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

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

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

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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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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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Collaborative Arrangements and Licensing Agreements, Biogen (Details) $ in Thousands	1 Months Ended	3 Months Ended
	Apr. 30, 2019 USD ($)	Mar. 31, 2019 USD ($) Drug	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue		$ 297,214		$ 144,419
SPINRAZA Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue		59,711		41,081
R&D [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue		$ 229,126		102,396
Biogen [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of additional drugs in clinical development to treat neurodegenerative diseases \| Drug		6
Cumulative payments received		$ 2,100,000
Revenue		84,200		$ 51,900
Deferred revenue		$ 556,400			$ 580,900
Biogen [Member] \| Revenue [Member] \| Strategic Partner [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Concentration percentage		28.00%		36.00%
Biogen [Member] \| SPINRAZA Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue		$ 59,700		$ 41,100
Biogen [Member] \| R&D [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue		$ 24,500		$ 10,800
2018 Strategic Neurology [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Upfront payment received			$ 1,000,000
2018 Strategic Neurology [Member] \| Subsequent Event [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Milestone payment received	$ 7,500
Next prospective milestone	$ 10,000