Share Based Compensation - Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stockholders Equity Note [Abstract]
Number of Shares Outstanding, Beginning Balance	11,313,909	9,480,497	7,245,485
Number of Shares Granted/Issued	2,985,667	4,029,500	2,984,010
Number of Shares Exercised	(1,311,293)	(142,493)	(20,000)
Number of Shares Cancelled/Forfeited	(839,455)	(2,053,595)	(728,998)
Number of Shares Outstanding, Ending Balance	12,148,828	11,313,909	9,480,497
Number of Shares Exercisable, Ending Balance	8,443,947
Weighted Average Exercise Price Outstanding, Beginning Balance	$ 1.84	$ 2.19	$ 2.25
Weighted Average Exercise Price Granted/Issued	2.67	1.14	2.23
Weighted Average Exercise Price Exercised	1.38	1.23	1.54
Weighted Average Exercise Price Cancelled/Forfeited	2.68	2.13	2.92
Weighted Average Exercise Price Outstanding, Ending Balance	2.04	$ 1.84	$ 2.19
Weighted Average Exercise Price Exercisable, Ending Balance	$ 1.99
Weighted Average Remaining Contractual Term (Years) Outstanding, Ending Balance	7 years 1 month 6 days
Weighted Average Remaining Contractual Term (Years) Exercisable, Ending Balance	6 years 3 months 18 days
Aggregate Intrinsic Value, Exercised	$ 2,226	$ 122
Aggregate Intrinsic Value Outstanding, Ending Balance	4,206
Aggregate Intrinsic Value Exercisable, Ending Balance	$ 3,201