Stock-Based Compensation - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option grant, description	Options granted pursuant to the 2008 Plan and the 2015 Plan may be either incentive stock options or non-statutory stock options. Options granted pursuant to the 2018 Plan are non-statutory stock options.
Common stock reserved for issuance	22,967,741	21,576,339
Weighted average grant date fair value of options granted, per share	$ 4.67	$ 4.89	$ 3.38
Grant date fair value of options vested	$ 14,400,000	$ 6,300,000	$ 4,800,000
Aggregate intrinsic value of options exercised	7,800,000	17,700,000	2,600,000
Unrecognized compensation cost	$ 25,200,000
Weighted average period for recognition of compensation costs	2 years 8 months 12 days
Weighted average grant date fair value of RSUs granted	$ 3.63
Stock-based compensation expense capitalized	$ 0	$ 0	$ 0
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for issuance	4,496,121	1,857,741
Weighted average period for recognition of compensation costs	2 years 1 month 6 days
Weighted average grant date fair value of RSUs granted		$ 9.65	$ 8.02
Total grant date fair value of RSUs	$ 12,200,000	$ 17,000,000	$ 400,000
Total fair value of RSUs vested	6,000,000	$ 300,000	$ 400,000
Unrecognized stock based compensation cost	$ 16,800,000
Shares expected to vest	4,145,163
Restricted Stock Units (RSUs) | Minimum | Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	2 years
Restricted Stock Units (RSUs) | Maximum | Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	3 years
Two Thousand Eight Stock Option Plan | Nonstatutory Options | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise price of fair value of common stock, percentage	85.00%
Two Thousand Eight, Two Thousand Fifteen and Two Thousand Eighteen Stock Option Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expiration period of stock option plan	10 years
Vesting period	4 years
Two Thousand Eight, Two Thousand Fifteen and Two Thousand Eighteen Stock Option Plan | Ten Percent Stockholder
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Voting power Percentage	10.00%
Expiration period of stock option plan	5 years
Two Thousand Eight, Two Thousand Fifteen and Two Thousand Eighteen Stock Option Plan | Ten Percent Stockholder | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise price of fair value of common stock, percentage	110.00%
2015 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for issuance	2,743,340	1,780,020
Shares issued under ESPP	0	0