Changes in Fair Value of Common Stock Warrant Liability (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Warrants And Rights Note Disclosure [Abstract]
Common stock warrant liability, beginning balance	$ 1.9	$ 7.5	$ 4.4	$ 6.9
Warrants exercised	0.0	(0.1)	0.0	(0.1)
Change in fair value of common stock warrant liability	0.2	(1.3)	(2.3)	(0.7)
Common stock warrant liability, ending balance	$ 2.1	$ 6.1	$ 2.1	$ 6.1