Stock-Based Compensation - Stock Options (Details) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares
Employee and non-employee stock options
Stock Based Compensation
Shares available for grant, beginning balance	6,416
Additional shares authorized	3,120
Options granted (in shares)	(446)
Options forfeited (in shares)	84
RSUs granted (in shares)	(4,670)
RSUs forfeited/cancelled (in shares)	210
Shares available for grant, end balance	4,714	6,416
Number of shares
Outstanding, beginning balance (in shares)	8,497
Options granted (in shares)	446
Options exercised (in shares)	(1,037)
Options forfeited (in shares)	(84)
Outstanding, end balance (in shares)	7,822	8,497
Exercisable (in shares)	5,523
Vested and expected to vest (in shares)	7,682
Weighted-Average Exercise Price
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 10.39
Granted (in dollars per share) | $ / shares	26.75
Exercised (in dollars per share) | $ / shares	10.42
Forfeited (in dollars per share) | $ / shares	16.00
Outstanding, end balance (in dollars per share) | $ / shares	11.26	$ 10.39
Exercisable (in dollars per share) | $ / shares	5.77
Vested and expected to vest (in dollars per share) | $ / shares	$ 11.16
Additional disclosures
Weighted average contractual term, options outstanding	6 years 6 months 18 days	6 years 10 months 17 days
Exercisable (in years)	8 years 10 months 2 days
Vested and expected to vest (in years)	6 years 6 months 7 days
Aggregate intrinsic value, options outstanding | $	$ 301,915	$ 197,955
Aggregate intrinsic value, options exercisable | $	226,552
Aggregate intrinsic value, vested and expected to vest | $	$ 297,314
Restricted stock units
Stock Based Compensation
RSUs granted (in shares)	(2,335)
RSUs forfeited/cancelled (in shares)	105