Alnylam Pharmaceuticals (ALNY) 10-Q7 Nov 132013 Q3 Quarterly reportFinancial data

Company Profile

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
Document and Entity Information	Sep. 30, 2013	Oct. 31, 2013
Document Information [Line Items]	'	'
Document Type	'10-Q	'
Amendment Flag	'false	'
Document Period End Date	30-Sep-13	'
Document Fiscal Year Focus	'2013	'
Document Fiscal Period Focus	'Q3	'
Trading Symbol	'ALNY	'
Entity Registrant Name	'ALNYLAM PHARMACEUTICALS, INC.	'
Entity Central Index Key	'0001178670	'
Current Fiscal Year End Date	'--12-31	'
Entity Filer Category	'Accelerated Filer	'
Entity Common Stock, Shares Outstanding	'	63,430,076

Condensed_Consolidated_Balance

Condensed Consolidated Balance Sheets (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Current assets:	'	'
Cash and cash equivalents	$11,840	$51,405
Marketable securities	181,939	71,407
Billed and unbilled collaboration receivables	609	104
Prepaid expenses and other current assets	4,776	2,641
Total current assets	199,164	125,557
Marketable securities	173,366	103,416
Investment in equity securities of Regulus Therapeutics Inc.	57,999	38,748
Property and equipment, net	16,974	19,799
Total assets	447,503	287,520
Current liabilities:	'	'
Accounts payable	2,728	4,420
Accrued expenses	15,386	11,558
Accrued intraperiod tax allocation	2,023	'
Deferred rent	1,200	950
Deferred revenue	33,558	31,417
Total current liabilities	54,895	48,345
Deferred rent, net of current portion	3,247	4,248
Deferred revenue, net of current portion	91,779	100,874
Total liabilities	149,921	153,467
Commitments and contingencies (Note 3)	'	'
Stockholders' equity:	'	'
Preferred stock, $0.01 par value, 5,000,000 shares authorized and no shares issued and outstanding at September 30, 2013 and December 31, 2012	'	'
Common stock, $0.01 par value, 125,000,000 shares authorized; 63,236,216 shares issued and outstanding at September 30, 2013; 52,489,936 shares issued and outstanding at December 31, 2012	632	525
Additional paid-in capital	833,660	624,876
Accumulated other comprehensive income	27,168	15,662
Accumulated deficit	-563,878	-507,010
Total stockholders' equity	297,582	134,053
Total liabilities and stockholders' equity	$447,503	$287,520

Condensed_Consolidated_Balance1

Condensed Consolidated Balance Sheets (Parenthetical) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Preferred stock, par value	$0.01	$0.01
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	'	'
Preferred stock, shares outstanding	'	'
Common stock, par value	$0.01	$0.01
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	63,236,216	52,489,936
Common stock, shares outstanding	63,236,216	52,489,936

Condensed_Consolidated_Stateme

Condensed Consolidated Statements of Comprehensive Loss (USD $)	3 Months Ended				9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Net revenues from research collaborators	$8,991		$16,759		$36,320		$58,230
Operating expenses:	'		'		'		'
Research and development	34,457	[1]	22,094	[1]	80,851	[1]	64,891	[1]
General and administrative	6,768	[1]	12,812	[1]	18,819	[1]	34,446	[1]
Total operating expenses	41,225		34,906		99,670		99,337
Loss from operations	-32,234		-18,147		-63,350		-41,107
Other income (expense):	'		'		'		'
Equity in loss of joint venture (Regulus Therapeutics Inc.)	'		-1,613		'		-3,641
Interest income	290		261		784		755
Other (expense) income	-12		-3		-18		167
Total other income (expense)	278		-1,355		766		-2,719
Loss before income taxes	-31,956		-19,502		-62,584		-43,826
Benefit from income taxes	2,270		'		5,716		'
Net loss	-29,686		-19,502		-56,868		-43,826
Net loss per common share - basic and diluted	($0.48)		($0.38)		($0.93)		($0.88)
Weighted average common shares used to compute basic and diluted net loss per common share	62,416		51,542		61,103		49,772
Comprehensive loss:	'		'		'		'
Net loss	-29,686		-19,502		-56,868		-43,826
Unrealized (loss) gain on marketable securities, net of tax	-962		170		11,506		273
Comprehensive loss	($30,648)		($19,332)		($45,362)		($43,553)

[1]	Non-cash stock-based compensation expenses included in operating expenses are as follows: Research and development $ 6,805 $ 2,271 $ 11,092 $ 6,357 General and administrative 2,040 1,115 4,205 3,281

Condensed_Consolidated_Stateme1

Condensed Consolidated Statements of Comprehensive Loss (Parenthetical) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Non-cash stock-based compensation expenses included in operating expenses are as follows:	'	'	'	'
Research and development	$6,805	$2,271	$11,092	$6,357
General and administrative	$2,040	$1,115	$4,205	$3,281

Condensed_Consolidated_Stateme2

Condensed Consolidated Statements of Cash Flows (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Cash flows from operating activities:	'	'
Net loss	($56,868)	($43,826)
Adjustments to reconcile net loss to net cash used in operating activities:	'	'
Depreciation and amortization	7,602	7,011
Non-cash stock-based compensation	15,297	9,638
Charge for 401(k) company stock match	351	310
Equity in loss of joint venture (Regulus Therapeutics Inc.)	'	3,641
Realized gain on sale of marketable securities	'	-179
Benefit from intraperiod tax allocation on marketable securities	-5,716	'
Changes in operating assets and liabilities:	'	'
Proceeds from landlord tenant improvements	192	1,214
Billed and unbilled collaboration receivables	-505	-450
Prepaid expenses and other assets	-2,222	-1,359
Accounts payable	-1,336	-707
Accrued expenses and other	3,871	2,678
Deferred revenue	-6,954	-24,668
Net cash used in operating activities	-46,288	-46,697
Cash flows from investing activities:	'	'
Purchases of property and equipment	-3,128	-7,662
Increase in restricted cash	'	-162
Purchases of marketable securities	-335,197	-195,672
Sales and maturities of marketable securities	151,772	163,357
Net cash used in investing activities	-186,553	-40,139
Cash flows from financing activities:	'	'
Proceeds from exercise of stock options and other types of equity	19,819	5,974
Proceeds from issuance of common stock, net of offering costs	173,572	86,800
Payments for repurchase of common stock for employee tax withholding	-115	-328
Net cash provided by financing activities	193,276	92,446
Net (decrease) increase in cash and cash equivalents	-39,565	5,610
Cash and cash equivalents, beginning of period	51,405	70,228
Cash and cash equivalents, end of period	$11,840	$75,838

Summary_of_Significant_Account

Summary of Significant Accounting Policies	9 Months Ended
Summary of Significant Accounting Policies	Sep. 30, 2013
Summary of Significant Accounting Policies	'
	1. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
	Basis of Presentation and Principles of Consolidation
	The accompanying condensed consolidated financial statements of Alnylam Pharmaceuticals, Inc. (the “Company” or “Alnylam”) are unaudited and have been prepared in accordance with accounting principles generally accepted in the United States of America (“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. The Company’s condensed consolidated financial statements have also been prepared on a basis substantially consistent with, and should be read in conjunction with, the Company’s audited consolidated financial statements for the year ended December 31, 2012, which were included in the Company’s Annual Report on Form 10-K that was filed with the Securities and Exchange Commission (the “SEC”) on February 19, 2013. The year-end condensed balance sheet data was derived from audited financial statements, but does not include all disclosures required by GAAP. The results of the Company’s operations for any interim period are not necessarily indicative of the results of the Company’s operations for any other interim period or for a full fiscal year.
	The accompanying condensed consolidated financial statements reflect the operations of the Company and Alnylam U.S., Inc., Alnylam Europe AG, Alnylam (Bermuda) Ltd. and Alnylam Securities Corporation, which are wholly-owned subsidiaries of the Company. All significant intercompany accounts and transactions have been eliminated.
	Use of Estimates
	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
	Net Loss Per Common Share
	The Company computes basic net loss per common share by dividing net loss by the weighted average number of common shares outstanding. The Company computes 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 (using the treasury stock method) and unvested restricted stock awards. Because the inclusion of potential common shares would be anti-dilutive for all periods presented, diluted net loss per common share is the same as basic net loss per common share.
	The following table sets forth for the periods presented the potential common shares (prior to consideration of the treasury stock method) excluded from the calculation of net loss per common share because their inclusion would be anti-dilutive, in thousands:


			Three and Nine Months Ended
			September 30,
			2013			2012
	Options to purchase common stock			8,021			8,156
	Unvested restricted common stock			593			638

				8,614			8,794

	Restricted Stock Awards
	In January 2012, as part of its post-restructuring retention program, the Company granted an aggregate of 508,928 shares of restricted stock to its retained employees, excluding the Company’s chief executive officer and president and chief operating officer. These restricted stock awards were valued at $5.3 million on the grant date and vest in full on the second anniversary of the grant date.

