Stockholders' Equity Common Stock - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Equity [Abstract]
Number of Shares, Beginning Balance	7,561,719
Number of Shares, Granted	2,148,250
Number of Shares, Exercised	(38,125)
Number of Shares, Forfeited	(763,412)
Number of Shares, Ending Balance	8,908,432	7,561,719
Number of Shares, Options vested and expected to vest	8,908,432
Number of Shares, Options exercisable	4,659,499
Weighted Average Exercise Price, Beginning Balance	$ 12.26
Weighted Average Exercise Price, Granted	6.16
Weighted Average Exercise Price, Exercised	3.14
Weighted Average Exercise Price, Forfeited	12.70
Weighted Average Exercise Price, Ending Balance	10.79	$ 12.26
Weighted Average Exercise Price, Options vested and expected to vest	10.79
Weighted Average Exercise Price, Options exercisable	$ 12.56
Weighted Average Remaining Contractual Term, Outstanding	7 years 6 months 25 days	7 years 2 months 23 days
Weighted Average Remaining Contractual Term, Options vested and expected to vest	7 years 6 months 25 days
Weighted Average Remaining Contractual Term, Options exercisable	6 years 1 month 20 days
Aggregate Intrinsic Value, Outstanding	$ 9,684	$ 4,958
Aggregate Intrinsic Value, Options vested and expected to vest	9,684
Aggregate Intrinsic Value, Options exercisable	$ 8,100