Loss Per Share - Summary of Potentially Dilutive Common Stock Equivalents (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Average unamortized share-based compensation expense:
Exercise prices on common stock options, lower range	$ 3.00	$ 3.00	$ 3.00
Exercise prices on common stock options, upper range	10.00	10.00	10.00
Weighted average exercise price of the Warrants	$ 5.64	$ 5.64	$ 5.73
Restricted Common Stock Awards
Average unamortized share-based compensation expense:
Average unamortized share-based compensation expense	$ 3.1	$ 2.5	$ 1.3
Restricted Stock Unit Awards
Average unamortized share-based compensation expense:
Average unamortized share-based compensation expense	$ 0.8	$ 1.4	$ 1.0