Stock-Based Compensation - Stock Option Activity (Details) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares
Shares Available for Grant
Beginning number of shares, available for grant (in shares)	2,726,225
2018 plan annual increase (in shares)	3,689,000
Granted (in shares)	(78,476)
Canceled (in shares)	12,406
Ending number of shares, available for grant (in shares)	2,864,528	2,726,225
Shares Subject to Options Outstanding
Beginning number of shares, outstanding (in shares)	4,494,889
Granted (in shares)	78,476
Exercised (in shares)	(654,557)
Canceled (in shares)	(49,806)
Ending number of shares, outstanding (in shares)	3,869,002	4,494,889
Options vested and exercisable, number of options (in shares)	2,014,893
Weighted-Average Exercise Price
Beginning balance of options outstanding (in usd per share) | $ / shares	$ 10.90
Granted (in usd per share) | $ / shares	76.01
Exercised (in usd per share) | $ / shares	5.42
Canceled (in usd per share) | $ / shares	8.79
Ending balance of options outstanding (in usd per share) | $ / shares	13.18	$ 10.90
Options vested and exercisable, weighted average exercise price per share (in usd per share) | $ / shares	$ 6.90
Weighted-Average Remaining Contractual Life and Aggregate Intrinsic Value
Options outstanding, weighted average remaining contractual term (in years)	7 years 3 months 18 days	7 years 8 months 12 days
Options outstanding, aggregate intrinsic value | $	$ 266,316	$ 306,392
Options vested and exercisable, weighted average remaining contractual term (in years)	6 years 10 months 24 days
Options vested and exercisable, aggregate intrinsic value | $	$ 150,268
Restricted Stock Units
Shares Available for Grant
Granted (in shares)	(126,834)
Canceled (in shares)	33,355
Market-based restricted stock units
Shares Available for Grant
Granted (in shares)	(3,391,148)