Share-Based Compensation - Stock Option Activity (Detail) - Stock Options - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Options
Options, Beginning Balance (in shares)	6,668,000	2,806,000
Options, Granted (in shares)	3,100,000	4,450,000
Options, Exercised (in shares)	(12,000)	0
Options, Forfeited (in shares)	(408,000)	(378,000)
Options, Cancelled and expired (in shares)	(2,129,000)	(210,000)
Options, Ending Balance (in shares)	7,219,000	6,668,000
Options, Vested or expected to vest (in shares)	6,321,000
Options, Exercisable (in shares)	2,597,000	2,500,000	1,913,000
Weighted Average Exercise Price
Weighted Average Exercise Price, Beginning balance (in USD per share)	$ 6.15	$ 11.44
Weighted Average Exercise Price, Granted (in USD per share)	3.04	3.68
Weighted Average Exercise Price, Exercised (in USD per share)	3.89	0.00
Weighted Average Exercise Price, Forfeited (in USD per share)	3.48	6.27
Weighted Average Exercise Price, Cancelled and expired (in USD per share)	9.18	24.33
Weighted Average Exercise Price, Ending balance (in USD per share)	4.08	6.15
Weighted Average Exercise Price, Vested or expected to vest (in USD per share)	4.32
Weighted Average Exercise Price, Exercisable (in USD per share)	$ 5.49	$ 9.83
Weighted Average Remaining Contractual Term, Outstanding at end of period	8 years 1 month 2 days
Weighted Average Remaining Contractual Term, Vested and expected to vest at end of period	7 years 8 months 1 day
Weighted Average Remaining Contractual Term, Exercisable at end of period	6 years 5 months 8 days
Aggregate Intrinsic Value, Outstanding at end of period	$ 0
Aggregate Intrinsic Value, Vested and expected to vest at end of period	0
Aggregate Intrinsic Value, Exercisable at end of period	$ 0