Collaborative Arrangements and Licensing Agreements, Strategic Partnerships - Biogen (Strategic Neurology) (Details) $ in Thousands	1 Months Ended	3 Months Ended								12 Months Ended
	Sep. 30, 2013 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	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
Collaboration Agreements with Biogen [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments received		500,000								500,000
Revenue earned										207,900	106,200	$ 123,200
Deferred revenue		67,500				$ 91,600				$ 67,500	$ 91,600
Collaboration Agreements with Biogen [Member] | Revenue [Member] | Strategic Partner [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Concentration percentage										60.00%	37.00%	58.00%
Strategic Neurology [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of drugs currently being advanced | Drug										3
Upfront payment recorded as deferred revenue	$ 100,000
Term of collaboration agreement										6 years
Cumulative payments received		142,000								$ 142,000
Next prospective milestone		10,000								10,000
Strategic Neurology [Member] | Antisense Molecule [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Maximum amount of payments receivable for license fee and substantive milestones		260,000								260,000
Maximum amount of payments receivable for development milestones		60,000								60,000
Maximum amount of payments receivable for regulatory milestones		130,000								130,000
Strategic Neurology [Member] | Other Modalities [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Maximum amount of payments receivable for license fee and substantive milestones		90,000								90,000
Maximum amount of payments receivable for development milestones		35,000								35,000
Maximum amount of payments receivable for regulatory milestones		$ 55,000								$ 55,000