, 2018 $ (32,792 ) $ — $ (421 ) $ — $ (33,213 ) Other comprehensive income (loss) before reclassifications — (4,282 ) 478 842 (2,962 ) Amounts reclassified from other comprehensive income — — 344 — 344 Net other comprehensive income (loss) — (4,282 ) 822 842 (2,618 ) Balance at June 30, 2019 $ (32,792 ) $ (4,282 ) $ 401 $ 842 $ (35,831 ) Loss on Investment in Joint Venture Defined Benefit Pension Plans Unrealized Gains (Losses) from Debt Securities Foreign Currency Translation Adjustment Total Accumulated Other Comprehensive Income (Loss) Balance at December 31, 2017 $ (32,792 ) $ — $ (1,641 ) $ — $ (34,433 ) Other comprehensive income before reclassifications — — 69 — 69 Amounts reclassified from other comprehensive income — — 557 — 557 Net other comprehensive income — — 626 — 626 Balance at June 30, 2018 $ (32,792 ) $ — $ (1,015 ) $ — $ (33,807 )

Basis of Presentation and Pri_3

Basis of Presentation and Principles of Consolidation - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Jan. 01, 2019
Accounting Policies [Abstract]
Operating lease liabilities	$ 306,558	$ 290,000
Operating lease right-of-use assets	$ 226,357	$ 230,000

Product Revenues, Net - Summary

Product Revenues, Net - Summary of Net Product Revenues Of ONPATTRO (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation Of Revenue [Line Items]
Revenues	$ 44,714	$ 29,907	$ 78,008	$ 51,806
Product Revenues, Net
Disaggregation Of Revenue [Line Items]
Revenues	38,231		64,522
Product Revenues, Net | United States
Disaggregation Of Revenue [Line Items]
Revenues	28,192		46,952
Product Revenues, Net | Rest of World
Disaggregation Of Revenue [Line Items]
Revenues	$ 10,039		$ 17,570

Product Revenues, Net - Additio

Product Revenues, Net - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Disaggregation Of Revenue [Line Items]
Accounts receivable, net	$ 30,739	$ 18,760
Product Revenues, Net
Disaggregation Of Revenue [Line Items]
Accounts receivable, net	$ 17,000	$ 13,100

Collaboration Agreements - Reve

Collaboration Agreements - Revenue from Collaborators (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	$ 44,714	$ 29,907	$ 78,008	$ 51,806
Net Revenues from Collaborators
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	6,483	29,907	13,486	51,806
Net Revenues from Collaborators | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	4,383	23,077	8,500	41,930
Net Revenues from Collaborators | Vir Biotechnology
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	1,091	6,113	2,019	7,356
Net Revenues from Collaborators | The Medicines Company
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues		662	1,745	1,957
Net Revenues from Collaborators | Regeneron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	700		700
Net Revenues from Collaborators | Other
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues	$ 309	$ 55	$ 522	$ 563

Collaboration Agreements - Bala

Collaboration Agreements - Balance of Receivables and Contract Liabilities Related to Collaboration Agreements (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Receivables included in “Accounts receivable, net”	$ 13,691	$ 5,625
Contract liabilities included in “Deferred revenue”	$ 403,033	$ 3,954

Collaboration Agreements - Chan

Collaboration Agreements - Change in Contract Liability Balance Related to Collaboration Agreements (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Revenue recognized in the period from:
Amounts included in contract liability at the beginning of the period	$ 1,091	$ 2,019

Collaboration Agreements - Sche

Collaboration Agreements - Schedule of Research and Development Expenses Incurred by Type that are Directly Attributable to Collaboration Agreements (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	$ 163,890	$ 137,582	$ 293,017	$ 234,439
Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	3,060	19,647	8,080	33,352
Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	471	11,940	1,130	13,382
Mdco
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	75		1,747	641
Regeneron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	991		991
Clinical trial and manufacturing | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	2,945	17,572	7,771	28,095
Clinical trial and manufacturing | Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	248	5,497	542	6,051
Clinical trial and manufacturing | Mdco
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	65		1,677	641
Clinical trial and manufacturing | Regeneron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	515		515
External services | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	81	1,834	216	4,507
External services | Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	12	6,151	248	6,351
External services | Mdco
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses			10
External services | Regeneron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	51		51
Other | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	34	241	93	750
Other | Vir
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	211	$ 292	340	$ 980
Other | Mdco
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	10		60
Other | Regeneron
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research and development expenses	$ 425		$ 425

