Stock-Based Compensation - Stock Option Activity (Details) - Options - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Number of Shares
Outstanding, beginning balance (in shares)	2,744,153
Options granted (in shares)	361,800
Options forfeited (in shares)	(11,250)
Options exercised (in shares)	(2,581)
Outstanding, ending balance (in shares)	3,092,122	2,744,153
Options vested and exercisable (in shares)	2,058,172
Weighted Average Exercise Price per Share
Outstanding, beginning balance (in dollars per shares)	$ 2.33
Options granted (in dollars per share)	9.95
Options forfeited (in dollars per shares)	8.45
Options exercised (in dollars per shares)	0.41
Outstanding, end balance (in dollars per shares)	3.20	$ 2.33
Options vested and exercisable (in dollars per shares)	$ 1.53
Options
Intrinsic value, Outstanding, beginning balance	$ 7,411
Intrinsic value, Outstanding, end balance	14,431	$ 7,411
Intrinsic value, vested and exercisable	$ 11,995
Weighted average remaining contractual life, Outstanding, Beginning	8 years 2 months 12 days	8 years 9 months 18 days
Weighted average remaining contractual life, granted	9 years 7 months 6 days
Weighted average remaining contractual life, vested and exercisable	8 years