Collaborative Arrangements and Licensing Agreements, Strategic Partnership - Biogen (SPINRAZA) (Details) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Jul. 31, 2016 USD ($)	Jan. 31, 2012 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) Agreement Drug	Sep. 30, 2015 USD ($)
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned			$ 110,927	$ 49,121	$ 186,272	$ 232,133
Biogen [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned			75,000
Collaboration Agreements [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of strategic collaboration agreements | Agreement					4
Number of drugs in clinical development to treat neurological diseases | Drug					6
Number of drugs in clinical development to treat undisclosed neurodegenerative diseases | Drug					3
Number of collaboration agreements with substantive changes | Agreement					2
Revenue earned			90,200	$ 18,100	$ 120,900	$ 75,100
SPINRAZA [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
License fee received	$ 75,000
Upfront payment recorded as deferred revenue		$ 29,000
Maximum amount of payments receivable for license fee and substantive milestones			346,000		346,000
Maximum amount of payments receivable for development milestones			121,000		121,000
Maximum amount of payments receivable for regulatory milestones			150,000		150,000
Milestone payments earned					11,500
SPINRAZA [Member] | Minimum [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments received			235,000		235,000
SPINRAZA [Member] | Maximum [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Next prospective milestone			$ 60,000		$ 60,000