Changes in Fair Value of Common Stock Warrant Liability (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Warrants And Rights Note Disclosure [Abstract]
Common stock warrant liability, beginning balance	$ 4.4	$ 6.9
Change in fair value of common stock warrant liability	(2.5)	0.6
Common stock warrant liability, ending balance	$ 1.9	$ 7.5