Collaboration Agreements (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Co-promotion and profit share income	$ 6,020,000	$ 3,750,000
AstraZeneca [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Term for co-promotion	4 years
Co-promotion and profit share income	29,400,000
Termination fee	5,000,000
Alnylam Pharmaceuticals, Inc. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Initial payment	25,000,000
Maximum payment for success-based development and commercialization milestones	180,000,000
Boston Scientific Corporation [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Co-promotion and profit share income	$ 300,000