Stock-Based Compensation - Summary of Stock Option and Related Information (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares Subject to Outstanding Options, Beginning Balance	2,964,549	2,141,868
Shares Subject to Outstanding Options, granted	1,370,381	1,735,875
Shares Subject to Outstanding Options, exercised	(489,153)	(333,822)
Shares Subject to Outstanding Options, canceled	(144,316)	(579,372)
Shares Subject to Outstanding Options, Ending Balance	3,701,461	2,964,549	2,141,868
Shares Subject to Outstanding Options, Options outstanding and exercisable	1,519,637
Weighted Average Exercise Price per Share, Beginning Balance	$ 11.54	$ 6.42
Weighted Average Exercise Price Per Share, granted	52.53	15.41
Weighted Average Exercise Price per Share, exercised	10.61	4.81
Weighted Average Exercise Price Per Share, cancelled	22.67	8.11
Weighted Average Exercise Price per Share, Ending Balance	26.40	$ 11.54	$ 6.42
Weighted Average Exercise Price Per Share, Options outstanding and exercisable	$ 14.35
Weighted Average Remaining Contractual Term	8 years	8 years 3 months 18 days	8 years 4 months 24 days
Weighted Average Remaining Contractual Term, Options outstanding and exercisable	7 years 3 months 18 days
Aggregate Intrinsic Value, Beginning Balance	$ 88,524	$ 90,780	$ 14,963
Options outstanding and exercisable as of December 31, 2018	$ 52,979