Stock-Based Compensation - Additional Information (Detail)	1 Months Ended		12 Months Ended
	Jul. 31, 2018 Officer	Nov. 30, 2017 Officer	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) Consultant $ / shares
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 | shares			21,576,339	14,209,402
Grant date fair value of options granted, per share | $ / shares			$ 4.89	$ 3.38	$ 2.72
Grant date fair value of options vested			$ 6,300,000	$ 4,800,000	$ 2,400,000
Aggregate intrinsic value of options exercised			17,700,000	2,600,000	2,300,000
Unrecognized compensation cost			$ 29,000,000
Weighted average period for recognition of compensation costs			3 years 2 months 12 days
Weighted average grant date fair value of RSUs granted | $ / shares			$ 9.65
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 | shares			1,857,741	149,636
Weighted average period for recognition of compensation costs			2 years 7 months 6 days
Number of consultant | Consultant					1
Number of executive officer | Officer	2	1
Vesting period	3 years
Weighted average grant date fair value of RSUs granted | $ / shares				$ 8.02	$ 3.52
Total grant date fair value of RSUs			$ 17,000,000	$ 400,000	$ 500,000
Total fair value of RSUs vested			300,000	$ 400,000	$ 500,000
Unrecognized stock based compensation cost			$ 13,300,000
Shares expected to vest | shares			1,616,819
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]
Voting power Percentage			10.00%
Expiration period of stock option plan			10 years
Exercisable period of stock option plan			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]
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 | shares			1,780,020	1,103,481
Shares issued under ESPP | shares			0	0