Collaboration Agreements (Details) (USD $)	12 Months Ended			1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Feb. 28, 2013	Dec. 31, 2014	Mar. 31, 2013
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Co-promotion and profit share income	$ 24,236,000	$ 17,383,000	$ 10,000,000
AstraZeneca [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum payment for success-based development and commercialization milestones	54,500,000.0
Co-promotion and profit share income	41,000,000
Alnylam Pharmaceuticals, Inc. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Initial payment				25,000,000		25,000,000
Maximum payment for success-based development and commercialization milestones	170,000,000			180,000,000
Alnylam Pharmaceuticals, Inc. [Member] | Collaborative Arrangement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payment					10,000,000
Boston Scientific Corporation [Member] | Collaborative Arrangement, Co-promotion [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Co-promotion and profit share income	$ 5,000,000