Collaborative Arrangements and Licensing Agreements, Biogen (Details) $ in Thousands	1 Months Ended			3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($) Drug Market	Sep. 30, 2013 USD ($)	Dec. 31, 2012 USD ($) Program Target	Dec. 31, 2017 USD ($) Drug Market	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Drug Market	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Collaborative Arrangement and Licensing Agreement [Abstract]
SPINRAZA royalties												$ 112,540	$ 883	$ 0
Revenue earned				$ 172,299	$ 120,911	$ 104,152	$ 110,304	$ 160,349	$ 110,927	$ 38,470	$ 36,874	$ 507,666	346,620	283,703
Biogen [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of additional drugs in clinical development to treat neurodegenerative diseases | Drug												6
Cumulative payments received	$ 745,000			745,000								$ 745,000
Cumulative payments received	745,000			745,000								745,000
Revenue earned												259,800	207,900	$ 106,200
Deferred revenue	69,300			69,300				$ 67,500				$ 69,300	$ 67,500
Biogen [Member] | Revenue [Member] | Strategic Partner [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Concentration percentage												51.00%	60.00%	37.00%
SPINRAZA [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments received	$ 436,000			$ 436,000								$ 436,000
Number of global markets were SPINRAZA is available | Market	30			30								30
SPINRAZA royalties												$ 112,540
Milestone payments received												90,000
Cumulative payments received	$ 436,000			$ 436,000								436,000
News Antisense Drugs for Treatment of SMA [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments received	800,000			800,000								800,000
Upfront payment recorded as deferred revenue	25,000
Maximum amount of payments receivable for license fees and substantive milestones	1,200,000			1,200,000								1,200,000
Maximum amount of payments receivable for development milestones	80,000			80,000								80,000
Maximum amount of payments receivable for commercialization milestones	180,000			180,000								180,000
Cumulative payments received	800,000			800,000								$ 800,000
Royalty percentage received on net sales of drug												20.00%
Next prospective milestone	45,000			45,000								$ 45,000
Neurology [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments received	56,000			56,000								56,000
Milestone payments received												10,000
Upfront payment recorded as deferred revenue			$ 30,000
Cumulative payments received	56,000			56,000								56,000
Next prospective milestone	7,500			7,500								7,500
Number of targets | Target			3
Number of programs under which drugs are to be developed and commercialized | Program			3
Maximum amount of payments receivable per program for license fee and substantive milestone payments	210,000			210,000								210,000
Maximum amount of payments receivable per program for development milestones	10,000			10,000								10,000
Maximum amount of payments receivable per program for regulatory milestones	130,000			130,000								130,000
Strategic Neurology [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments received	165,400			165,400								165,400
Milestone payments received												15,000
Upfront payment recorded as deferred revenue		$ 100,000
Cumulative payments received	165,400			165,400								165,400
Next prospective milestone	$ 10,000			$ 10,000								$ 10,000
Number of drugs currently being advanced | Drug	5			5								5
Term of collaboration agreement												6 years
Strategic Neurology [Member] | Antisense Molecule [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Maximum amount of payments receivable for license fees and substantive milestones	$ 260,000			$ 260,000								$ 260,000
Maximum amount of payments receivable for development milestones	60,000			60,000								60,000
Maximum amount of payments receivable for regulatory milestones	130,000			130,000								130,000
Strategic Neurology [Member] | Other Modalities [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Maximum amount of payments receivable for license fees and substantive milestones	90,000			90,000								90,000
Maximum amount of payments receivable for development milestones	35,000			35,000								35,000
Maximum amount of payments receivable for regulatory milestones	$ 55,000			$ 55,000								$ 55,000