Significant Agreements - Additional Information (Detail) (USD $)	12 Months Ended		12 Months Ended					6 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended							6 Months Ended		12 Months Ended	1 Months Ended			6 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended		12 Months Ended	1 Months Ended			12 Months Ended	1 Months Ended			12 Months Ended		1 Months Ended		12 Months Ended		1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Research and Development Expense	Dec. 31, 2011 Research and Development Expense	Dec. 31, 2010 Research and Development Expense	Dec. 31, 2012 Minimum	Dec. 31, 2012 Maximum	Jul. 31, 2007 Roche/Arrowhead	Dec. 31, 2012 Novartis	Oct. 31, 2012 Genzyme	Dec. 31, 2012 Genzyme Minimum	Dec. 31, 2012 Genzyme Maximum	Dec. 31, 2012 Cubist Alliance	Dec. 31, 2012 Platform Alliances Joint Technology Transfer Committee	Dec. 31, 2012 Platform Alliances Joint Delivery Collobartion Committee	Dec. 31, 2012 Platform Alliances Roche/Arrowhead	Jul. 31, 2007 Platform Alliances Roche/Arrowhead	Jul. 31, 2012 Platform Alliances Roche/Arrowhead Alnylam Europe Purchase Agreement	Jul. 31, 2007 Platform Alliances Roche/Arrowhead Common Stock Purchase Agreement	Dec. 31, 2012 Platform Alliances Arrowhead	Mar. 31, 2011 Platform Alliances Takeda	Mar. 31, 2010 Platform Alliances Takeda	Oct. 31, 2008 Platform Alliances Takeda	Jun. 30, 2008 Platform Alliances Takeda	Dec. 31, 2012 Platform Alliances Takeda	Aug. 31, 2012 Platform Alliances Monsanto Alliance	Dec. 31, 2012 Platform Alliances Monsanto Alliance	Dec. 31, 2012 Platform Alliances Monsanto Alliance Loss Of Patent Rights	Dec. 31, 2012 Platform Alliances Cubist Alliance	Sep. 30, 2010 Discovery and Development Alliances	Mar. 31, 2004 Discovery and Development Alliances Isis	Apr. 30, 2009 Discovery and Development Alliances Isis Research and Development Expense	Dec. 31, 2012 Discovery and Development Alliances Isis Research and Development Expense	Mar. 31, 2004 Discovery and Development Alliances Isis Up Front Payment Research and Development Expense	Mar. 31, 2004 Discovery and Development Alliances Isis Milestone Payments Research and Development Expense	Sep. 30, 2010 Discovery and Development Alliances Novartis	Dec. 31, 2012 Discovery and Development Alliances Novartis	Dec. 31, 2005 Discovery and Development Alliances Novartis	Dec. 31, 2012 Discovery and Development Alliances Novartis Technology Support Agreement	Dec. 31, 2012 Discovery and Development Alliances Novartis Maximum	Dec. 31, 2012 Product Alliances Kyowa Hakko Kirin	Dec. 31, 2012 Product Alliances Genzyme	Jan. 31, 2009 Product Alliances Cubist Alliance
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of therapeutic areas																4
Payments for additional therapeutic areas																$ 50,000,000
Upfront fee received								273,500,000					20,000,000											100,000,000			29,200,000				500,000						10,000,000				15,000,000	22,500,000	20,000,000
Maximum number of potential future milestones																100,000,000									171,000,000		5,000,000			75,000,000							64,000,000				78,000,000	50,000,000
Potential future payment for the achievement of specified development milestones																17,500,000									26,000,000																	25,000,000
Potential future payment for the achievement of specified regulatory milestones																62,500,000									40,000,000																	25,000,000
Potential future payment for the achievement of specified commercialization milestones																20,000,000									105,000,000
Next potential milestone payment																1,000,000									2,000,000		2,500,000															7,000,000
License and collaborations agreements, expiration period																10 years																						5 years		10 years
License and collaborations agreements, prior written notice period before termination																180 days									180 days
Upfront fees relating to common stock purchase agreement and the Alnylam Europe Purchase Agreement																		15,000,000	42,500,000
Aggregate upfront and near term payments								331,000,000																	150,000,000
Deferred revenue determined at contract execution																	278,200,000
Revenue recognizing period																5 years				5 years
Granted period of royalty bearing license																									5 years
Profit sharing agreement ratio																									50.00%
Number of licensed products																									4						10					31
Additional number of licensed products																									2						1
Milestone fees to company upon achievement of specified technology transfer milestones																								50,000,000
Milestones considered probable at inception of collaborations																					10,000,000	20,000,000	20,000,000
Maximum life of collaboration committee														7 years	7 years
Deferred revenue													9,700,000												52,800,000	29,200,000	28,700,000	5,000,000									200,000				15,500,000	22,500,000
Period of exclusivity in the collaboration																										10 years
Milestone payment earned																											1,500,000
Estimated range of expiration for fundamental patents						2016	2025				2016	2021
Period for services under contract																				5 years
Future payment of liquidated damages																											5,000,000
Technology transfer milestone																											2,500,000
License fee			5,700,000	1,400,000	2,400,000																											11,000,000	2,500,000	5,000,000	3,400,000
Potential future payment upon the occurrence of achievement of specified development and regulatory milestones																															3,400,000
Premium received on sale of common stock	$ 624,876,000	$ 518,731,000																																			$ 6,400,000
Collaboration and license agreement, term																																							5 years
Percentage of outstanding stock owned by Novartis									7.70%
Estimated year of patent expiration for compounds										2032
Percentage of sharing in development cost	50.00%												50.00%
Life of development and manufacturing services																													8 years
Upfront payment recognition period													8 years