Equity Incentive Plans - Summary of Stock Option Activity Under 2006 and 2016 Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Options Outstanding
Beginning balance	2,094,137	2,601,181
Options granted	20,010	366,928
Options exercised	(426,655)	(798,424)
Options forfeited	(61,865)	(75,548)
Ending balance	1,625,627	2,094,137	2,601,181
Options exercisable	1,191,485
Options vested and expected to vest	1,605,077
Weighted- Average Exercise Price Per Share
Beginning Balance (in USD per share)	$ 23.20	$ 12.24
Options granted (in USD per share)	82.77	68.32
Options exercised (in USD per share)	9.98	7.19
Options forfeited (in USD per share)	53.96	34.30
Ending Balance (in USD per share)	26.23	$ 23.20	$ 12.24
Options exercisable (in USD per share)	17.54
Options vested and expected to vest (in USD per share)	$ 25.82
Weighted- Average Remaining Contractual Life (years)
Beginning Balance	6 years 4 months 13 days	7 years 7 days	7 years 2 months 1 day
Ending Balance	6 years 4 months 13 days	7 years 7 days	7 years 2 months 1 day
Options exercisable	5 years 9 months 25 days
Options vested and expected to vest	6 years 4 months 6 days
Aggregate Intrinsic Value (in thousands)
Beginning Balance (in USD)	$ 97,976	$ 113,958
Ending Balance (in USD)	78,664	$ 97,976	$ 113,958
Options exercisable (in USD)	67,568
Options vested and expected to vest (in USD)	$ 78,284