Stock-Based Compensation - Stock Options (Details) - Employee and non-employee stock options - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock Based Compensation
Shares available for grant, beginning balance	4,737
Additional shares authorized	2,161
Options granted (in shares)	(2,319)
Options forfeited (in shares)	852
Shares available for grant, end balance	5,431	4,737
Number of shares
Outstanding, beginning balance (in shares)	9,963
Options granted (in shares)	2,319
Options exercised (in shares)	(1,967)
Options forfeited (in shares)	(852)
Outstanding, end balance (in shares)	9,463	9,963
Exercisable (in shares)	5,484
Vested and expected to vest (in shares)	9,224
Weighted-Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 6.54
Granted (in dollars per share)	11.77
Exercised (in dollars per share)	6.78
Forfeited (in dollars per share)	7.54
Outstanding, end balance (in dollars per share)	7.69	$ 6.54
Exercisable (in dollars per share)	5.25
Vested and expected to vest (in dollars per share)	$ 7.60
Additional disclosures
Weighted average contractual term, options outstanding	6 years 10 months 28 days	7 years 11 days
Exercisable (in years)	5 years 6 months 26 days
Vested and expected to vest (in years)	6 years 10 months 13 days
Aggregate intrinsic value, options outstanding	$ 61,718	$ 31,902
Aggregate intrinsic value, options exercisable	48,055
Aggregate intrinsic value, vested and expected to vest	60,895
Aggregate intrinsic value, options exercised	25,600	3,700	$ 16,900
Fair value, options vested	$ 11,300	$ 12,100	$ 8,500
Weighted-average grant date fair value, options granted	$ 4.85	$ 5.71	$ 5.92