Collaboration Agreements (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 AstraZeneca [Member]	Dec. 31, 2013 Alnylam Pharmaceuticals, Inc. [Member]	Mar. 31, 2013 Alnylam Pharmaceuticals, Inc. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Term for co-promotion				4 years
Co-promotion and profit share income	$ 17,383,000	$ 10,000,000	$ 0	$ 25,000,000
Termination fee				5,000,000
Initial payment					25,000,000	25,000,000
Maximum payment for success-based development and commercialization milestones				$ 54,500,000.0	$ 180,000,000.0