	The Company recognized an aggregate of $1.3 million and $0.7 million of stock-based compensation expense related to all outstanding restricted stock awards for the three months ended September 30, 2013 and 2012, respectively. The Company recognized an aggregate of $2.6 million and $1.9 million of stock-based compensation expense related to all outstanding restricted stock awards for the nine months ended September 30, 2013 and 2012, respectively.
	Public Offering
	In February 2012, the Company sold an aggregate of 8,625,000 shares of its common stock through an underwritten public offering at a price to the public of $10.75 per share. As a result of the offering, the Company received aggregate net proceeds of approximately $86.8 million, after deducting underwriting discounts and commissions and other estimated offering expenses of approximately $5.9 million.
	In January 2013, the Company sold an aggregate of 9,200,000 shares of its common stock through an underwritten public offering at a price to the public of $20.13 per share. As a result of this offering, the Company received aggregate net proceeds of approximately $173.6 million, after deducting underwriting discounts and commissions and other estimated offering expenses of approximately $11.6 million.
	Fair Value Measurements
	The following tables present information about the Company’s assets that are measured at fair value on a recurring basis at September 30, 2013 and December 31, 2012, and indicate the fair value hierarchy of the valuation techniques the Company utilized to determine such fair value. In general, fair values determined by Level 1 inputs utilize quoted prices (unadjusted) in active markets for identical assets or liabilities. Fair values determined by Level 2 inputs utilize data points that are observable, such as quoted prices (adjusted), interest rates and yield curves. Fair values determined by Level 3 inputs utilize unobservable data points for the asset or liability, and include situations where there is little, if any, market activity for the asset or liability. The fair value hierarchy level is determined by the lowest level of significant input. Financial assets measured at fair value on a recurring basis are summarized as follows, in thousands:


	Description		At			Quoted Prices			Significant			Significant
			September 30,			in Active			Observable			Unobservable
			2013			Markets			Inputs			Inputs
						(Level 1)			(Level 2)			(Level 3)
	Cash equivalents		$	10,515		$	10,515		$	—		$	—
	Marketable securities (fixed income)
	Corporate notes			266,418			—			266,418			—
	U.S. Government obligations			69,901			—			69,901			—
	Commercial paper			18,986			—			18,986			—
	Marketable securities (Regulus equity holdings)			57,999			—			57,999			—

	Total		$	423,819		$	10,515		$	413,304		$	—



	Description		At			Quoted Prices			Significant			Significant
			December 31,			in Active			Observable			Unobservable
			2012			Markets			Inputs			Inputs
						(Level 1)			(Level 2)			(Level 3)
	Cash equivalents		$	50,213		$	50,213		$	—		$	—
	Marketable securities (fixed income)
	Corporate notes			91,523			—			91,523			—
	U.S. Government obligations			60,661			—			60,661			—
	Commercial paper			22,639			—			22,639			—
	Marketable securities (Regulus equity holdings)			38,748			—			38,748			—

	Total		$	263,784		$	50,213		$	213,571		$	—

	During the nine months ended September 30, 2013, there were no transfers between Level 1, Level 2 and Level 3 financial assets. The carrying amounts reflected in the Company’s condensed consolidated balance sheets for cash, collaboration receivables, other current assets, accounts payable and accrued expenses approximate fair value due to their short-term maturities.

	Investments in Marketable Securities
	The Company invests its excess cash balances in short-term and long-term marketable debt and equity securities. The Company classifies its investments in marketable debt securities as either held-to-maturity or available-for-sale based on facts and circumstances present at the time it purchased the securities. At each balance sheet date presented, the Company classified all of its investments in debt and equity securities as available-for-sale. The Company reports available-for-sale investments at fair value at each balance sheet date and includes any unrealized holding gains and losses (the adjustment to fair value) in accumulated other comprehensive loss, a component of stockholders’ equity. Realized gains and losses are determined using the specific identification method and are included in other income. The Company did not recognize any realized gains or losses from sales of its available-for-sale securities during the nine months ended September 30, 2013, and as a result, did not reclassify any amount out of accumulated other comprehensive income for the same period. If any adjustment to fair value reflects a decline in the value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other than temporary” and, if so, marks the investment to market through a charge to its condensed consolidated statements of comprehensive loss. The Company did not record any impairment charges related to its fixed income marketable securities during the current period. The Company’s marketable securities are classified as cash equivalents if the original maturity, from the date of purchase, is 90 days or less, and as marketable securities if the original maturity, from the date of purchase, is in excess of 90 days. At September 30, 2013, the Company’s cash equivalents are composed of money market funds.
	In the fourth quarter of 2012, the Company began accounting for its investment in Regulus Therapeutics Inc. (“Regulus”) as an available-for-sale marketable security. Intraperiod tax allocation rules require the Company to allocate its provision for income taxes between continuing operations and other categories of earnings, such as other comprehensive income. In periods in which the Company has a year-to-date pre-tax loss from continuing operations and pre-tax income in other categories of earnings, such as other comprehensive income, the Company must allocate the tax provision to the other categories of earnings. The Company then records a related tax benefit in continuing operations. The following tables summarize the fair value, accumulated other comprehensive income and intraperiod tax allocation regarding the Company’s investment in Regulus available-for-sale marketable securities at September 30, 2013 and the activity for the three and nine months ended September 30, 2013, in thousands:


Description		At			Three Months			Balance at
		June 30,			Ended			September 30,
		2013			September 30,			2013
					2013
Carrying value		$	12,449		$	—		$	12,449
Accumulated other comprehensive income (loss), before tax			47,887			(2,337	)		45,550

Investment in equity securities of Regulus Therapeutics Inc., as reported		$	60,336		$	(2,337	)	$	57,999


Accumulated other comprehensive income (loss), before tax		$	47,887		$	(2,337	)	$	45,550
Intraperiod tax allocation recorded as a benefit from income taxes			(14,018	)		(2,270	)		(16,288	)
Intraperiod tax allocation recorded as an accrued liability			(5,232	)		3,209			(2,023	)

Accumulated other comprehensive income, net of tax		$	28,637		$	(1,398	)	$	27,239



Description		At			Nine Months			Balance at
		December 31,			Ended			September 30,
		2012			September 30,			2013
					2013
Carrying value		$	12,449		$	—		$	12,449
Accumulated other comprehensive income, before tax			26,299			19,251			45,550

Investment in equity securities of Regulus Therapeutics Inc., as reported		$	38,748		$	19,251		$	57,999


Accumulated other comprehensive income, before tax		$	26,299		$	19,251		$	45,550
Intraperiod tax allocation recorded as a benefit from income taxes			(10,572	)		(5,716	)		(16,288	)
Intraperiod tax allocation recorded as an accrued liability			—			(2,023	)		(2,023	)

Accumulated other comprehensive income, net of tax		$	15,727		$	11,512		$	27,239

The Company obtains fair value measurement data for its fixed income marketable securities from independent pricing services. The Company performs 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, the Company performs its 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.
The following tables summarize the Company’s fixed income marketable securities at September 30, 2013 and December 31, 2012, in thousands:


		September 30, 2013
		Amortized			Gross			Gross			Fair Value
		Cost			Unrealized			Unrealized
					Gains			Losses
Commercial paper (Due within 1 year)		$	18,987		$	3		$	(4	)	$	18,986
Corporate notes (Due within 1 year)			148,864			46			(42	)		148,868
Corporate notes (Due after 1 year through 2 years)			117,644			20			(114	)		117,550
U.S. Government obligations (Due within 1 year)			14,079			7			—			14,086
U.S. Government obligations (Due after 1 year through 2 years)			55,802			17			(4	)		55,815

Total		$	355,376		$	93		$	(164	)	$	355,305



		December 31, 2012
		Amortized			Gross			Gross			Fair Value
		Cost			Unrealized			Unrealized
					Gains			Losses
Commercial paper (Due within 1 year)		$	22,650		$	1		$	(12	)	$	22,639
Corporate notes (Due within 1 year)			41,249			23			(4	)		41,268
Corporate notes (Due after 1 year through 2 years)			50,322			5			(72	)		50,255
U.S. Government obligations (Due within 1 year)			7,500			—			—			7,500
U.S. Government obligations (Due after 1 year through 2 years)			53,168			2			(9	)		53,161

Total		$	174,889		$	31		$	(97	)	$	174,823

Subsequent Events
The Company evaluated all events or transactions that occurred after September 30, 2013 through the date these condensed consolidated financial statements were issued. During this period, the Company did not have any material recognized or nonrecognized subsequent events.
Recent Accounting Pronouncements
In February 2013, the Financial Accounting Standards Board (“FASB”) issued amendments to the accounting guidance for presentation of comprehensive income to improve the reporting of reclassifications out of accumulated other comprehensive income. The amendments did not change the current requirements for reporting net income or other comprehensive income, but require an entity to provide information about the amounts reclassified out of accumulated other comprehensive income by component. In addition, an entity is now required to present, either on the face of the statement where the net income is presented or in the notes, significant amounts reclassified out of accumulated other comprehensive income by the respective line items of net income but only if the amount reclassified is required under GAAP to be reclassified to net income in its entirety in the same reporting period. For other amounts that are not required under GAAP to be reclassified in their entirety to net income, an entity is required to cross-reference to other disclosures required under GAAP that provide additional detail about these amounts. For public companies, these amendments were effective prospectively for reporting periods beginning after December 15, 2012. The Company adopted this guidance on January 1, 2013. Other than a change in presentation, the adoption of this guidance did not have a material impact on the Company’s condensed consolidated financial statements.
In July 2013, the FASB issued new accounting guidance specific to income taxes. The new guidance requires an entity to present an unrecognized tax benefit and a net operating loss carryforward, a similar tax loss, or a tax credit carryforward on a net basis as part of a deferred tax asset, unless the unrecognized tax benefit is not available to reduce the deferred tax asset component or would not be utilized for that purpose, then a liability would be recognized. The updated accounting guidance is effective for fiscal years beginning after December 15, 2013. The Company does not expect the adoption of this guidance to have a material impact on the Company’s condensed consolidated financial statements.

