Significant Agreements - Additional Information (Detail) (USD $)	3 Months Ended		1 Months Ended		3 Months Ended	1 Months Ended							3 Months Ended	1 Months Ended		1 Months Ended	3 Months Ended		3 Months Ended		1 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 05, 2014 Sirna Therapeutics	Jan. 10, 2014 Sirna Therapeutics	Mar. 31, 2014 Sirna Therapeutics	Jan. 10, 2014 Sirna Therapeutics Intellectual Property	Jan. 10, 2014 Sirna Therapeutics Additional	Jan. 10, 2014 Clinical Development and Regulatory Milestone Sirna Therapeutics	Jan. 10, 2014 Commercial Milestones Sirna Therapeutics	Mar. 31, 2014 Genzyme	Feb. 28, 2014 Genzyme	Jan. 11, 2014 Genzyme	Mar. 31, 2014 Genzyme	Jan. 11, 2014 Genzyme Minimum	Jan. 11, 2014 Genzyme Maximum	Oct. 31, 2012 Genzyme Product Alliances	Mar. 31, 2014 Genzyme Product Alliances	Dec. 31, 2013 Genzyme Product Alliances	Dec. 31, 2013 Genzyme Product Alliances Phase Two Clinical Trial ALN-TTR	Dec. 31, 2013 Genzyme Product Alliances Phase Three Clinical Trial	Feb. 28, 2014 Genzyme Regional Collaboration Product	Jan. 11, 2014 Genzyme Co-developed/ Co-promoted Collaboration Product	Jan. 11, 2014 Genzyme Co-developed/ Co-promoted Collaboration Product Phase Three Clinical Trial	Jan. 11, 2014 Genzyme Global Collaboration Product
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront fee received																$ 22,500,000
Next potential milestone payment																			7,000,000	4,000,000	50,000,000		25,000,000
Deferred revenue																		33,500,000
Percentage of sharing in development cost																					20.00%	50.00%		100.00%
Maximum number of potential future milestones																					75,000,000	75,000,000		200,000,000
Potential future payment for the achievement of specified development milestones																					55,000,000			60,000,000
Potential future payment for the achievement of specified commercialization milestones																					20,000,000			140,000,000
Royalty rate																					20.00%	20.00%		20.00%
Number of shares under agreement to sell stock										344,448	8,766,338
Aggregate cash consideration										23,000,000	700,000,000
Common stock price			$ 81.36								$ 85.72
Fair value of the shares issued			174,300,000								751,500,000
Excess of fair value over cash received for stock issuance											51,500,000
Performance period											5 years						5 years
Income tax benefit	(17,870,000)	(562,000)											15,200,000
Agreement to acquire outstanding shares of common stock percentage														5.00%	30.00%
Voting percentage												20.00%
Minimum percentage of ownership interest terminate												7.50%
Percentage of ownership interest														30.00%
Approximate percentage of ownership interest										12.00%
Asset acquisition cash consideration	25,000,000		25,000,000
Asset acquisition issuable common stock, shares			2,142,037
Number of shares under agreement to issue in the future					378,007
Liability for stock issuance	25,379,000				25,400,000
In-process research and development	224,656,000		199,300,000		224,700,000
Asset acquisition consideration upon achievement of milestones				$ 105,000,000		$ 10,000,000		$ 40,000,000	$ 65,000,000
Lock up period				6 months			6 months
Subsequent lock up period stock holding percentage				50.00%