Stock-Based Compensation - Option activity (Details) - Stock options. - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Number of shares
Outstanding at beginning of period	14,784,770
Options granted	3,059,004
Exercised	(1,449,309)
Forfeited	(1,290,558)
Outstanding at end of period	15,103,907	14,784,770
Vested and expected to vest as of December 31, 2019	15,103,907
Options exercisable as of December 31, 2019	5,182,808
Weighted average exercise price
Outstanding at the beginning (in dollars per share)	$ 8.93
Granted (in dollars per share)	12.67
Exercised (in dollars per share)	1.68
Forfeited (in dollars per share)	8.58
Outstanding at the end (in dollars per share)	10.41	$ 8.93
Vested and expected to vest (in dollars per share)	10.41
Options exercisable (in dollars per share	$ 6.33
Weighted average contractual term and aggregate intrinsic value
Outstanding at the beginning (in years)	8 years 2 months 19 days	8 years 10 months 21 days
Outstanding at the end (in years)	8 years 2 months 19 days	8 years 10 months 21 days
Vested and expected to vest (in years)	8 years 2 months 19 days
Options exercisable (in years)	7 years 5 months 27 days
Outstanding at the beginning (in dollars)	$ 126,367
Outstanding at the end (in dollars)	36,006	$ 126,367
Vested and expected to vest (in dollars)	36,006
Options exercisable (in dollars)	$ 21,743