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Stock-based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	9 Months Ended
	Jan. 31, 2019	Sep. 30, 2019	Sep. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense		$ 67,700
Unrecognized stock-based compensation, weighted-average period		2 years 9 months 18 days
Proceeds from exercise of stock options		$ 5,513	$ 9,051
2015 Equity Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional shares to be issued	1,444,808
Common stock shares authorized for issuance		10,933,221
Shares available for future grants		2,189,562
Weighted-average fair values of options granted		$ 31.84
Exercise of stock options, Shares		684,218
Total intrinsic value of options exercised		$ 27,800
2015 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares authorized for issuance		254,000
Shares available for future grants		133,586
Common stock shares issued to participants		35,994