Collaborative Arrangements and Licensing Agreements, Biogen (Details) $ / shares in Units, $ in Thousands	1 Months Ended					3 Months Ended		12 Months Ended
	Apr. 30, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) PerformanceObligation Target	Jul. 31, 2016 USD ($)	Sep. 30, 2013 USD ($) PerformanceObligation	Dec. 31, 2012 USD ($) PerformanceObligation Target Program	Mar. 31, 2018 USD ($) Drug	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) PerformanceObligation Target	Dec. 31, 2016 PerformanceObligation	Jul. 31, 2015 PerformanceObligation
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of separate performance obligations | PerformanceObligation										1
Revenue earned						$ 144,419	$ 115,800
Deferred revenue		$ 233,362						$ 233,362
SPINRAZA Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned						41,081	5,211
Licensing and Other Royalty Revenue [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned						942	2,590
R&D Revenue Under Collaborative Agreements [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned						$ 102,396	107,999
Biogen [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of additional drugs in clinical development to treat neurodegenerative diseases | Drug						6
Cumulative payments received						$ 800,000
Revenue earned						51,900	$ 28,700
Deferred revenue		$ 93,600				$ 84,600		93,600
Biogen [Member] | Revenue [Member] | Strategic Partner [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Concentration percentage						36.00%	25.00%
Biogen [Member] | SPINRAZA Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned						$ 41,100	$ 5,200
Biogen [Member] | R&D Revenue Under Collaborative Agreements [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned						10,800	$ 23,500
SPINRAZA [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments received						436,000
Number of separate performance obligations | PerformanceObligation									2
SPINRAZA [Member] | Licensing and Other Royalty Revenue [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative revenue earned						155,000
Revenue earned			$ 75,000
SPINRAZA [Member] | R&D Revenue Under Collaborative Agreements [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned								90,000
Neurology [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments received						58,000
Upfront payment received					$ 30,000
Next prospective payment						7,500
Number of separate performance obligations | PerformanceObligation					1
Transaction price					$ 30,000
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
Maximum amount of payments receivable per program for development milestones						10,000
Maximum amount of payments receivable per program for regulatory milestones						130,000
Neurology [Member] | R&D Revenue Under Collaborative Agreements [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Payments included in transaction price for performance obligation						40,000
Strategic Neurology [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments received						165,000
Milestone payments received								$ 15,000
Upfront payment received				$ 100,000
Next prospective payment						$ 10,000
Number of separate performance obligations | PerformanceObligation				1
Transaction price				$ 100,000
Number of targets | Target		2						2
Number of drugs currently being advanced | Drug						5
Strategic Neurology [Member] | R&D Revenue Under Collaborative Agreements [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Payments included in transaction price for performance obligation						$ 145,000
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
Maximum amount of payments receivable for development milestones						60,000
Maximum amount of payments receivable for regulatory milestones						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
Maximum amount of payments receivable for development milestones						35,000
Maximum amount of payments receivable for regulatory milestones						55,000
New Antisense Drugs for Treatment of SMA [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Upfront payment received		$ 25,000
Maximum amount of payments receivable for license fees and substantive milestones						1,200,000
Maximum amount of payments receivable for development milestones						80,000
Maximum amount of payments receivable for commercialization milestones						180,000
Maximum amount of payments receivable for sales milestones						$ 800,000
Royalty percentage received on net sales of drug						20.00%
Next prospective payment						$ 60,000
Number of separate performance obligations | PerformanceObligation		1						1
Transaction price		$ 25,000						$ 25,000
Expanded Strategic Neurology [Member] | Subsequent Event [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Upfront payment, including purchase of stock	$ 1,000,000
Purchase of stock	625,000
Upfront payment	$ 375,000
Percentage cash premium to be paid on shares purchased	25.00%
Royalty percentage received on net sales of drug	20.00%
Maximum amount of payments receivable per program for license fee and substantive milestone payments	$ 270,000
Stock to be purchased (in shares) | shares	11,501,153
Share price (in dollars per share) | $ / shares	$ 54.34