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Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Detail) - Biogen Inc [Member] - Change in estimated performance period [Member] - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2016
Change In Accounting Estimate [Line Items]
Increase in net loss	$ 0.9	$ 2.7
Increase in basic net loss per share	$ 0.02	$ 0.04

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