Stockholders' Equity Common Stock - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Equity [Abstract]
Number of Shares, Beginning Balance	6,125,692
Number of Shares, Granted	1,820,776
Number of Shares, Exercised	(196,508)
Number of Shares, Forfeited	(305,920)
Number of Shares, Ending Balance	7,444,040	6,125,692
Number of Shares, Options vested and expected to vest	7,444,040
Number of Shares, Options exercisable	4,176,275
Weighted Average Exercise Price, Beginning Balance	$ 13.00
Weighted Average Exercise Price, Granted	10.08
Weighted Average Exercise Price, Exercised	0.69
Weighted Average Exercise Price, Forfeited	16.96
Weighted Average Exercise Price, Ending Balance	12.45	$ 13.00
Weighted Average Exercise Price, Options vested and expected to vest	12.45
Weighted Average Exercise Price, Options exercisable	$ 12.23
Weighted Average Remaining Contractual Term, Outstanding	7 years 6 months 18 days	7 years 8 months 12 days
Weighted Average Remaining Contractual Term, Options vested and expected to vest	7 years 6 months 18 days
Weighted Average Remaining Contractual Term, Options exercisable	6 years 7 months 9 days
Aggregate Intrinsic Value, Outstanding	$ 9,360,614	$ 15,808
Aggregate Intrinsic Value, Options vested and expected to vest	9,360,614
Aggregate Intrinsic Value, Options exercisable	$ 9,357,014