Stock-based Compensation - Schedule of Stock-based Awards (Details) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Shares Subject to Options Outstanding
Stock options outstanding, beginning balance (in shares)	25,087
Stock options granted (in shares)	394
Stock options exercised (in shares)	(2,589)
Repurchases of unvested shares (in shares)	0
Stock options cancelled, forfeited, expired (in shares)	(1,014)
Stock options outstanding, ending balance (in shares)	21,878	25,087
Stock options vested and expected to vest (in shares)	21,878
Stock options exercisable (in shares)	21,865
Weighted- Average Exercise Price per Option
Stock options outstanding, weighted-average exercise price, beginning balance (in dollars per share) | $ / shares	$ 2.18
Stock options granted, weighted-average exercise price (in dollars per share) | $ / shares	9.60
Stock options exercised, weighted-average exercise price (in dollars per share) | $ / shares	2.24
Stock options cancelled, forfeited, expired, weighted-averaged exercise price (in dollars per share) | $ / shares	2.46
Stock options outstanding, weighted-average exercise price, ending balance (in dollars per share) | $ / shares	2.30	$ 2.18
Stock options vested and expected to vest, weighted-average exercise price (in dollars per share) | $ / shares	2.30
Stock options exercisable, weighted-average exercise price (in dollars per share) | $ / shares	$ 2.30
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Stock options outstanding, weighted-average remaining contractual term	7 years 9 months 18 days	8 years 4 months 24 days
Stock options vested and expected to vest, weighted-average remaining contractual term	7 years 9 months 18 days
Stock options exercisable, weighted-average remaining contractual term	7 years 9 months 18 days
Share-Based Payment Award, Options, Aggregate Intrinsic Value [Abstract]
Stock options outstanding, aggregate intrinsic value | $	$ 355,967	$ 159,945
Stock options exercised, aggregate intrinsic value | $	21,188
Stock options vested and expected to vest, aggregate intrinsic value | $	355,967
Stock options exercisable, aggregate intrinsic value | $	$ 355,755