Changes in Fair Value of Common Stock Warrant Liability (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Warrants And Rights Note Disclosure [Abstract]
Common stock warrant liability, beginning balance	$ 7.5	$ 4.9	$ 6.9	$ 5.6
Warrants exercised	(0.1)	(0.1)	(0.1)	(0.1)
Change in fair value of common stock warrant liability	(1.3)	6.3	(0.7)	5.6
Common stock warrant liability, ending balance	$ 6.1	$ 11.1	$ 6.1	$ 11.1