Collaborative Arrangements and Licensing Agreements, Traditional Pharmaceutical Alliances and Licensing - Biogen Idec (ISIS-SMN) (Details) (Biogen Idec [Member], USD $) In Millions, unless otherwise specified	3 Months Ended	1 Months Ended
	Mar. 31, 2015	Jan. 31, 2012
ISIS-SMN [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Increase in payments that the Company is eligible to receive under collaboration	$ 57.0
Cumulative payments received	110.0
Next prospective milestone	8.5
ISIS-SMN [Member] | Phase 3 Study in Infants with SMA [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Milestone payment earned	9
ISIS-SMN [Member] | Open-Label Extension Study in Children with SMA [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Milestone payment earned	7
ISIS-SMN [Member] | License Fee and Substantive Milestones [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Maximum amount of payments receivable	327.0
ISIS-SMN [Member] | Development Milestones [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Maximum amount of payments receivable	102.0
ISIS-SMN [Member] | Regulatory Milestones [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Maximum amount of payments receivable	150.0
Collaborations and Licensing Agreements [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of collaborative agreements	4
Agreement Entered into in January 2012 [Member] | ISIS-SMN [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Upfront fee recorded as deferred revenue		$ 29