Collaboration Agreements - Addi

Collaboration Agreements - Additional Information (Detail)	Apr. 25, 2019USD ($)shares	Apr. 08, 2019USD ($)Program$ / sharesshares	Jan. 31, 2019$ / sharesshares	Mar. 31, 2019	Jun. 30, 2019USD ($)shares	May 03, 2019shares	Jan. 17, 2019shares	Dec. 31, 2018USD ($)shares
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Issuance of common stock, net of offering costs (in shares) | shares			5,000,000
Offering proceeds, net of costs					$ 381,900,000
Common stock at a price per share | $ / shares			$ 77.50
Deferred revenue					$ 403,033,000			$ 3,954,000
Common Stock Shares owned | shares					111,114,000			101,177,000
Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common Stock Shares owned | shares						0	10,554,134
Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Issuance of common stock, net of offering costs (in shares) | shares	4,444,445	4,444,445
Offering proceeds, net of costs	$ 400,000,000	$ 400,000,000
Common stock at a price per share | $ / shares		$ 90
Discovery period of programs development		5 years
Extended additional discovery period of programs development		2 years
Maximum royalties and commercial milestone payments upon potential product sale		$ 325,000,000
Upfront fee received		400,000,000
Maximum additional milestone payments to be receive upon achievement of certain criteria		$ 200,000,000
Number of targeted programs | Program		30
Royalty rate		20.00%
Maximum percentage of royalty payments		20.00%
Transaction price		$ 521,600,000
Variable consideration related to research, development, manufacturing and supply activities		121,600,000
Deferred revenue					$ 401,800,000
Transactional price remaining performance obligation					271,300,000
Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | Research Services Obligation
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction price					178,500,000
Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | C5 License Obligation
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction price					93,500,000
Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | C5 Co-Co Obligation
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction price					$ 249,600,000
Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | Funding At Program Initiation
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential proceeds from collaboration arrangement		2,500,000
Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | Funding At Lead Candidate Identification
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential proceeds from collaboration arrangement		2,500,000
Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | Funding At Steady State
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential proceeds from collaboration arrangement		30,000,000
Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research term extension fee		400,000,000
Collaborative arrangement milestone payments		$ 150,000,000
Collaboration Amendment | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Investor agreement description				until the earlier of (i) the fifth anniversary of the expiration of the last to expire royalty term or the earlier termination of the collaboration agreement, as amended by the Collaboration Amendment, and (ii) the date after December 31, 2021 on which the beneficial ownership of Sanofi Genzyme and its affiliates no longer represents at least 5% of the outstanding shares of common stock, Sanofi Genzyme and its affiliates will be bound by certain “standstill” provisions, including an agreement not to propose or support a proposal to acquire us.
Collaboration Amendment | Minimum | Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage ownership interest owned by noncontrolling owners					5.00%

Inventory - Schedule of Invento

Inventory - Schedule of Inventory (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Inventory Disclosure [Abstract]
Raw materials	$ 8,446	$ 8,709
Work in progress	31,696	15,262
Finished goods	445	97
Total inventory	$ 40,587	$ 24,068

