Stock-Based Compensation - Stock Options (Details) - Employee and non-employee stock options - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Stock Based Compensation
Shares available for grant, beginning balance	5,431
Additional shares authorized	2,483
Options granted (in shares)	(1,195)
Options forfeited (in shares)	55
Shares available for grant, end balance	6,774	5,431
Number of shares
Outstanding, beginning balance (in shares)	9,463
Options granted (in shares)	1,195
Options exercised (in shares)	(1,070)
Options forfeited (in shares)	(55)
Outstanding, end balance (in shares)	9,533	9,463
Exercisable (in shares)	5,016
Vested and expected to vest (in shares)	9,312
Weighted-Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 7.69
Granted (in dollars per share)	16.57
Exercised (in dollars per share)	2.41
Forfeited (in dollars per share)	11.24
Outstanding, end balance (in dollars per share)	9.37	$ 7.69
Exercisable (in dollars per share)	6.36
Vested and expected to vest (in dollars per share)	$ 9.29
Additional disclosures
Weighted average contractual term, options outstanding	7 years 4 months 21 days	6 years 10 months 28 days
Exercisable (in years)	6 years 7 days
Vested and expected to vest (in years)	7 years 4 months 6 days
Aggregate intrinsic value, options outstanding	$ 107,874	$ 61,718
Aggregate intrinsic value, options exercisable	71,540
Aggregate intrinsic value, vested and expected to vest	$ 106,097