Stock-Based Compensation - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	4 years
Weighted average grant date fair value of RSUs granted	$ 2.80
Weighted average period for recognition of compensation costs	2 years
Expiration period of stock option plan	10 years
Weighted average grant date fair value of options granted, per share	$ 1.20	$ 4.67	$ 4.89
Grant date fair value of options vested	$ 9,200,000	$ 14,400,000	$ 6,300,000
Aggregate intrinsic value of options exercised	0	7,800,000	17,700,000
Unrecognized compensation cost	10,100,000
Stock-based compensation expense	$ 22,805,000	$ 19,445,000	$ 14,169,000
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted average grant date fair value of RSUs granted	$ 2.80	$ 3.63	$ 9.65
Total grant date fair value of RSUs	$ 17,000,000.0	$ 12,200,000	$ 17,000,000.0
Total fair value of RSUs vested	10,100,000	$ 6,000,000.0	300,000
Unrecognized stock based compensation cost	$ 17,400,000
Weighted average period for recognition of compensation costs	1 year 10 months 24 days
Shares expected to vest	7,700,000
Restricted Stock Units (RSUs) | Employees | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	3 years
Restricted Stock Units (RSUs) | Employees | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	2 years
2015 and 2018 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares available for grant	7,900,000
2015 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock based compensation cost	$ 0
Percentage of market value price of common stock under Employee Stock Purchase Plan	85.00%
Common stock reserved for issuance	3,500,000	2,700,000
Shares issued under ESPP	200,000
Common stock remained available for issuance pursuant to the purchase plan	3,300,000
Stock-based compensation expense	$ 100,000	$ 0	$ 0
2015 Employee Stock Purchase Plan | Offering Period
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum contribution percentage amount of employee's base compensation	15.00%
Maximum employee contribution in to share purchase plan per offering	$ 30,000
Number of common stock authorized for issuance	3,000
2015 Employee Stock Purchase Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common stock authorized for issuance	3,500,000