Share-Based Compensation - Schedule of stock option activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of Stock Options
Outstanding, beginning balance (in shares)	5,120,293	4,207,029	4,879,850
Granted (in shares)	1,571,925	2,136,012	382,476
Exercised (in shares)	(1,429,756)	(1,139,962)	(446,201)
Cancelled/Forfeited (in shares)	(91,486)	(82,786)	(609,096)
Outstanding, ending balance (in shares)	5,170,976	5,120,293	4,207,029	4,879,850
Weighted Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 3.13	$ 0.88	$ 0.83
Granted (in dollars per share)	39.01	6.49	1.56
Exercised (in dollars per share)	1.87	1.20	0.85
Cancelled/Forfeited (in dollars per share)	9.33	2.03	0.97
Outstanding, ending balance (in dollars per share)	$ 14.28	$ 3.13	$ 0.88	$ 0.83
Weighted Average Remaining Contractual Life (Years)
Weighted average remaining contractual life, outstanding	7 years 6 months	7 years 3 months 18 days	7 years 2 months 12 days	8 years 2 months 12 days
Weighted average remaining contractual life, vested and exercisable	6 years
Weighted average remaining contractual life, vested and expected to vest	6 years 10 months 24 days
Aggregate Intrinsic Value
Outstanding, beginning balance	$ 81,371	$ 8,936	$ 3,557
Exercised	121,591	5,722	347
Outstanding, ending balance	$ 329,879	$ 81,371	$ 8,936	$ 3,557
Number of Stock Options, Vested and exercisable (in shares)	2,533,199
Number of Stock Options, Vested and expected to vest (in shares)	3,761,031
Weighted Average Exercise Price, Vested and exercisable (in dollars per share)	$ 2.32
Weighted Average Exercise Price, Vested and expected to vest (in dollars per share)	$ 8.55
Aggregate intrinsic value, vested and exercisable	$ 185,671
Aggregate intrinsic value, vested and expected to vest	$ 256,967