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

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The number of medicines currently being developed under the collaboration agreement(s).

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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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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 (Details) $ in Thousands	1 Months Ended	3 Months Ended
	Apr. 30, 2021 USD ($)	Mar. 31, 2021 USD ($) Medicine	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue		$ 111,607	$ 133,367
SPINRAZA Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue		59,986	66,008
R&D Revenue [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue		$ 27,159	49,406
Biogen [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of medicines currently being developed \| Medicine		8
Cumulative payments received		$ 2,900,000
Revenue		78,100	$ 87,400
Deferred revenue		$ 447,700		$ 465,800
Biogen [Member] \| Revenue [Member] \| Strategic Partner [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Concentration percentage		70.00%	66.00%
Biogen [Member] \| SPINRAZA Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue		$ 60,000	$ 66,000
Biogen [Member] \| R&D Revenue [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue		$ 18,100	$ 21,400
2013 Strategic Neurology [Member] \| Plan [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Next prospective payment	$ 8,000
2013 Strategic Neurology [Member] \| ION541 [Member] \| Subsequent Event [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Milestone payment achieved	$ 10,000