Collaborative Arrangements and Licensing Agreements, Biogen Idec (ISIS-SMN) (Details) (Biogen Idec [Member], USD $)	9 Months Ended		3 Months Ended						9 Months Ended	1 Months Ended
	Sep. 30, 2014 ISIS-SMN [Member]	Jan. 31, 2014 ISIS-SMN [Member]	Sep. 30, 2014 ISIS-SMN [Member] Phase 3 [Member]	Jan. 31, 2014 ISIS-SMN [Member] Phase 2 [Member]	Jan. 31, 2014 ISIS-SMN [Member] Open-label extension [Member]	Sep. 30, 2014 ISIS-SMN [Member] License fee and substantive milestones [Member]	Sep. 30, 2014 ISIS-SMN [Member] Development milestones [Member]	Sep. 30, 2014 ISIS-SMN [Member] Regulatory milestones [Member]	Sep. 30, 2014 Collaborations and Licensing Agreements [Member] Agreement	Jan. 31, 2012 Agreement entered into in January 2012 [Member] ISIS-SMN [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of collaborative agreements									4
Upfront fee recorded as deferred revenue										$ 29,000,000
Increase in payments that the Company is eligible to receive under collaboration	57,000,000
Milestone payment earned			18,000,000
Cumulative milestone payments received under collaborative arrangement at period end	37,300,000
Payment received related to the amendment of clinical development plan	11,300,000
Deferred revenue		11,300,000		3,800,000	7,500,000				127,400,000
Maximum amount of payment receivable						327,200,000	102,200,000	150,000,000
Next prospective milestone	$ 27,000,000