Significant_Agreements

Significant Agreements	9 Months Ended
Significant Agreements	Sep. 30, 2013
Significant Agreements	'
	2. SIGNIFICANT AGREEMENTS
	The following table summarizes the Company’s total consolidated net revenues from research collaborators, for the periods indicated, in thousands:

		Three Months				Nine Months
		Ended September 30,				Ended September 30,
		2013		2012		2013		2012
	Takeda	$	5,494	$	5,494	$	16,481	$	16,481
	Monsanto		1,410		519		4,230		519
	The Medicines Company		1,253		—		3,343		—
	Cubist		—		694		9,721		2,084
	Roche/Arrowhead		—		9,330		—		37,318
	Other		834		722		2,545		1,828

	Total net revenues from research collaborators	$	8,991	$	16,759	$	36,320	$	58,230

	Platform Alliances
	Takeda Alliance
	In May 2008, the Company entered into a license and collaboration agreement (the “Takeda Agreement”) with Takeda to pursue the development and commercialization of RNAi therapeutics. Under the Takeda Agreement, the Company granted to Takeda a non-exclusive, worldwide, royalty-bearing license to the Company’s intellectual property, including delivery-related intellectual property, controlled by the Company as of the date of the agreement or during the five years thereafter, to develop, manufacture, use and commercialize RNAi therapeutics, subject to the Company’s existing contractual obligations to third parties. The license initially is limited to the fields of oncology and metabolic disease and may be expanded at Takeda’s option to include other therapeutic areas, subject to specified conditions.
	In consideration for the rights granted to Takeda under the Takeda Agreement, Takeda agreed to pay the Company $150.0 million in upfront and near-term technology transfer payments. In addition, the Company has the option, exercisable until the start of Phase III development, to opt-in under a 50-50 profit sharing agreement to the development and commercialization in the United States of up to four Takeda licensed products, and would be entitled to opt-in rights for two additional products for each additional field expansion, if any, elected by Takeda under the Takeda Agreement. In June 2008, Takeda paid the Company an upfront payment of $100.0 million and agreed to pay to the Company an additional $50.0 million upon achievement of specified technology transfer milestones. Of this $50.0 million, $20.0 million was paid to the Company in October 2008, $20.0 million was paid to the Company in March 2010, and $10.0 million was paid to the Company in March 2011 (collectively, the “Technology Transfer Milestones”). If Takeda elects to expand its license to additional therapeutic areas, Takeda will be required to pay the Company $50.0 million for each additional field selected, if any. In addition, for each RNAi therapeutic product developed by Takeda, its affiliates and sublicensees, the Company is entitled to receive specified development, regulatory and commercialization milestone payments, totaling up to $171.0 million per product, together with up to a double-digit percentage royalty payment based on worldwide annual net sales, if any. The potential future milestone payments per product include up to $26.0 million for the achievement of specified development milestones, up to $40.0 million for the achievement of specified regulatory milestones and up to $105.0 million for the achievement of specified commercialization milestones. The Company could potentially earn the next milestone payment of $2.0 million under the Takeda Agreement based upon the achievement of a specified pre-clinical event by Takeda for an RNAi therapeutic product. For purposes of potential future revenue recognition, the Company does not believe this milestone or any future milestones are substantive. Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with drug development, the Company may not receive any additional milestone payments or any royalty payments from Takeda.
	Pursuant to the Takeda Agreement, the Company and Takeda are also collaborating on the research of RNAi therapeutics directed to one or two disease targets agreed to by the parties (the “Research Collaboration”), subject to the Company’s existing contractual obligations with third parties. Takeda also has the option, subject to certain conditions, to collaborate with the Company on the research and development of RNAi drug delivery technology for targets agreed to by the parties. In addition, the Company has a right of first negotiation to participate with Takeda in the development and commercialization of licensed products in the United States. The collaboration is governed by a joint technology transfer committee (the “JTTC”), a joint research collaboration committee (the “JRCC”) and a joint delivery collaboration committee (the “JDCC”), each of which is comprised of an equal number of representatives from each party.
	The Company has determined that the deliverables under the Takeda Agreement include the license, the joint committees (the JTTC, JRCC and JDCC), the technology transfer activities and the services that the Company will be obligated to perform under the Research Collaboration. The Company also determined that, pursuant to the accounting guidance governing revenue recognition on multiple element arrangements, the license and undelivered services (i.e., the joint committees and the Research Collaboration) are not separable and, accordingly, the license and services are being treated as a single unit of accounting. When multiple deliverables are accounted for as a single unit of accounting, the Company bases its revenue recognition pattern on the final deliverable. Under the Takeda Agreement, the last elements to be delivered are the JDCC and JTTC services, each of which has a life of no more than seven years.
	The Company is recognizing the upfront payment of $100.0 million and the Technology Transfer Milestones of $50.0 million, the receipt of which the Company believed was probable at the commencement of the collaboration, on a straight-line basis over seven years because the Company is unable to reasonably estimate the level of effort to fulfill these obligations, primarily because the effort required under the Research Collaboration is largely unknown, and therefore, cannot utilize a proportional performance model. As future milestones are achieved, if any, the Company will recognize as revenue a portion of the milestone payment equal to the percentage of the performance period completed when the milestone is achieved, multiplied by the amount of the milestone payment. At September 30, 2013, deferred revenue under the Takeda Agreement was $36.3 million.
	Monsanto Alliance
	In August 2012, the Company and Monsanto Company (“Monsanto”) entered into a license and collaboration agreement (the “Monsanto Agreement”), pursuant to which the Company granted to Monsanto a worldwide, exclusive, royalty bearing right and license, including the right to grant sublicenses, to the Company’s RNAi platform technology and intellectual property controlled by the Company as of the date of the Monsanto Agreement or during the 30 months thereafter, in the field of agriculture. The Monsanto Agreement also includes the transfer of technology from the Company to Monsanto and a collaborative research project (the “Monsanto Discovery Collaboration”). Under the Monsanto Agreement, Monsanto will be the Company’s exclusive collaborator in the agriculture field for a ten-year period (the “Exclusivity Period”).
	In consideration for the rights granted to Monsanto under the Monsanto Agreement, Monsanto paid the Company $29.2 million in upfront cash payments. Monsanto is also required to make near-term milestone payments to the Company upon the achievement of specified technology transfer and patent-related milestones. The Company is also entitled to receive additional funding for collaborative research efforts. In the aggregate, the Company can earn up to $5.0 million in potential future milestone payments and research funding under the Monsanto Agreement. In addition, Monsanto is required to pay to the Company a percentage of specified fees from certain sublicense agreements Monsanto may enter into that include access to the Company’s intellectual property, as well as low single-digit royalty payments on worldwide, net sales by Monsanto, its affiliates and sublicensees of certain Licensed Products (as defined in the Monsanto Agreement), if any. In December 2012, the Company received a milestone payment of $1.5 million of the $5.0 million in potential milestone payments under the Monsanto Agreement based upon the achievement of a specified patent-related event. In August 2013, the Company received an additional milestone payment of $2.5 million based upon the completion of technology transfer activities. The Company could potentially earn the next and final milestone payment of $1.0 million under the Monsanto Agreement in connection with the Monsanto Discovery Collaboration. For purposes of potential future revenue recognition, the Company does not believe this final milestone is substantive. Due to the uncertainty of the application of RNAi technology in the field of agriculture, the Company may not receive the final milestone payment or any royalty payments from Monsanto.
	The Company determined that the significant deliverables under the Monsanto Agreement include the license, the technology transfer activities and the services that the Company will be obligated to perform under the Monsanto Discovery Collaboration. The Company also determined that, pursuant to the accounting guidance governing revenue recognition on multiple element arrangements, the license and undelivered technical transfer activities and Monsanto Discovery Collaboration services do not have standalone value due to the specialized nature of the services to be provided by the Company. In addition, while Monsanto has the ability to grant sublicenses, it cannot grant access to certain of the Company’s proprietary technology. The uniqueness of the Company’s services and the limited sublicense right are indicators that standalone value is not present in the arrangement. Therefore the deliverables are not separable and, accordingly, the license and undelivered technical transfer activities and Monsanto Discovery Collaboration services are being treated as a single unit of accounting. When multiple deliverables are accounted for as a single unit of accounting, the Company bases its revenue recognition model on the final deliverable. Under the Monsanto Agreement, the last deliverable to be completed is the Monsanto Discovery Collaboration, which must be completed within five years. The Company is recognizing revenue under the Monsanto Agreement on a straight-line basis over five years. The Company is not utilizing a proportional performance model since it is unable to reasonably estimate the level of effort to fulfill these obligations, primarily because the effort required under the Monsanto Discovery Collaboration is largely unknown.
	The Company received a payment of $29.2 million from Monsanto in August 2012, which was initially recorded as deferred revenue. Under the terms of the Monsanto Agreement, in the event that during the Exclusivity Period the Company ceases to own or otherwise exclusively control certain licensed patent rights in the agriculture field, for any reason other than Monsanto’s breach of the Monsanto Agreement or its negligence or willful misconduct, resulting in the loss of exclusivity with respect to Monsanto’s rights to such patent rights, and such loss of exclusivity has a material adverse effect on the Licensed Products, then the Company would be required to pay Monsanto up to $5.0 million as liquidated damages, and Monsanto’s royalty obligations to the Company under the Monsanto Agreement would be reduced or, under certain circumstances, terminated. The Company has the right to cure any such loss of patent rights under the Monsanto Agreement. The Company has determined that this amount is not fixed and determinable and therefore, the Company has excluded this amount from its revenue model and is deferring the recognition of $5.0 million of revenue. The Company will continue to reassess when this amount can be considered fixed and determinable. If the achievement of a milestone is considered probable at the inception of the collaboration, the Company’s policy is to include the related payment in its revenue model. The Company concluded that the receipt of the technology transfer payment of $2.5 million was probable, and has therefore included this amount in the Company’s revenue model. As future milestones are achieved, if any, the Company will recognize as revenue a portion of the milestone payment equal to the percentage of the performance period completed when the milestone is achieved, multiplied by the amount of the milestone payment. At September 30, 2013, deferred revenue under the Monsanto Agreement was $27.0 million.
	Product Alliances
	Cubist Alliance
	In January 2009, the Company entered into a license and collaboration agreement with Cubist Pharmaceuticals, Inc. (the “Cubist Agreement”) to develop and commercialize therapeutic products based on certain of the Company’s RNAi technology for the treatment of RSV infection. Licensed products initially included ALN-RSV01, as well as several other second-generation RNAi-based RSV inhibitors. In November 2009, the Company and Cubist entered into an amendment to the Cubist Agreement (the “Amendment”), which provided that the Company and Cubist would focus their collaboration and joint development efforts on ALN-RSV02, a second-generation compound, intended for use in pediatric patients. In December 2010, the Company and Cubist jointly made a portfolio decision to put the development of ALN-RSV02 on hold. Pursuant to the terms of the Amendment, the Company continued to develop ALN-RSV01 for adult transplant patients at its sole discretion and expense and Cubist had the right to opt into collaborating with the Company on ALN-RSV01, subject to specified conditions.
