Stock-Based Compensation - Schedule of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Original Stock Option Plan [Member]
Stock options
Shares, Beginning Balance	3,014,004
Shares, Granted	0	0
Shares, Exercised	(528,261)
Shares, Forfeited	(62,153)
Shares, Cancelled	(125,999)
Shares, Ending Balance	2,297,591		3,014,004
Shares, Vested and expected to vest	2,299,283
Shares, Vested and exercisable	2,198,632
Weighted-average exercise price
Weighted-average exercise price, Beginning Balance	$ 3.04
Weighted-average exercise price, Exercised	1.15
Weighted-average exercise price, Forfeited	4.44
Weighted-average exercise price, Cancelled	3.30
Weighted-average exercise price, Ending Balance	3.38		$ 3.04
Weighted-average exercise price, Vested and expected to vest	3.37
Weighted-average exercise price, Vested and exercisable	$ 3.22
Weighted-average remaining contractual term (years)
Weighted-average remaining contractual term	3 years 9 months 18 days		5 years 9 months 18 days
Weighted-average remaining contractual term, Vested and expected to vest	3 years 9 months 18 days
Weighted-average remaining contractual term, Vested and exercisable	3 years 8 months 12 days
Aggregate intrinsic value
Aggregate intrinsic value	$ 32,717		$ 44,108
Aggregate intrinsic value, Vested and expected to vest	32,742
Aggregate intrinsic value, Vested and exercisable	$ 31,750
2018 Equity Incentive Plan [Member] | Time-based Stock Options [Member]
Stock options
Shares, Beginning Balance	5,307,130
Shares, Granted	0	10,000
Shares, Exercised	(286,948)
Shares, Forfeited	(245,222)
Shares, Cancelled	(193,127)
Shares, Ending Balance	4,581,833		5,307,130
Shares, Vested and expected to vest	4,511,481
Shares, Vested and exercisable	3,276,183
Weighted-average exercise price
Weighted-average exercise price, Beginning Balance	$ 14.04
Weighted-average exercise price, Exercised	7.76
Weighted-average exercise price, Forfeited	13.93
Weighted-average exercise price, Cancelled	13.11
Weighted-average exercise price, Ending Balance	14.48		$ 14.04
Weighted-average exercise price, Vested and expected to vest	14.32
Weighted-average exercise price, Vested and exercisable	$ 12.47
Weighted-average remaining contractual term (years)
Weighted-average remaining contractual term	7 years 7 months 6 days		8 years 4 months 24 days
Weighted-average remaining contractual term, Vested and expected to vest	7 years 6 months
Weighted-average remaining contractual term, Vested and exercisable	7 years 7 months 6 days
Aggregate intrinsic value
Aggregate intrinsic value			$ 44,297
2018 Equity Incentive Plan [Member] | Performance-based Stock Options [Member]
Stock options
Shares, Beginning Balance	520,000
Shares, Granted	0	0
Shares, Exercised	(320,000)
Shares, Ending Balance	200,000		520,000
Shares, Vested and expected to vest	200,000
Shares, Vested and exercisable	200,000
Weighted-average exercise price
Weighted-average exercise price, Beginning Balance	$ 7.76
Weighted-average exercise price, Exercised	7.76
Weighted-average exercise price, Ending Balance	7.76		$ 7.76
Weighted-average exercise price, Vested and expected to vest	7.76
Weighted-average exercise price, Vested and exercisable	$ 7.76
Weighted-average remaining contractual term (years)
Weighted-average remaining contractual term	7 years 7 months 6 days		8 years 4 months 24 days
Weighted-average remaining contractual term, Vested and expected to vest	7 years 7 months 6 days
Weighted-average remaining contractual term, Vested and exercisable	7 years 7 months 6 days
Aggregate intrinsic value
Aggregate intrinsic value			$ 4,872