Stock-Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Shares
Number of shares outstanding, beginning of period (in shares)	14,573	9,336	7,616
Number of shares, granted (in shares)	6,108	9,022	2,800
Number of shares, exercised (in shares)	(740)	(1,870)	(495)
Number of shares, forfeited/canceled (in shares)	(722)	(1,915)	(585)
Number of shares outstanding, end of period (in shares)	19,219	14,573	9,336	7,616
Number of shares exercisable (in shares)	6,758
Weighted Average Exercise Price
Weighted average exercise price, outstanding, beginning of period (in USD per share)	$ 4.38	$ 2.77	$ 2.15
Weighted average exercise price, granted (in USD per share)	15.17	5.22	4.15
Weighted average exercise price, exercised (in USD per share)	2.26	1.62	1.00
Weighted average exercise price, forfeited/canceled (in USD per share)	7.23	3.21	2.84
Weighted average exercise price, outstanding, end of period (in USD per share)	7.78	$ 4.38	$ 2.77	$ 2.15
Weighted average exercise price, exercisable (in USD per share)	$ 3.75
Weighted average remaining contractual term (in years), outstanding	8 years	8 years 2 months 12 days	7 years 2 months 12 days	7 years 4 months 24 days
Weighted average remaining contractual term (in years), exercisable	6 years 7 months 6 days
Aggregate intrinsic value, outstanding, beginning of period	$ 77,020	$ 15,374	$ 17,237
Aggregate intrinsic value, exercised	9,902	7,667	1,567
Aggregate intrinsic value, outstanding, end of period	277,114	$ 77,020	$ 15,374	$ 17,237
Aggregate intrinsic value, exercisable	$ 124,629