	In consideration for the rights granted to Cubist under the Cubist Agreement, in January 2009, Cubist paid the Company an upfront cash payment of $20.0 million. Under the terms of the Cubist Agreement, the Company and Cubist shared responsibility for developing licensed products in North America and each was responsible for one-half of the related development costs, subject to the terms of the Amendment. The Company’s collaboration with Cubist for the development of licensed products in North America was governed by a joint steering committee comprised of an equal number of representatives from each party.
	The Company determined that the deliverables under the Cubist Agreement included the licenses, technology transfer related to the ALN-RSV program, the joint steering committee and the development and manufacturing services that the Company was obligated to perform during the development period. The Company also determined that, pursuant to the accounting guidance governing revenue recognition on multiple element arrangements, the licenses and undelivered services were not separable and, accordingly, the licenses and services were treated as a single unit of accounting. When multiple deliverables are accounted for as a single unit of accounting, the Company bases its revenue recognition pattern on the final deliverable. Under the Cubist Agreement, the last element to be delivered was the development and manufacturing services, which had an expected life of approximately eight years.
	In February 2013, Cubist notified the Company that it would not exercise its opt-in right for ALN-RSV01. In light of this determination, the Company and Cubist mutually agreed to terminate the license and collaboration agreement effective as of February 6, 2013 (the “Cubist Effective Date”). As of the Cubist Effective Date, the parties have no further rights and obligations under the Cubist Agreement, as amended, notwithstanding anything to the contrary in the Cubist Agreement, as amended.
	The Company was recognizing the upfront payment of $20.0 million on a straight-line basis over approximately eight years because the Company was unable to reasonably estimate the level of effort to fulfill its performance obligations, and therefore, could not utilize a proportional performance model. As a result of the termination of the Cubist Agreement in February 2013 and the end of the Company’s performance obligations thereunder, the Company recognized the remaining deferred revenue of $9.7 million during the three months ended March 31, 2013.
	Genzyme Alliance
	In October 2012, the Company and Genzyme entered into a license and collaboration agreement (the “Genzyme Agreement”) pursuant to which the Company granted to Genzyme an exclusive license in Japan and the Asia-Pacific region (“the Genzyme Territory”) to develop and commercialize RNAi therapeutics targeting transthyretin (“TTR”) for the treatment of transthyretin-mediated amyloidosis (“ATTR”) and other human diseases. The Genzyme Agreement covers patisiran (the recommended International Nonproprietary Name (“INN”) for ALN-TTR02) and ALN-TTRsc, and may in the future cover additional TTR-specific RNAi therapeutic compounds that comprise the Company’s TTR program (together, “ALN-TTR Licensed Products”), subject, in the case of Improvement Products (as defined in the Genzyme Agreement), to specified additional terms and conditions. Under the Genzyme Agreement, the Company retains all development and commercialization rights worldwide outside of the Genzyme Territory.
	In consideration for the rights granted to Genzyme under the Genzyme Agreement, Genzyme paid the Company an upfront cash payment of $22.5 million. Upon achievement of certain milestones, the Company will be entitled to receive milestone payments, up to an aggregate of $50.0 million, including up to $25.0 million in specified development milestones and $25.0 million in specified regulatory milestones. In addition, the Company will be entitled to tiered royalties expected to yield an effective royalty rate percentage ranging from the mid-teens to mid-twenties based on annual net sales, if any, of ALN-TTR Licensed Products in the Genzyme Territory by Genzyme, its affiliates and sublicensees. The Company could potentially earn the next development milestone payment of $7.0 million under the Genzyme Agreement based upon the completion of a successful Phase II ALN-TTR clinical trial, as defined in the Genzyme Agreement. For purposes of potential future revenue recognition, the Company does not believe this milestone or any future milestones are substantive. Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with drug development, the Company may not receive any milestone or royalty payments from Genzyme.
	Under the Genzyme Agreement, the parties will collaborate in the development of ALN-TTR Licensed Products, with Genzyme assuming primary responsibility for the development and commercialization of ALN-TTR Licensed Products in the Genzyme Territory and the Company retaining primary responsibility for the development and commercialization of ALN-TTR Licensed Products in the rest of the world. The collaboration between Genzyme and the Company is governed by a joint steering committee that will be comprised of an equal number of representatives from each party. Under the agreement, Genzyme is establishing a development plan for the ALN-TTR program relating to the development activities to be undertaken in the Genzyme Territory. Genzyme is responsible, at its expense, for all development activities under the development plan that are reasonably necessary for the regulatory approval and commercialization of an RNAi therapeutic for the treatment of ATTR in the Genzyme Territory. The Company and Genzyme intend to enter into a supply agreement to provide for supply of ALN-TTR Licensed Products to Genzyme for clinical trials, and, at Genzyme’s request, commercial sales. Genzyme may elect, at any time during the term of the Genzyme Agreement, to manufacture ALN-TTR Licensed Products itself or arrange for a third party to manufacture ALN-TTR Licensed Products.
	Genzyme also has a right of first negotiation in the event that the Company desires to grant any third party rights to develop and/or commercialize an ALN-TTR Licensed Product for the treatment of ATTR or other human diseases outside of the Genzyme Territory.
	The Genzyme Agreement originally provided that if development of an ALN-TTR Licensed Product was terminated by the Company or Genzyme under certain limited circumstances, Genzyme would have the right to terminate the Genzyme Agreement and the Company would be required to refund certain amounts paid by Genzyme to the Company under the Genzyme Agreement prior to such termination. In February 2013, the Company and Genzyme amended the Genzyme Agreement to remove this provision.
	The Company has determined that the significant deliverables under the Genzyme Agreement include the license, the joint steering committee and any additional TTR-specific RNAi therapeutic compounds that comprise the ALN-TTR program. The Company also determined that, pursuant to the accounting guidance governing revenue recognition on multiple element arrangements, the license and undelivered joint steering committee and any additional TTR-specific RNAi therapeutic compounds do not have standalone value due to the specialized nature of the services to be provided by the Company. In addition, while Genzyme has the ability to grant sublicenses, it cannot sublicense all or substantially all of its rights under the Genzyme Agreement. The uniqueness of the Company’s services and the limited sublicense right are indicators that standalone value is not present in the arrangement. Therefore the deliverables are not separable and, accordingly, the license and undelivered services are being treated as a single unit of accounting. When multiple deliverables are accounted for as a single unit of accounting, the Company bases its revenue recognition pattern on the final deliverable.
	The Company is currently unable to reasonably estimate its period of performance under the Genzyme Agreement, as it is unable to estimate the timeline of its deliverables related to the deliverable for any additional TTR-specific RNAi therapeutic compounds. The Company is deferring all revenue under the Genzyme Agreement until it is able to reasonably estimate its period of performance. The Company will continue to reassess whether it can reasonably estimate the period of performance to fulfill its obligations under the Genzyme Agreement. At September 30, 2013, deferred revenue under the Genzyme Agreement was $22.5 million.
	The Medicines Company Agreement
	In February 2013, the Company and The Medicines Company (“MDCO”) entered into a license and collaboration agreement (the “MDCO Agreement”) pursuant to which the Company granted to MDCO an exclusive, worldwide license to develop, manufacture and commercialize RNAi therapeutics targeting proprotein convertase subtilisin/kexin type 9 (“PCSK9”), including ALN-PCS02 and ALN-PCSsc, for the treatment of hypercholesterolemia and other human diseases (collectively, “ALN-PCS Licensed Products”). ALN-PCS02 is an intravenously administered RNAi therapeutic for which the Company completed a Phase I clinical trial, and ALN-PCSsc is a subcutaneously administered RNAi therapeutic currently in pre-clinical development.
	In consideration for the rights granted to MDCO under the MDCO Agreement, MDCO paid the Company an upfront cash payment of $25.0 million. Upon achievement of certain milestones, the Company will be entitled to receive milestone payments, up to an aggregate of $180.0 million, including up to $30.0 million in specified development milestones, $50.0 million in specified regulatory milestones and $100.0 million in specified commercialization milestones. In addition, the Company will be entitled to scaled double-digit royalties based on annual worldwide net sales, if any, of ALN-PCS Licensed Products by MDCO, its affiliates and sublicensees, subject to reduction under specified circumstances. The Company could potentially earn the next development milestone payment of $10.0 million under the MDCO Agreement based upon the initiation of the next clinical trial. For purposes of potential future revenue recognition, the Company does not believe this milestone or any future milestones are substantive. Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with drug development, the Company may not receive any milestone or royalty payments from MDCO.
	Under the MDCO Agreement, the parties will collaborate in the further development of ALN-PCS Licensed Products. The Company will retain responsibility for the development of ALN-PCS Licensed Products until Phase I Completion (as defined in the MDCO Agreement) at its cost, up to an agreed upon initial development cost cap. MDCO will assume all other responsibility for the development and commercialization of ALN-PCS Licensed Products, at its sole cost. Initially the collaboration included the development of both ALN-PCS02 and ALN-PCSsc in parallel. In October 2013, the parties announced the selection of ALN-PCSsc for ongoing development, in accordance with the terms of the MDCO Agreement. The collaboration between MDCO and the Company will be governed by a joint steering committee that will be comprised of an equal number of representatives from each party. The Company will be solely responsible for obtaining supply of finished product reasonably required for the conduct of its obligations through Phase I Completion, and supplying MDCO with finished product reasonably required for the first Phase II clinical trial of an ALN-PCS Licensed Product conducted by MDCO, at the Company’s expense, provided such costs do not exceed the development costs cap, subject to certain exceptions. After such time, MDCO will have the sole right and responsibility to manufacture and supply ALN-PCS Licensed Product for development and commercialization under the MDCO development plan, subject to the terms of the MDCO Agreement. The Company also has obligations under the MDCO Agreement to transfer certain technology to MDCO.
	The Company has determined that the significant deliverables under the MDCO Agreement include the license, the joint steering committee, technology transfer obligations, development activities through Phase I Completion and supply of product for a Phase II clinical trial. The Company also determined that, pursuant to the accounting guidance governing revenue recognition on multiple element arrangements, the license and collective undelivered activities and services do not have standalone value due to the specialized nature of the activities and services to be provided by the Company. In addition, while MDCO has the ability to grant sublicenses, it must receive the Company’s prior written consent to sublicense all or substantially all of its rights. The uniqueness of the Company’s services and the limited sublicense right are indicators that standalone value is not present in the arrangement. Therefore the deliverables are not separable and, accordingly, the license and undelivered services are being treated as a single unit of accounting. When multiple deliverables are accounted for as a single unit of accounting, the Company bases its revenue recognition pattern on the final deliverable. Under the MDCO Agreement, all deliverables are expected to be completed within five years. The Company is recognizing revenue under the MDCO Agreement on a straight-line basis over five years. The Company is not utilizing a proportional performance model since it is unable to reasonably estimate the level of effort to fulfill these obligations, primarily because the effort required under the development activities is largely unknown.
	The Company received the upfront payment of $25.0 million from MDCO in February 2013, which was initially recorded as deferred revenue. As future milestones are achieved, if any, the Company will recognize as revenue a portion of the milestone payment equal to the percentage of the performance period completed when the milestone is achieved, multiplied by the amount of the milestone payment. At September 30, 2013, deferred revenue under the MDCO Agreement was $21.8 million.