Fair Value Measurements - Fair

Fair Value Measurements - Fair Value of Assets Measured on a Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 798,784	$ 221,281
Available for sale debt securities, Fair value disclosure	1,589,326	884,084
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable equity securities		1,206
Total	1,685,302	989,212
Recurring | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable equity securities		1,206
Total	95,976	105,128
Recurring | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	1,589,326	884,084
Recurring | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	2,989
Available for sale debt securities, Fair value disclosure	19,169	57,197
Recurring | Commercial paper | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	2,989
Available for sale debt securities, Fair value disclosure	19,169	57,197
Recurring | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	692,310	221,281
Available for sale debt securities, Fair value disclosure	608,109	325,227
Recurring | U.S. treasury securities | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	692,310	221,281
Available for sale debt securities, Fair value disclosure	608,109	325,227
Recurring | U.S. government-sponsored enterprise securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	103,485
Available for sale debt securities, Fair value disclosure	70,607	39,018
Recurring | U.S. government-sponsored enterprise securities | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	103,485
Available for sale debt securities, Fair value disclosure	70,607	39,018
Recurring | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	94,496	102,445
Restricted cash	1,480	1,477
Recurring | Money Market Funds | Quoted Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	94,496	102,445
Restricted cash	1,480	1,477
Recurring | Certificate of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, Fair value disclosure	6,500	8,951
Recurring | Certificate of deposit | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, Fair value disclosure	6,500	8,951
Recurring | Corporate notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, Fair value disclosure	86,157	232,410
Recurring | Corporate notes | Significant Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, Fair value disclosure	$ 86,157	$ 232,410

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) $ in Millions	Jun. 30, 2019USD ($)
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of long-term debt	$ 30.1

Marketable Debt Securities - Ad

Marketable Debt Securities - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Marketable Securities [Abstract]
Impairment charges of marketable debt securities	$ 0	$ 0

Marketable Debt Securities - Su

Marketable Debt Securities - Summary of Company's Marketable Debt Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,588,925	$ 884,505
Gross Unrealized Gains	419	1
Gross Unrealized Losses	(18)	(422)
Fair Value	1,589,326	884,084
Certificate of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,500	8,951
Fair Value	6,500	8,951
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,158	57,197
Fair Value	22,158	57,197
Corporate notes
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	86,122	232,695
Gross Unrealized Gains	45
Gross Unrealized Losses	(10)	(285)
Fair Value	86,157	232,410
U.S. government-sponsored enterprise securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	174,061	39,031
Gross Unrealized Gains	34
Gross Unrealized Losses	(3)	(13)
Fair Value	174,092	39,018
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,300,084	546,631
Gross Unrealized Gains	340	1
Gross Unrealized Losses	(5)	(124)
Fair Value	$ 1,300,419	$ 546,508

Marketable Debt Securities - _2

Marketable Debt Securities - Summary of Fair Value of Marketable Debt Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Cash equivalents	$ 798,784	$ 221,281
Marketable debt securities	790,542	662,803
Total	$ 1,589,326	$ 884,084

Leases - Additional Information

Leases - Additional Information (Detail) $ in Millions	Sep. 26, 2003ft²RenewalOption	May 31, 2015ft²RenewalOption	Apr. 30, 2015ft²RenewalOption	Mar. 31, 2015	Jun. 30, 2019USD ($)
Operating Leased Assets [Line Items]
Net cash paid included in operating activities in cash flow | $					$ 16
Operating lease, weighted-average remaining lease term					13 years 7 months 6 days
Operating lease, weighted-average discount rate					8.20%
BMR-675 West Kendall Lease
Operating Leased Assets [Line Items]
Area of office and laboratory space held under lease | ft²			295,000
Lease term			15 years
Number of lease extension options | RenewalOption			2
Lease renewal options period			5 years
Lessee, operating lease, option to extend					with options to renew for two terms of five years each.
Lessee, operating lease, existence of option to terminate [true false]					true
Third Street Lease
Operating Leased Assets [Line Items]
Area of office and laboratory space held under lease | ft²	129,000
Number of lease extension options | RenewalOption	2
Lease renewal options period	5 years
Lessee, operating lease, option to extend					with an option to extend for two additional five-year terms
Lessee, operating lease, existence of option to terminate [true false]					true
Lease extended expiration date	Jan. 31, 2034
101 Main Street Lease
Operating Leased Assets [Line Items]
Area of office and laboratory space held under lease | ft²		72,000
Number of lease extension options | RenewalOption		1
Lessee, operating lease, option to extend					each with an option to renew for one five-year term.
Lessee, operating lease, existence of option to terminate [true false]					true
Lease extended expiration month and year		2021-06		2024-03
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	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Lessee Disclosure [Abstract]
Operating lease cost	$ 9,291	$ 18,581
Variable lease cost	4,486	9,533
Total	$ 13,777	$ 28,114

