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Cumulative amount of payments received under the collaboration agreement as of the end of the reporting period.

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Cumulative amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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The maximum amount of payments receivable for commercialization milestones under the collaboration agreement.

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No definition available.

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The maximum amount of payments receivable for development milestones under the collaboration agreement.

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- Definition

The maximum amount of payments receivable for license fees and substantive milestones under the collaboration agreement.

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No definition available.

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The maximum amount of payments receivable for regulatory milestones under the collaboration agreement.

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No definition available.

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The maximum amount of payments receivable for sales milestones under the collaboration agreement.

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No definition available.

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The maximum amount of payments receivable per drug or program for development milestone payments under the collaboration agreement.

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No definition available.

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The maximum amount of payments receivable per drug or program for license fees and/or substantive milestone payments under the collaboration agreement.

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No definition available.

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The maximum amount of payments receivable per drug or program for regulatory milestone payments under the collaboration agreement.

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No definition available.

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The next potential payment to be earned under the collaboration agreement.

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No definition available.

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The number of additional drugs in clinical development to treat neurodegenerative diseases under the collaboration agreement.

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No definition available.

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The number of drugs currently being advanced under the collaboration agreement.

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The number of programs under which drugs are to be developed and commercialized under the collaboration agreement.

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No definition available.

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The number of targets for the research and development of drugs under the collaboration agreement.

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No definition available.

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Total payments received since inception of the collaboration agreement included in the transaction price for the performance obligation.

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No definition available.

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The percentage cash premium paid per share of the Company's common stock under the collaboration agreement.

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No definition available.

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The cash premium paid on the purchase of the Company's common stock under the collaboration agreement.

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The number of separate performance obligations under the collaboration agreement.

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No definition available.

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The amount of consideration the Company is eligible to earn under the collaboration agreement, including any variable consideration.

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No definition available.

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Royalty percentage received on the net sales of a drug by a collaboration partner under the collaboration agreement.

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No definition available.

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Term of the collaboration agreement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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No definition available.

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The amount of upfront payments received during the period under the collaboration agreement.

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No definition available.

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The total amount of the upfront payment received, including the purchase of shares of the Company's stock, under the collaboration agreement.

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No definition available.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable.

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The cash inflow from the additional capital contribution to the entity.

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Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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Collaborative Arrangements and Licensing Agreements, Biogen (Details) $ in Thousands	1 Months Ended					3 Months Ended			9 Months Ended		12 Months Ended
	Apr. 30, 2018 USD ($) PerformanceObligation	Dec. 31, 2017 USD ($) PerformanceObligation	Jul. 31, 2016 USD ($)	Sep. 30, 2013 USD ($) PerformanceObligation	Dec. 31, 2012 USD ($) PerformanceObligation Target Program	Sep. 30, 2018 USD ($) Drug	Jun. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Drug	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) PerformanceObligation	Dec. 31, 2016 PerformanceObligation
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue						$ 145,395		$ 118,314	$ 407,559	$ 346,387
Deferred revenue		$ 233,362									$ 233,362
SPINRAZA Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue						70,010		32,890	167,743	60,467
R&D [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue						$ 62,639		83,697	$ 225,584	280,281
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						$ 1,900,000			$ 1,900,000
Revenue						104,800		$ 87,800	234,600	$ 197,600
Deferred revenue		$ 93,600				$ 593,600			$ 593,600		93,600
Biogen [Member] \| Revenue [Member] \| Strategic Partner [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Concentration percentage						72.00%		74.00%	58.00%	57.00%
Biogen [Member] \| SPINRAZA Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue						$ 70,000		$ 32,900	$ 167,700	$ 60,500
Biogen [Member] \| R&D [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue						34,800		$ 54,900	66,900	$ 137,100
SPINRAZA [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative revenue earned						717,000			$ 717,000
Number of separate performance obligations \| PerformanceObligation												2
Revenue						10,000
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						281,000			$ 281,000
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
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] \| 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