Changes in Fair Value of Common Stock Warrant Liability (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Warrants And Rights Note Disclosure [Abstract]
Common stock warrant liability, beginning balance	$ 6.1	$ 11.1	$ 6.9	$ 5.6
Warrants exercised	0.0	(0.1)	(0.1)	(0.2)
Change in fair value of common stock warrant liability	(1.9)	(3.4)	(2.6)	2.2
Common stock warrant liability, ending balance	$ 4.2	$ 7.6	$ 4.2	$ 7.6