Collaborative Arrangements and Licensing Agreements, Biogen (Details) $ in Thousands	3 Months Ended
Mar. 31, 2017 USD ($) Drug	Mar. 31, 2016 USD ($)
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned	$ 110,304	$ 36,874
Collaboration Agreements with Biogen [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of drugs in clinical development to treat neurological diseases \| Drug	4
Number of drugs in clinical development to treat undisclosed neurodegenerative diseases \| Drug	2
Revenue earned	$ 20,400	$ 21,300
Milestone payment earned	5,000
Deferred revenue	$ 57,700
Collaboration Agreements with Biogen [Member] \| Revenue [Member] \| Strategic Partner [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Concentration percentage	18.00%	58.00%
Collaboration Agreements with Biogen [Member] \| Minimum [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments received	$ 550,000

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

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