Collaborative Arrangements and Licensing Agreements, Strategic Partnerships - Biogen (SPINRAZA) (Details) $ in Thousands	1 Months Ended			3 Months Ended								12 Months Ended
	Feb. 28, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 31, 2012 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) Drug Agreement	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned				$ 160,349	$ 110,927	$ 38,470	$ 36,874	$ 51,571	$ 49,121	$ 120,428	$ 62,583	$ 346,620	$ 283,703	$ 214,161
Biogen [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned					$ 75,000
Collaboration Agreements with Biogen [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of strategic collaboration agreements | Agreement												4
Number of additional drugs in clinical development to treat neurological diseases | Drug												4
Number of drugs in clinical development to treat undisclosed neurodegenerative diseases | Drug												3
Cumulative payments received		$ 500,000		500,000								$ 500,000
Revenue earned												207,900	$ 106,200	$ 123,200
SPINRAZA [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments received		259,000		259,000								259,000
Milestone payment earned		60,000
Upfront payment recorded as deferred revenue			$ 29,000
Maximum amount of payments receivable for license fee and substantive milestones		346,000		346,000								346,000
Maximum amount of payments receivable for development milestones		121,000		121,000								121,000
Maximum amount of payments receivable for regulatory milestones		150,000		150,000								150,000
Next prospective milestone		$ 50,000		$ 50,000								$ 50,000
SPINRAZA [Member] | Subsequent Event [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Milestone payment received	$ 60,000