Derivative and Other Financial Instruments and Fair Value Measurements - Changes in Fair Value of Common Stock Warrant Liability (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Warrants And Rights Note Disclosure
Common stock warrant liability, beginning balance	$ 4.4	$ 6.9	$ 5.6
Warrants exercised		(0.1)	(0.2)
Change in fair value of common stock warrant liability	$ (4.4)	(2.4)	1.5
Common stock warrant liability, ending balance		$ 4.4	$ 6.9