Commitments_and_Contingencies

Commitments and Contingencies	9 Months Ended
Commitments and Contingencies	Sep. 30, 2013
Commitments and Contingencies	'
	3. COMMITMENTS AND CONTINGENCIES
	Litigation
	University of Utah Litigation
	On March 22, 2011, The University of Utah (“Utah”) filed a civil complaint in the United States District Court for the District of Massachusetts against the Company, Max Planck Gesellschaft Zur Foerderung Der Wissenschaften e.V. and Max Planck Innovation GmbH (together, “Max Planck”), the Whitehead Institute for Biomedical Research (“Whitehead”), the Massachusetts Institute of Technology (“MIT”) and the University of Massachusetts (“UMass”), claiming a professor at Utah is the sole inventor or, in the alternative, a joint inventor, of the Tuschl patents. Utah did not serve the original complaint on the Company or the other defendants. On July 6, 2011, Utah filed an amended complaint alleging substantially the same claims against the Company, Max Planck, Whitehead, MIT and UMass. The amended complaint was served on the Company on July 14, 2011. Utah is seeking changes to the inventorship of the Tuschl patents, unspecified damages and other relief. On October 31, 2011, the Company, Max Planck, Whitehead, MIT and UMass filed a motion to dismiss. Also on October 31, 2011, UMass filed a motion to dismiss on separate grounds, which the Company, Max Planck, Whitehead and MIT have joined. On December 31, 2011, Utah filed a second amended complaint dropping UMass as a defendant and adding as defendants several UMass officials. In June 2012, the Court denied both motions to dismiss. The Company, Max Planck, Whitehead, MIT and UMass filed an appeal of the Court’s ruling on the motion to dismiss for lack of jurisdiction and a motion requesting that the Court stay the case pending the outcome of the appeal. In July 2012, the Court stayed discovery in the case pending the outcome of the defendants’ appeal. Oral arguments in the appeal were heard in early March 2013 in the United States Court of Appeals for the Federal Circuit (the “CAFC”). In August 2013, the CAFC affirmed the lower Court’s ruling, in a split decision. The Company believes the majority made an error in law when affirming the lower Court’s decision, and in September 2013, the Company filed a petition with the CAFC for rehearing or rehearing en banc. In October 2013, the CAFC invited Utah to file an answer to the petition. The Company is awaiting a decision on the petition.
	Although the Company believes it has meritorious defenses and intends to vigorously defend itself in this matter, litigation is subject to inherent uncertainty and a court could ultimately rule against the Company. In addition, the defense of litigation and related matters are costly and may divert the attention of the Company’s management and other resources that would otherwise be engaged in other activities. The Company has not recorded an estimate of the possible loss associated with this legal proceeding due to the uncertainties related to both the likelihood and the amount of any possible loss or range of loss.
	The Company’s accounting policy for accrual of legal costs is to recognize such expenses as incurred.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	9 Months Ended
Summary of Significant Accounting Policies (Policies)	Sep. 30, 2013
Basis of Presentation and Principles of Consolidation	'
	Basis of Presentation and Principles of Consolidation
	The accompanying condensed consolidated financial statements of Alnylam Pharmaceuticals, Inc. (the “Company” or “Alnylam”) are unaudited and have been prepared in accordance with accounting principles generally accepted in the United States of America (“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. The Company’s condensed consolidated financial statements have also been prepared on a basis substantially consistent with, and should be read in conjunction with, the Company’s audited consolidated financial statements for the year ended December 31, 2012, which were included in the Company’s Annual Report on Form 10-K that was filed with the Securities and Exchange Commission (the “SEC”) on February 19, 2013. The year-end condensed balance sheet data was derived from audited financial statements, but does not include all disclosures required by GAAP. The results of the Company’s operations for any interim period are not necessarily indicative of the results of the Company’s operations for any other interim period or for a full fiscal year.
	The accompanying condensed consolidated financial statements reflect the operations of the Company and Alnylam U.S., Inc., Alnylam Europe AG, Alnylam (Bermuda) Ltd. and Alnylam Securities Corporation, which are wholly-owned subsidiaries of the Company. All significant intercompany accounts and transactions have been eliminated.
Use of Estimates	'
	Use of Estimates
	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Net Loss Per Common Share	'
	Net Loss Per Common Share
	The Company computes basic net loss per common share by dividing net loss by the weighted average number of common shares outstanding. The Company computes 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 (using the treasury stock method) and unvested restricted stock awards. Because the inclusion of potential common shares would be anti-dilutive for all periods presented, diluted net loss per common share is the same as basic net loss per common share.
	The following table sets forth for the periods presented the potential common shares (prior to consideration of the treasury stock method) excluded from the calculation of net loss per common share because their inclusion would be anti-dilutive, in thousands:


			Three and Nine Months Ended
			September 30,
			2013				2012
	Options to purchase common stock			8,021				8,156
	Unvested restricted common stock			593				638

				8,614				8,794

Restricted Stock Awards	'
	Restricted Stock Awards
	In January 2012, as part of its post-restructuring retention program, the Company granted an aggregate of 508,928 shares of restricted stock to its retained employees, excluding the Company’s chief executive officer and president and chief operating officer. These restricted stock awards were valued at $5.3 million on the grant date and vest in full on the second anniversary of the grant date.

	The Company recognized an aggregate of $1.3 million and $0.7 million of stock-based compensation expense related to all outstanding restricted stock awards for the three months ended September 30, 2013 and 2012, respectively. The Company recognized an aggregate of $2.6 million and $1.9 million of stock-based compensation expense related to all outstanding restricted stock awards for the nine months ended September 30, 2013 and 2012, respectively.
Public Offering	'
	Public Offering
	In February 2012, the Company sold an aggregate of 8,625,000 shares of its common stock through an underwritten public offering at a price to the public of $10.75 per share. As a result of the offering, the Company received aggregate net proceeds of approximately $86.8 million, after deducting underwriting discounts and commissions and other estimated offering expenses of approximately $5.9 million.
	In January 2013, the Company sold an aggregate of 9,200,000 shares of its common stock through an underwritten public offering at a price to the public of $20.13 per share. As a result of this offering, the Company received aggregate net proceeds of approximately $173.6 million, after deducting underwriting discounts and commissions and other estimated offering expenses of approximately $11.6 million.
Fair Value Measurements	'
	Fair Value Measurements
	The following tables present information about the Company’s assets that are measured at fair value on a recurring basis at September 30, 2013 and December 31, 2012, and indicate the fair value hierarchy of the valuation techniques the Company utilized to determine such fair value. In general, fair values determined by Level 1 inputs utilize quoted prices (unadjusted) in active markets for identical assets or liabilities. Fair values determined by Level 2 inputs utilize data points that are observable, such as quoted prices (adjusted), interest rates and yield curves. Fair values determined by Level 3 inputs utilize unobservable data points for the asset or liability, and include situations where there is little, if any, market activity for the asset or liability. The fair value hierarchy level is determined by the lowest level of significant input. Financial assets measured at fair value on a recurring basis are summarized as follows, in thousands:


	Description		At				Quoted Prices				Significant				Significant
			September 30,				in Active				Observable				Unobservable
			2013				Markets				Inputs				Inputs
							(Level 1)				(Level 2)				(Level 3)
	Cash equivalents		$	10,515			$	10,515			$	—			$	—
	Marketable securities (fixed income)
	Corporate notes			266,418				—				266,418				—
	U.S. Government obligations			69,901				—				69,901				—
	Commercial paper			18,986				—				18,986				—
	Marketable securities (Regulus equity holdings)			57,999				—				57,999				—

	Total		$	423,819			$	10,515			$	413,304			$	—



	Description		At				Quoted Prices				Significant				Significant
			December 31,				in Active				Observable				Unobservable
			2012				Markets				Inputs				Inputs
							(Level 1)				(Level 2)				(Level 3)
	Cash equivalents		$	50,213			$	50,213			$	—			$	—
	Marketable securities (fixed income)
	Corporate notes			91,523				—				91,523				—
	U.S. Government obligations			60,661				—				60,661				—
	Commercial paper			22,639				—				22,639				—
	Marketable securities (Regulus equity holdings)			38,748				—				38,748				—

	Total		$	263,784			$	50,213			$	213,571			$	—

	During the nine months ended September 30, 2013, there were no transfers between Level 1, Level 2 and Level 3 financial assets. The carrying amounts reflected in the Company’s condensed consolidated balance sheets for cash, collaboration receivables, other current assets, accounts payable and accrued expenses approximate fair value due to their short-term maturities.
Investments in Marketable Securities	'
	Investments in Marketable Securities
	The Company invests its excess cash balances in short-term and long-term marketable debt and equity securities. The Company classifies its investments in marketable debt securities as either held-to-maturity or available-for-sale based on facts and circumstances present at the time it purchased the securities. At each balance sheet date presented, the Company classified all of its investments in debt and equity securities as available-for-sale. The Company reports available-for-sale investments at fair value at each balance sheet date and includes any unrealized holding gains and losses (the adjustment to fair value) in accumulated other comprehensive loss, a component of stockholders’ equity. Realized gains and losses are determined using the specific identification method and are included in other income. The Company did not recognize any realized gains or losses from sales of its available-for-sale securities during the nine months ended September 30, 2013, and as a result, did not reclassify any amount out of accumulated other comprehensive income for the same period. If any adjustment to fair value reflects a decline in the value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other than temporary” and, if so, marks the investment to market through a charge to its condensed consolidated statements of comprehensive loss. The Company did not record any impairment charges related to its fixed income marketable securities during the current period. The Company’s marketable securities are classified as cash equivalents if the original maturity, from the date of purchase, is 90 days or less, and as marketable securities if the original maturity, from the date of purchase, is in excess of 90 days. At September 30, 2013, the Company’s cash equivalents are composed of money market funds.
	In the fourth quarter of 2012, the Company began accounting for its investment in Regulus Therapeutics Inc. (“Regulus”) as an available-for-sale marketable security. Intraperiod tax allocation rules require the Company to allocate its provision for income taxes between continuing operations and other categories of earnings, such as other comprehensive income. In periods in which the Company has a year-to-date pre-tax loss from continuing operations and pre-tax income in other categories of earnings, such as other comprehensive income, the Company must allocate the tax provision to the other categories of earnings. The Company then records a related tax benefit in continuing operations. The following tables summarize the fair value, accumulated other comprehensive income and intraperiod tax allocation regarding the Company’s investment in Regulus available-for-sale marketable securities at September 30, 2013 and the activity for the three and nine months ended September 30, 2013, in thousands:


	Description		At				Three Months				Balance at
			June 30,				Ended				September 30,
			2013				September 30,				2013
							2013
	Carrying value		$	12,449			$	—			$	12,449
	Accumulated other comprehensive income (loss), before tax			47,887				(2,337	)			45,550

	Investment in equity securities of Regulus Therapeutics Inc., as reported		$	60,336			$	(2,337	)		$	57,999


	Accumulated other comprehensive income (loss), before tax		$	47,887			$	(2,337	)		$	45,550
	Intraperiod tax allocation recorded as a benefit from income taxes			(14,018	)			(2,270	)			(16,288	)
	Intraperiod tax allocation recorded as an accrued liability			(5,232	)			3,209				(2,023	)

	Accumulated other comprehensive income, net of tax		$	28,637			$	(1,398	)		$	27,239



	Description		At				Nine Months				Balance at
			December 31,				Ended				September 30,
			2012				September 30,				2013
							2013
	Carrying value		$	12,449			$	—			$	12,449
	Accumulated other comprehensive income, before tax			26,299				19,251				45,550

	Investment in equity securities of Regulus Therapeutics Inc., as reported		$	38,748			$	19,251			$	57,999


	Accumulated other comprehensive income, before tax		$	26,299			$	19,251			$	45,550
	Intraperiod tax allocation recorded as a benefit from income taxes			(10,572	)			(5,716	)			(16,288	)
	Intraperiod tax allocation recorded as an accrued liability			—				(2,023	)			(2,023	)

	Accumulated other comprehensive income, net of tax		$	15,727			$	11,512			$	27,239

	The Company obtains fair value measurement data for its fixed income marketable securities from independent pricing services. The Company performs 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, the Company performs its 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.
	The following tables summarize the Company’s fixed income marketable securities at September 30, 2013 and December 31, 2012, in thousands:


			September 30, 2013
			Amortized				Gross				Gross				Fair Value
			Cost				Unrealized				Unrealized
							Gains				Losses
	Commercial paper (Due within 1 year)		$	18,987			$	3			$	(4	)		$	18,986
	Corporate notes (Due within 1 year)			148,864				46				(42	)			148,868
	Corporate notes (Due after 1 year through 2 years)			117,644				20				(114	)			117,550
	U.S. Government obligations (Due within 1 year)			14,079				7				—				14,086
	U.S. Government obligations (Due after 1 year through 2 years)			55,802				17				(4	)			55,815

