Collaborative Arrangements and Licensing Agreements, Biogen (Details) $ in Thousands	1 Months Ended					3 Months Ended				12 Months Ended
	Dec. 31, 2017 USD ($) PerformanceObligation	Jul. 31, 2016 USD ($)	May 31, 2015 USD ($)	Sep. 30, 2013 USD ($) PerformanceObligation	Dec. 31, 2012 USD ($) PerformanceObligation Target Program	Dec. 31, 2018 USD ($) Drug Country Target	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2018 USD ($) Drug Country Target	Dec. 31, 2017 USD ($) PerformanceObligation	Dec. 31, 2016 USD ($) PerformanceObligation	Apr. 30, 2018 USD ($) PerformanceObligation
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue			$ 91,200							$ 599,674	$ 514,179	$ 372,776
Deferred revenue	$ 233,362										233,362
SPINRAZA Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue										237,930	112,540	883
Licensing and Other Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue										14,755	7,474	21,884
R&D [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue										$ 344,752	394,165	350,009
Biogen [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of additional drugs in clinical development to treat neurodegenerative diseases | Drug						6				6
Cumulative payments received						$ 2,000,000				$ 2,000,000
Revenue										375,000	263,100	$ 249,700
Deferred revenue	$ 93,600					580,900				$ 580,900	$ 93,600
Biogen [Member] | Revenue [Member] | Strategic Partner [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Concentration percentage										63.00%	51.00%	67.00%
Biogen [Member] | SPINRAZA Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue										$ 237,900	$ 112,500	$ 900
Biogen [Member] | R&D [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue										137,100	150,600	$ 248,800
SPINRAZA [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative revenue earned						$ 785,000				$ 785,000
Number of separate performance obligations | PerformanceObligation												2
SPINRAZA [Member] | Minimum [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of countries where SPINRAZA is approved for use | Country						40				40
Royalty percentage received on net sales of drug										11.00%
SPINRAZA [Member] | Maximum [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Royalty percentage received on net sales of drug										15.00%
SPINRAZA [Member] | SPINRAZA Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative revenue earned						$ 350,000				$ 350,000
SPINRAZA [Member] | Licensing and Other Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue		$ 75,000
SPINRAZA [Member] | R&D [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative revenue earned						435,000				$ 435,000
Revenue											$ 90,000
New Antisense Drugs for the Treatment of SMA [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Royalty percentage received on net sales of drug										20.00%
Number of separate performance obligations | PerformanceObligation	1										1
Upfront payment received	$ 25,000
Maximum amount of payments receivable for license fees and substantive milestones						1,200,000				$ 1,200,000
Maximum amount of payments receivable for development milestones						80,000				80,000
Maximum amount of payments receivable for commercialization milestones						180,000				180,000
Maximum amount of payments receivable for sales milestones						800,000				800,000
Next prospective payment						60,000				60,000
Transaction price	$ 25,000										$ 25,000
2018 Strategic Neurology [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments received						$ 1,000,000				$ 1,000,000
Royalty percentage received on net sales of drug										20.00%
Number of separate performance obligations | PerformanceObligation													1
Number of targets | Target						2				2
Upfront payment received								$ 375,000
Next prospective payment						$ 7,500				$ 7,500
Transaction price													$ 552,000
Maximum amount of payments receivable per drug for substantive milestone payments						270,000				$ 270,000
Term of collaboration agreement										10 years
Upfront payment received, including purchase of stock								1,000,000
Proceeds from issuance of common stock								$ 625,000
Percentage cash premium paid on shares purchased								25.00%
Premium paid on shares purchased								$ 177,000
Milestone payments received and added to transaction price						15,000
2013 Strategic Neurology [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments received						210,000				$ 210,000
Number of separate performance obligations | PerformanceObligation				1
Upfront payment received				$ 100,000
Next prospective payment						$ 10,000				$ 10,000
Transaction price				$ 100,000
Number of drugs currently being advanced | Drug						5				5
Revenue							$ 10,000
2013 Strategic Neurology [Member] | Licensing and Other Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue						$ 35,000
2013 Strategic Neurology [Member] | R&D [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Payments included in transaction price for performance obligation						145,000				$ 145,000
Revenue						5,000
2013 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
Maximum amount of payments receivable for development milestones						60,000				60,000
Maximum amount of payments receivable for regulatory milestones						130,000				130,000
Neurology [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments received						58,000				58,000
Number of separate performance obligations | PerformanceObligation					1
Number of targets | Target					3
Number of programs under which drugs are to be developed and commercialized | Program					3
Upfront payment received					$ 30,000
Next prospective payment						7,500				7,500
Transaction price					$ 30,000
Maximum amount of payments receivable per program for license fee and substantive milestone payments						210,000				210,000
Maximum amount of payments receivable per program for development milestones						10,000				10,000
Maximum amount of payments receivable per program for regulatory milestones						130,000				130,000
Milestone payments received and added to transaction price									$ 10,000
Neurology [Member] | R&D [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Payments included in transaction price for performance obligation						$ 40,000				$ 40,000