Changes in Fair Value of Common Stock Warrant Liability (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Warrants And Rights Note Disclosure [Abstract]
Common stock warrant liability, beginning balance	$ 6.9	$ 5.6	$ 9.3
Warrants exercised	(0.1)	(0.2)	0.0
Change in fair value of common stock warrant liability	(2.4)	1.5	(3.7)
Common stock warrant liability, ending balance	$ 4.4	$ 6.9	$ 5.6