	Total		$	355,376			$	93			$	(164	)		$	355,305



			December 31, 2012
			Amortized				Gross				Gross				Fair Value
			Cost				Unrealized				Unrealized
							Gains				Losses
	Commercial paper (Due within 1 year)		$	22,650			$	1			$	(12	)		$	22,639
	Corporate notes (Due within 1 year)			41,249				23				(4	)			41,268
Corporate notes (Due after 1 year through 2 years)			50,322				5				(72	)			50,255
U.S. Government obligations (Due within 1 year)			7,500				—				—				7,500
U.S. Government obligations (Due after 1 year through 2 years)			53,168				2				(9	)			53,161

Total		$	174,889			$	31			$	(97	)		$	174,823

Subsequent Events	'
	Subsequent Events
	The Company evaluated all events or transactions that occurred after September 30, 2013 through the date these condensed consolidated financial statements were issued. During this period, the Company did not have any material recognized or nonrecognized subsequent events.
Recent Accounting Pronouncements	'
	Recent Accounting Pronouncements
	In February 2013, the Financial Accounting Standards Board (“FASB”) issued amendments to the accounting guidance for presentation of comprehensive income to improve the reporting of reclassifications out of accumulated other comprehensive income. The amendments did not change the current requirements for reporting net income or other comprehensive income, but require an entity to provide information about the amounts reclassified out of accumulated other comprehensive income by component. In addition, an entity is now required to present, either on the face of the statement where the net income is presented or in the notes, significant amounts reclassified out of accumulated other comprehensive income by the respective line items of net income but only if the amount reclassified is required under GAAP to be reclassified to net income in its entirety in the same reporting period. For other amounts that are not required under GAAP to be reclassified in their entirety to net income, an entity is required to cross-reference to other disclosures required under GAAP that provide additional detail about these amounts. For public companies, these amendments were effective prospectively for reporting periods beginning after December 15, 2012. The Company adopted this guidance on January 1, 2013. Other than a change in presentation, the adoption of this guidance did not have a material impact on the Company’s condensed consolidated financial statements.
	In July 2013, the FASB issued new accounting guidance specific to income taxes. The new guidance requires an entity to present an unrecognized tax benefit and a net operating loss carryforward, a similar tax loss, or a tax credit carryforward on a net basis as part of a deferred tax asset, unless the unrecognized tax benefit is not available to reduce the deferred tax asset component or would not be utilized for that purpose, then a liability would be recognized. The updated accounting guidance is effective for fiscal years beginning after December 15, 2013. The Company does not expect the adoption of this guidance to have a material impact on the Company’s condensed consolidated financial statements.

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Tables)	9 Months Ended
Summary of Significant Accounting Policies (Tables)	Sep. 30, 2013
Common Shares Excluded From the Calculation of Net Loss Per Common Share	'
	The following table sets forth for the periods presented the potential common shares (prior to consideration of the treasury stock method) excluded from the calculation of net loss per common share because their inclusion would be anti-dilutive, in thousands:


		Three and Nine Months Ended
		September 30,
		2013			2012
	Options to purchase common stock		8,021			8,156
	Unvested restricted common stock		593			638

			8,614			8,794

Fair Value Measured on Recurring Basis	'
	Financial assets measured at fair value on a recurring basis are summarized as follows, in thousands:


	Description	At			Quoted Prices			Significant			Significant
		September 30,			in Active			Observable			Unobservable
		2013			Markets			Inputs			Inputs
					(Level 1)			(Level 2)			(Level 3)
	Cash equivalents	$	10,515		$	10,515		$	—		$	—
	Marketable securities (fixed income)
	Corporate notes		266,418			—			266,418			—
	U.S. Government obligations		69,901			—			69,901			—
	Commercial paper		18,986			—			18,986			—
	Marketable securities (Regulus equity holdings)		57,999			—			57,999			—

	Total	$	423,819		$	10,515		$	413,304		$	—



	Description	At			Quoted Prices			Significant			Significant
		December 31,			in Active			Observable			Unobservable
		2012			Markets			Inputs			Inputs
					(Level 1)			(Level 2)			(Level 3)
	Cash equivalents	$	50,213		$	50,213		$	—		$	—
	Marketable securities (fixed income)
	Corporate notes		91,523			—			91,523			—
	U.S. Government obligations		60,661			—			60,661			—
	Commercial paper		22,639			—			22,639			—
	Marketable securities (Regulus equity holdings)		38,748			—			38,748			—

	Total	$	263,784		$	50,213		$	213,571		$	—

Summary of Fair Value Accumulated Other Comprehensive Income and Intraperiod Tax Allocation in Available-for-Sale Marketable Securities	'
	The following tables summarize the fair value, accumulated other comprehensive income and intraperiod tax allocation regarding the Company’s investment in Regulus available-for-sale marketable securities at September 30, 2013 and the activity for the three and nine months ended September 30, 2013, in thousands:


	Description	At			Three Months			Balance at
		June 30,			Ended			September 30,
		2013			September 30,			2013
					2013
	Carrying value	$	12,449		$	—		$	12,449
	Accumulated other comprehensive income (loss), before tax		47,887			(2,337	)		45,550

	Investment in equity securities of Regulus Therapeutics Inc., as reported	$	60,336		$	(2,337	)	$	57,999


	Accumulated other comprehensive income (loss), before tax	$	47,887		$	(2,337	)	$	45,550
	Intraperiod tax allocation recorded as a benefit from income taxes		(14,018	)		(2,270	)		(16,288	)
	Intraperiod tax allocation recorded as an accrued liability		(5,232	)		3,209			(2,023	)

	Accumulated other comprehensive income, net of tax	$	28,637		$	(1,398	)	$	27,239



	Description	At			Nine Months			Balance at
		December 31,			Ended			September 30,
		2012			September 30,			2013
					2013
	Carrying value	$	12,449		$	—		$	12,449
	Accumulated other comprehensive income, before tax		26,299			19,251			45,550

	Investment in equity securities of Regulus Therapeutics Inc., as reported	$	38,748		$	19,251		$	57,999


	Accumulated other comprehensive income, before tax	$	26,299		$	19,251		$	45,550
	Intraperiod tax allocation recorded as a benefit from income taxes		(10,572	)		(5,716	)		(16,288	)
	Intraperiod tax allocation recorded as an accrued liability		—			(2,023	)		(2,023	)

	Accumulated other comprehensive income, net of tax	$	15,727		$	11,512		$	27,239

Summary of Company's Marketable Securities	'
	The following tables summarize the Company’s fixed income marketable securities at September 30, 2013 and December 31, 2012, in thousands:


		September 30, 2013
		Amortized			Gross			Gross			Fair Value
		Cost			Unrealized			Unrealized
					Gains			Losses
	Commercial paper (Due within 1 year)	$	18,987		$	3		$	(4	)	$	18,986
	Corporate notes (Due within 1 year)		148,864			46			(42	)		148,868
	Corporate notes (Due after 1 year through 2 years)		117,644			20			(114	)		117,550
	U.S. Government obligations (Due within 1 year)		14,079			7			—			14,086
	U.S. Government obligations (Due after 1 year through 2 years)		55,802			17			(4	)		55,815

	Total	$	355,376		$	93		$	(164	)	$	355,305



		December 31, 2012
		Amortized			Gross			Gross			Fair Value
		Cost			Unrealized			Unrealized
					Gains			Losses
	Commercial paper (Due within 1 year)	$	22,650		$	1		$	(12	)	$	22,639
	Corporate notes (Due within 1 year)		41,249			23			(4	)		41,268
	Corporate notes (Due after 1 year through 2 years)		50,322			5			(72	)		50,255
	U.S. Government obligations (Due within 1 year)		7,500			—			—			7,500
	U.S. Government obligations (Due after 1 year through 2 years)		53,168			2			(9	)		53,161

	Total	$	174,889		$	31		$	(97	)	$	174,823

Significant_Agreements_Tables

Significant Agreements (Tables)	9 Months Ended
Significant Agreements (Tables)	Sep. 30, 2013
Revenue from Research Collaborators	'
	The following table summarizes the Company’s total consolidated net revenues from research collaborators, for the periods indicated, in thousands:


		Three Months				Nine Months
		Ended September 30,				Ended September 30,
		2013		2012		2013		2012
	Takeda	$	5,494	$	5,494	$	16,481	$	16,481
	Monsanto		1,410		519		4,230		519
	The Medicines Company		1,253		—		3,343		—
	Cubist		—		694		9,721		2,084
	Roche/Arrowhead		—		9,330		—		37,318
	Other		834		722		2,545		1,828

	Total net revenues from research collaborators	$	8,991	$	16,759	$	36,320	$	58,230

