Derivative and Other Financial Instruments and Fair Value Measurements - Changes in Fair Value of Common Stock Warrant Liability (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Warrants And Rights Note Disclosure
Common stock warrant liability, beginning balance	$ 2.1	$ 6.1	$ 4.4	$ 6.9
Warrants exercised				(0.1)
Change in fair value of common stock warrant liability	(0.9)	(1.9)	(3.2)	(2.6)
Common stock warrant liability, ending balance	$ 1.2	$ 4.2	$ 1.2	$ 4.2