Stock-Based Compensation - Summary of Options Granted (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shares Available for Grant
Begining balance	1,855,240	1,283,375	871,226
Options authorized	3,570,000	1,568,397	1,133,987
Options granted	(1,645,489)	(1,063,019)	(746,825)
Options forfeited or canceled	10,769	5,797
Options repurchased	10,671	60,690	24,987
Ending balance	3,801,191	1,855,240	1,283,375	871,226
Shares Subject to Outstanding Options
Begining balance	544,116	129,918	20,200
Options granted	1,645,489	1,063,019	746,825
Options exercised	(720,756)	(643,024)	(637,107)
Options forfeited or canceled	(10,769)	(5,797)
Ending balance	1,458,080	544,116	129,918	20,200
Options outstanding and exercisable as of December 31, 2018	238,721
Options vested and expected to vest as of December 31, 2018	1,458,080
Weighted Average Exercise Price Per Share
Begining balance	$ 1.71	$ 1.20	$ 0.40
Options granted	8.19	1.61	0.59
Options exercised	4.64	1.45	0.48
Options forfeited or canceled	5.25	1.28
Options repurchased		0.49	0.40
Ending balance	7.55	$ 1.71	$ 1.20	$ 0.40
Options outstanding and exercisable as of December 31, 2018	6.49
Options vested and expected to vest as of December 31, 2018	$ 7.55
Weighted Average Remaining Contractual Term (in years)	8 years 11 months 26 days	9 years 3 months 10 days	9 years 10 months 2 days	8 years 9 months 14 days
Options outstanding and exercisable, Weighted Average Remaining Contractual Term (in years)	8 years 10 months 13 days
Options vested and expected to vest, Weighted Average Remaining Contractual Term (in years)	8 years 11 months 26 days