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	Jun. 30, 2019	Jan. 01, 2019
Lessee Disclosure [Abstract]
2019 (excluding the six months ended June 30, 2019)	$ 16,378
2020	28,686
2021	36,357
2022	36,469
2023	35,585
2024	34,676
2025 and thereafter	356,302
Total undiscounted payments due under non-cancellable operating leases	544,453
Less imputed interest	(237,895)
Total	306,558	$ 290,000
Operating lease liability	29,347
Operating lease liability, net of current portion	$ 277,211

Leases - Future Minimum Payment

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

Credit Agreement - Additional I

Credit Agreement - Additional Information (Detail) - USD ($)	Apr. 29, 2016	Jun. 30, 2019	Dec. 31, 2018
Line Of Credit Facility [Line Items]
Restricted investments		$ 44,825,000	$ 44,825,000
Wells Fargo Bank, National Association
Line Of Credit Facility [Line Items]
Cash collateral required for principal amount outstanding, percentage	100.00%
Restricted investments		$ 30,000,000	$ 30,000,000
Wells Fargo Bank, National Association | Term Loan Facility
Line Of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 30,000,000
Line of credit facility, expiration date	Apr. 29, 2021
Wells Fargo Bank, National Association | Term Loan Facility | LIBOR
Line Of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	0.45%

Equity - Additional Information

Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Apr. 25, 2019	Apr. 08, 2019	Jan. 31, 2019	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Equity [Line Items]
Issuance of common stock, net of offering costs (in shares)			5,000,000
Shares issued price per share			$ 77.50
Issuance of common stock, net of offering costs			$ 381,900	$ 390,577	$ 772,477
Underwriting discounts and commissions and other offering expenses			$ 5,600
Common stock, par value				$ 0.01	$ 0.01	$ 0.01
Offering proceeds, net of costs					$ 381,900
Preferred stock, par value				$ 0.01	$ 0.01	$ 0.01
Common stock, shares authorized				250,000,000	250,000,000	125,000,000
Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc.
Equity [Line Items]
Issuance of common stock, net of offering costs (in shares)	4,444,445	4,444,445
Shares issued price per share		$ 90
Common stock, par value		$ 0.01
Offering proceeds, net of costs	$ 400,000	$ 400,000
Preferred stock, par value		$ 90
Date of investor agreement					Apr. 8, 2019
Ownership percentage of shares outstanding, upper limit		30.00%
Gain on sale of common stock	$ 9,400
Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | Maximum
Equity [Line Items]
Agreement termination period		2 years
Beneficial ownership percentage of shares outstanding		5.00%
Common stock, shares authorized	250,000,000
Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | Minimum
Equity [Line Items]
Ownership percentage of shares outstanding		19.99%
Beneficial ownership percentage following the expiration of lock-up period		9.90%
Common stock, shares authorized	125,000,000
Global Strategic Collaboration | Regeneron Pharmaceuticals, Inc. | Series A Redeemable Convertible Preferred Stock
Equity [Line Items]
Issuance of common stock, net of offering costs (in shares)		1,481,482
Shares issued price per share		$ 270
Preferred stock, par value		$ 0.01

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Share-Based Compensation Expenses Included Operating Costs and Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	$ 30,603	$ 22,241	$ 62,635	$ 41,825
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	15,282	11,616	31,407	21,753
Selling, General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation	$ 15,321	$ 10,625	$ 31,228	$ 20,072

Net Loss Per Common Share - Com

Net Loss Per Common Share - Common Share Equivalents Excluded from Calculation of Net Loss Per Common Share (Detail) - shares shares in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share	14,467	12,944
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share	13,771	12,794
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	696	150

