Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Detail) - Biogen Inc [Member] - Change in estimated performance period [Member] $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($) $ / shares
Change In Accounting Estimate [Line Items]
Increase in net loss | $	$ 0.9
Increase in basic net loss per share | $ / shares	$ 0.01