Related Party Transactions - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Alnylam
Related Party Transaction [Line Items]
Expenses incurred for services performed or out-of-pocket expenses	$ 0.0	$ 0.0	$ 0.6
Potential milestone payments to related party	33.0
Sanofi
Related Party Transaction [Line Items]
Expenses incurred for services performed or out-of-pocket expenses	$ 0.4	$ 0.0	$ 0.0