Reconciliation of Cash, Cash _3

Reconciliation of Cash, Cash Equivalents and Restricted Cash - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Cash Cash Equivalents And Restricted Cash [Abstract]
Cash and cash equivalents	$ 1,136,289	$ 420,146	$ 361,457
Restricted cash included in prepaid expenses and other current assets	$ 332
Restricted Cash and Cash Equivalents, Current, Asset, Statement of Financial Position [Extensible List]	us-gaap:PrepaidExpenseAndOtherAssetsCurrent		us-gaap:PrepaidExpenseAndOtherAssetsCurrent
Restricted cash included in long-term other assets	$ 2,983		$ 1,934
Restricted Cash and Cash Equivalents, Noncurrent, Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssetsNoncurrent		us-gaap:OtherAssetsNoncurrent
Total cash, cash equivalents, and restricted cash shown in the condensed consolidated statements of cash flows	$ 1,139,604	$ 422,631	$ 363,391	$ 646,832

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) $ in Thousands	Apr. 18, 2018USD ($)shares	Jan. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2018USD ($)	Jun. 30, 2019USD ($)	Apr. 30, 2016a
Commitments And Contingencies [Line Items]
Property, plant and equipment, net			$ 320,658	$ 320,658	$ 371,769
Litigation settlement, common stock issuable from other party | shares	983,208
Dicerna Pharmaceuticals, Inc
Commitments And Contingencies [Line Items]
Litigation settlement, settlement agreement date	April 18, 2018
Litigation settlement amount	$ 25,000	$ 10,500	2,500	13,000
Litigation settlement, common stock value	10,000
Additional payment due from litigation settlement	$ 13,000
Litigation settlement payment period	4 years
Up Front Payment | Dicerna Pharmaceuticals, Inc
Commitments And Contingencies [Line Items]
Litigation settlement amount	$ 2,000
Norton, Massachusetts
Commitments And Contingencies [Line Items]
Undeveloped land acquired | a						12
Norton, Massachusetts | Land and costs related to the construction of manufacturing facility
Commitments And Contingencies [Line Items]
Property, plant and equipment, net			$ 227,700	$ 227,700	$ 259,400

Defined Benefit Plans - Additio

Defined Benefit Plans - Additional Information (Details) $ in Millions	Jun. 30, 2019USD ($)
Unfunded Plan | Other Liabilities
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	$ 4

Other Comprehensive Income an_3

Other Comprehensive Income and Accumulated Other Comprehensive Income - Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Balance	$ 1,546,986	$ 1,754,805	$ 1,301,965	$ 1,766,431
Other comprehensive income (loss) before reclassifications			(2,962)	69
Amounts reclassified from other comprehensive income			344	557
Net other comprehensive income (loss)	(2,978)	1,046	(2,618)	626
Balance	1,757,195	1,620,802	1,757,195	1,620,802
Loss on Investment in Joint Venture
Balance			(32,792)	(32,792)
Balance	(32,792)	(32,792)	(32,792)	(32,792)
Defined Benefit Pension Plans
Other comprehensive income (loss) before reclassifications			(4,282)
Net other comprehensive income (loss)			(4,282)
Balance	(4,282)		(4,282)
Unrealized Gains (Losses) from Debt Securities
Balance			(421)	(1,641)
Other comprehensive income (loss) before reclassifications			478	69
Amounts reclassified from other comprehensive income			344	557
Net other comprehensive income (loss)			822	626
Balance	401	(1,015)	401	(1,015)
Foreign Currency Translation Adjustment
Other comprehensive income (loss) before reclassifications			842
Net other comprehensive income (loss)			842
Balance	842		842
Accumulated Other Comprehensive Income (Loss)
Balance	(32,853)	(34,853)	(33,213)	(34,433)
Net other comprehensive income (loss)	(2,978)	1,046	(2,618)	626
Balance	$ (35,831)	$ (33,807)	$ (35,831)	$ (33,807)