Stockholders' Deficit Common Stock - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Equity [Abstract]
Number of Shares, Beginning Balance | shares	7,561,719
Number of Shares, Granted | shares	2,346,350
Number of Shares, Exercised | shares	(90,125)
Number of Shares, Forfeited | shares	(1,559,913)
Number of Shares, Ending Balance | shares	8,258,031	7,561,719
Number of Shares, Options vested and expected to vest | shares	8,258,031
Number of Shares, Options exercisable | shares	4,641,642
Weighted Average Exercise Price, Beginning Balance | $ / shares	$ 12.26
Weighted Average Exercise Price, Granted | $ / shares	5.90
Weighted Average Exercise Price, Exercised | $ / shares	1.61
Weighted Average Exercise Price, Forfeited | $ / shares	12.69
Weighted Average Exercise Price, Ending Balance | $ / shares	10.49	$ 12.26
Weighted Average Exercise Price, Options vested and expected to vest | $ / shares	10.49
Weighted Average Exercise Price, Options exercisable | $ / shares	$ 12.49
Weighted Average Remaining Contractual Term, Outstanding	7 years 2 months 15 days	7 years 2 months 23 days
Weighted Average Remaining Contractual Term, Options vested and expected to vest	7 years 2 months 15 days
Weighted Average Remaining Contractual Term, Options exercisable	5 years 11 months 8 days
Aggregate Intrinsic Value, Outstanding | $	$ 4,230	$ 4,958
Aggregate Intrinsic Value, Options vested and expected to vest | $	4,230
Aggregate Intrinsic Value, Options exercisable | $	$ 4,028