Changes in Fair Value of Common Stock Warrant Liability (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Warrants And Rights Note Disclosure [Abstract]
Common stock warrant liability, beginning balance	$ 4.9	$ 8.5	$ 5.6	$ 9.3
Warrants exercised	(0.1)		(0.1)
Change in fair value of common stock warrant liability	6.3	(0.2)	5.6	(1.0)
Common stock warrant liability, ending balance	$ 11.1	$ 8.3	$ 11.1	$ 8.3