Stock-Based Compensation - Stock Options (Details) - Employee and non-employee stock options $ / 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
Stock Based Compensation
Shares available for grant, beginning balance	5,431
Additional shares authorized	2,483
Options granted (in shares)	(1,837)
Options forfeited (in shares)	364
Shares available for grant, end balance	6,441	5,431
Number of shares
Outstanding, beginning balance (in shares)	9,463
Options granted (in shares)	1,837
Options exercised (in shares)	(2,138)
Options forfeited (in shares)	(364)
Outstanding, end balance (in shares)	8,798	9,463
Exercisable (in shares)	4,811
Vested and expected to vest (in shares)	8,603
Weighted-Average Exercise Price
Outstanding, beginning balance (in dollars per share) | $ / shares	$ 7.69
Granted (in dollars per share) | $ / shares	18.00
Exercised (in dollars per share) | $ / shares	4.87
Forfeited (in dollars per share) | $ / shares	13.93
Outstanding, end balance (in dollars per share) | $ / shares	10.27	$ 7.69
Exercisable (in dollars per share) | $ / shares	7.09
Vested and expected to vest (in dollars per share) | $ / shares	$ 10.18
Additional disclosures
Weighted average contractual term, options outstanding	7 years 1 month 6 days	6 years 10 months 28 days
Exercisable (in years)	5 years 8 months 27 days
Vested and expected to vest (in years)	7 years 22 days
Aggregate intrinsic value, options outstanding | $	$ 198,252	$ 61,718
Aggregate intrinsic value, options exercisable | $	123,680
Aggregate intrinsic value, vested and expected to vest | $	$ 194,605