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, 2019	Dec. 31, 2018	Dec. 31, 2017
Stock Based Compensation
Shares available for grant, beginning balance	5,431
Additional shares authorized	2,483
Options granted (in shares)	(1,838)
Options forfeited (in shares)	340
Shares available for grant, end balance	6,416	5,431
Number of shares
Outstanding, beginning balance (in shares)	9,463
Options granted (in shares)	1,838
Options exercised (in shares)	(2,464)
Options forfeited (in shares)	(340)
Outstanding, end balance (in shares)	8,497	9,463
Exercisable (in shares)	4,835
Vested and expected to vest (in shares)	8,288
Weighted-Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 7.69
Granted (in dollars per share)	18.01
Exercised (in dollars per share)	5.29
Forfeited (in dollars per share)	13.13
Outstanding, end balance (in dollars per share)	10.39	$ 7.69
Exercisable (in dollars per share)	7.27
Vested and expected to vest (in dollars per share)	$ 10.29
Additional disclosures
Weighted average contractual term, options outstanding	6 years 10 months 17 days	6 years 10 months 28 days
Exercisable (in years)	5 years 6 months 21 days
Vested and expected to vest (in years)	6 years 9 months 29 days
Aggregate intrinsic value, options outstanding	$ 197,955	$ 61,718
Aggregate intrinsic value, options exercisable	127,753
Aggregate intrinsic value, vested and expected to vest	193,940
Aggregate intrinsic value, options exercised	70,000	25,600	$ 3,700
Fair value, options vested	$ 11,000	$ 11,300	$ 12,100
Weighted-average grant date fair value, options granted	$ 8.01	$ 4.85	$ 5.71