Collaborative Arrangements and Licensing Agreements, Biogen (Details) $ in Thousands	1 Months Ended					3 Months Ended		6 Months Ended		12 Months Ended
	Apr. 30, 2018 USD ($) PerformanceObligation	Dec. 31, 2017 USD ($) PerformanceObligation Target	Jul. 31, 2016 USD ($)	Sep. 30, 2013 USD ($) PerformanceObligation	Dec. 31, 2012 USD ($) PerformanceObligation Target Program	Jun. 30, 2018 USD ($) Drug	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Drug	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) PerformanceObligation Target	Dec. 31, 2016 PerformanceObligation
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue						$ 117,747	$ 112,273	$ 262,165	$ 228,073
Deferred revenue		$ 233,362								$ 233,362
SPINRAZA Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue						56,653	22,366	97,734	27,577
R&D [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue						$ 60,549	88,585	$ 162,944	196,584
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						$ 1,800,000		$ 1,800,000
Revenue						78,000	$ 81,200	129,900	$ 109,800
Deferred revenue		$ 93,600				$ 618,300		$ 618,300		93,600
Biogen [Member] | Revenue [Member] | Strategic Partner [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Concentration percentage						66.00%	72.00%	50.00%	48.00%
Biogen [Member] | SPINRAZA Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue						$ 56,700	$ 22,400	$ 97,700	$ 27,600
Biogen [Member] | R&D [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue						21,300	$ 58,800	32,200	$ 82,200
SPINRAZA [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative revenue earned						647,000		$ 647,000
Number of separate performance obligations | PerformanceObligation											2
SPINRAZA [Member] | Minimum [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
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						211,000		$ 211,000
Revenue			$ 75,000
SPINRAZA [Member] | R&D [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative revenue earned						436,000		436,000
Revenue										$ 90,000
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
Neurology [Member] | R&D [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Payments included in transaction price for performance obligation						40,000		40,000
2013 Strategic Neurology [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments received						170,000		170,000
Number of separate performance obligations | PerformanceObligation				1
Number of targets | Target		2								2
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
Milestone payments received										$ 15,000
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
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
2013 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
Maximum amount of payments receivable for development milestones						35,000		35,000
Maximum amount of payments receivable for regulatory milestones						55,000		$ 55,000
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]
Royalty percentage received on net sales of drug						20.00%
Number of separate performance obligations | PerformanceObligation	1
Upfront payment received						$ 375,000
Next prospective payment						7,500		7,500
Transaction price	$ 552,000
Maximum amount of payments receivable per program for license fee and 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