Changes in Fair Value of Common Stock Warrant Liability (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Warrants And Rights Note Disclosure [Abstract]
Common stock warrant liability, beginning balance	$ 11.1	$ 8.3	$ 5.6	$ 9.3
Warrants exercised	(0.1)		(0.2)
Change in fair value of common stock warrant liability	(3.4)	(2.4)	2.2	(3.4)
Common stock warrant liability, ending balance	$ 7.6	$ 5.9	$ 7.6	$ 5.9