Potential_Common_Shares_Common

Potential Common Shares Common Shares Excluded from Calculation of Net Loss Per Common Share (Detail)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'	'
Anti-dilutive securities excluded from computation of earnings	8,614	8,794	8,614	8,794
Options to Purchase Common Stock	'	'	'	'
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'	'
Anti-dilutive securities excluded from computation of earnings	8,021	8,156	8,021	8,156
Unvested Restricted Common Stock	'	'	'	'
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'	'
Anti-dilutive securities excluded from computation of earnings	593	638	593	638

Summary_of_Significant_Account3

Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	1 Months Ended		3 Months Ended		9 Months Ended		1 Months Ended
In Millions, except Share data, unless otherwise specified	Jan. 31, 2013	Feb. 29, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Jan. 31, 2012
							Unvested Restricted Common Stock
							Current Employees
Significant Accounting Policies [Line Items]	'	'	'	'	'	'	'
Equity awards granted	'	'	'	'	'	'	508,928
Fair value of equity awards granted	'	'	'	'	'	'	$5.30
Stock awards vesting period	'	'	'	'	'	'	'2 years
Restricted stock expense	'	'	1.3	0.7	2.6	1.9	'
Common stocks issued through underwritten public offering	9,200,000	8,625,000	'	'	'	'	'
Underwritten public offering amount per share	$20.13	$10.75	'	'	'	'	'
Net proceeds from public issue, after deducting expenses	173.6	86.8	'	'	'	'	'
Underwriting discounts and commissions and other estimated offering expenses	$11.60	$5.90	'	'	'	'	'
Marketable securities classified as cash equivalents, original maturity	'	'	'	'	'90 days	'	'
Policy for marketable securities	'	'	'	'	'90 days	'	'

Fair_Value_Measured_on_Recurri

Fair Value Measured on Recurring Basis (Detail) (Recurring, USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Cash equivalents	$10,515	$50,213
Total	423,819	263,784
Corporate notes	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Available for sale securities, Fair value disclosure	266,418	91,523
U.S. Government obligations	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Available for sale securities, Fair value disclosure	69,901	60,661
Commercial paper	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Available for sale securities, Fair value disclosure	18,986	22,639
Equity Securities	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Available for sale securities, Fair value disclosure	57,999	38,748
Quoted Prices in Active Markets (Level 1)	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Cash equivalents	10,515	50,213
Total	10,515	50,213
Significant Observable Inputs (Level 2)	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total	413,304	213,571
Significant Observable Inputs (Level 2) \| Corporate notes	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Available for sale securities, Fair value disclosure	266,418	91,523
Significant Observable Inputs (Level 2) \| U.S. Government obligations	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Available for sale securities, Fair value disclosure	69,901	60,661
Significant Observable Inputs (Level 2) \| Commercial paper	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Available for sale securities, Fair value disclosure	18,986	22,639
Significant Observable Inputs (Level 2) \| Equity Securities	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Available for sale securities, Fair value disclosure	$57,999	$38,748

Summary_of_Fair_Value_Accumula

Summary of Fair Value Accumulated Other Comprehensive Income and Intraperiod Tax Allocation in Available-for-Sale Marketable Securities (Detail) (USD $)	3 Months Ended		9 Months Ended			3 Months Ended	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Regulus Therapeutics Inc	Regulus Therapeutics Inc	Regulus Therapeutics Inc	Regulus Therapeutics Inc
Schedule of Available-for-sale Securities [Line Items]	'	'	'	'	'	'	'	'	'
Carrying value	'	'	'	'	'	$12,449	$12,449	$12,449	$12,449
Accumulated other comprehensive income (loss), before tax	'	'	'	'	'	45,550	45,550	47,887	26,299
Investment in equity securities of Regulus Therapeutics Inc., as reported	57,999	'	57,999	'	38,748	57,999	57,999	60,336	38,748
Accumulated other comprehensive income (loss), before tax	'	'	'	'	'	45,550	45,550	47,887	26,299
Intraperiod tax allocation recorded as a benefit from income taxes	'	'	'	'	'	-16,288	-16,288	-14,018	-10,572
Intraperiod tax allocation recorded as an accrued liability	-2,023	'	-2,023	'	'	-2,023	-2,023	-5,232	'
Accumulated other comprehensive income, net of tax	'	'	'	'	'	27,239	27,239	28,637	15,727
Accumulated other comprehensive income (loss), before tax	'	'	'	'	'	-2,337	19,251	'	'
Investment in equity securities of Regulus Therapeutics Inc., as reported	'	'	'	'	'	-2,337	19,251	'	'
Accumulated other comprehensive income (loss), before tax	'	'	'	'	'	-2,337	19,251	'	'
Intraperiod tax allocation recorded as a benefit from income taxes	'	'	-5,716	'	'	-2,270	-5,716	'	'
Intraperiod tax allocation recorded as an accrued liability	'	'	'	'	'	3,209	-2,023	'	'
Accumulated other comprehensive income, net of tax	($962)	$170	$11,506	$273	'	($1,398)	$11,512	'	'

Summary_of_Companys_Marketable

Summary of Company's Marketable Securities (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	$355,376	$174,889
Gross Unrealized Gains	93	31
Gross Unrealized Losses	-164	-97
Fair Value	355,305	174,823
Commercial paper (Due within 1 year)	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	18,987	22,650
Gross Unrealized Gains	3	1
Gross Unrealized Losses	-4	-12
Fair Value	18,986	22,639
Corporate notes (Due within 1 year)	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	148,864	41,249
Gross Unrealized Gains	46	23
Gross Unrealized Losses	-42	-4
Fair Value	148,868	41,268
Corporate notes (Due after 1 year through 2 years)	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	117,644	50,322
Gross Unrealized Gains	20	5
Gross Unrealized Losses	-114	-72
Fair Value	117,550	50,255
U.S. Government obligations (Due within 1 year)	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	14,079	7,500
Gross Unrealized Gains	7	'
Fair Value	14,086	7,500
U.S. Government obligations (Due after 1 year through 2 years)	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized Cost	55,802	53,168
Gross Unrealized Gains	17	2
Gross Unrealized Losses	-4	-9
Fair Value	$55,815	$53,161

Revenue_from_Research_Collabor

Revenue from Research Collaborators (Detail) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'	'	'
Total net revenues from research collaborators	$8,991	$16,759	$36,320	$58,230
Takeda	'	'	'	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'	'	'
Total net revenues from research collaborators	5,494	5,494	16,481	16,481
Monsanto	'	'	'	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'	'	'
Total net revenues from research collaborators	1,410	519	4,230	519
MDCO	'	'	'	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'	'	'
Total net revenues from research collaborators	1,253	'	3,343	'
Cubist	'	'	'	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'	'	'
Total net revenues from research collaborators	'	694	9,721	2,084
Roche/Arrowhead	'	'	'	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'	'	'
Total net revenues from research collaborators	'	9,330	'	37,318
Other	'	'	'	'
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]	'	'	'	'
Total net revenues from research collaborators	$834	$722	$2,545	$1,828

Significant_Agreements_Additio

Significant Agreements - Additional Information (Detail) (USD $)

3 Months Ended

9 Months Ended

1 Months Ended

9 Months Ended

1 Months Ended

6 Months Ended

9 Months Ended

1 Months Ended

9 Months Ended

1 Months Ended

9 Months Ended

In Millions, unless otherwise specified

Mar. 31, 2013

Sep. 30, 2013

Aug. 31, 2013

Dec. 31, 2012

Aug. 31, 2012

Sep. 30, 2013

Mar. 31, 2011

Mar. 31, 2010

Oct. 31, 2008

Jun. 30, 2008

Sep. 30, 2013

Jan. 31, 2009

Oct. 31, 2012

Sep. 30, 2013

Feb. 28, 2013

Sep. 30, 2013

Cubist Alliance

Platform Alliances

Product Alliances

Joint Technology Transfer Committee

Joint Delivery Collaboration Committee

Monsanto Alliance

Arrowhead

Cubist Alliance

Takeda

Roche/Arrowhead

Cubist Alliance

Genzyme

MDCO

Loss Of Patent Rights

Product

Phase Two Clinical Trial ALN-TTR

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

Granted period of royalty bearing license

'5 years

Aggregate upfront and near term payments

$150

Profit sharing agreement ratio

50.00%

Number of licensed products

Additional number of licensed products

Upfront fee received

29.2

100

22.5

Milestone fees to company upon achievement of specified technology transfer milestones

Milestones considered probable at inception of collaborations

Maximum number of potential future milestones

171

180

Potential future payment for the achievement of specified development milestones

Potential future payment for the achievement of specified regulatory milestones

Potential future payment for the achievement of specified commercialization milestones

105

100

Next potential milestone payment

2.5

Payments for additional therapeutic areas

Maximum life of collaboration committee

'7 years

Deferred revenue

29.2

36.3

22.5

21.8

Period of exclusivity in the collaboration

'10 years

Milestone payment earned

1.5

Period for services under contract

'5 years

Future payment of liquidated damages

Technology transfer milestone

2.5

Percentage of sharing in development cost

50.00%

Life of development and manufacturing services

'8 years

Upfront payment recognition period

'8 years

Revenue recognized upon termination of collaboration

$9.70

Revenue recognizing period

'5 years