Changes in Fair Value of Common Stock Warrant Liability (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Warrants And Rights Note Disclosure [Abstract]
Common stock warrant liability, beginning balance	$ 5.6	$ 9.3	$ 2.4
Warrants exercised	(0.2)
Change in fair value of common stock warrant liability	1.5	(3.7)	6.9
Common stock warrant liability, ending balance	$ 6.9	$ 5.6	$ 9.3