Stockholders' Equity Common Stock - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Equity [Abstract]
Number of shares, Beginning Balance	5,069,133
Number of Shares, Granted	1,749,000
Number of Shares, Exercised	(174,386)
Number of Shares, Forfeited	(518,055)
Number of shares, Ending Balance	6,125,692	5,069,133
Number of Shares, Options vested and expected to vest	6,125,692
Number of Shares, Options exercisable	3,235,166
Weighted Average Exercise Price, Beginning Balance	$ 14.36
Weighted Average Exercise Price, Granted	10.21
Weighted Average Exercise Price, Exercised	0.67
Weighted Average Exercise Price, Forfeited	21.07
Weighted Average Exercise Price, Ending Balance	13.00	$ 14.36
Weighted Average Exercise Price, Options vested and expected to vest	13.00
Weighted Average Exercise Price, Options exercisable	$ 11.37
Weighted Average Remaining Contractual Term, Outstanding	7 years 8 months 12 days	8 years 3 months 3 days
Weighted Average Remaining Contractual Term, Options vested and expected to vest	7 years 8 months 12 days
Weighted Average Remaining Contractual Term, Options exercisable	7 years
Aggregate Intrinsic Value, Outstanding	$ 15,808	$ 16,736
Aggregate Intrinsic Value, Options vested and expected to vest	15,808
Aggregate Intrinsic Value, Options exercisable	$ 14,074