Equity-Based Compensation - Stock Options (Details) - Stock options $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Number of Units
Outstanding, beginning (in shares) | shares	1,627,947
Granted (in shares) | shares	976,835
Exercised (in shares) | shares	(14,931)
Cancelled (in shares) | shares	(119,381)
Outstanding, end (in shares) | shares	2,470,470	1,627,947
Options exercisable, end (in shares) | shares	761,359
Weighted Average Exercise Price
Outstanding, beginning (in dollars per share)	$ 10.86
Granted (in dollars per share)	15.35
Exercised (in dollars per share)	6.23
Cancelled (in dollars per share)	15.08
Outstanding, end (in dollars per share)	12.46	$ 10.86
Options exercisable, end (in dollars per share)	$ 10.09
Stock options
Weighted average remaining contractual term	8 years 2 months 1 day	9 years 3 months 4 days
Weighted average remaining contractual term, exercisable	6 years 3 months 4 days
Aggregate intrinsic value, beginning | $	$ 19,831
Aggregate intrinsic value, end | $	2,577	$ 19,831
Aggregate intrinsic value, exercisable | $	$ 1,189
Weighted average grant date fair value (in dollars per share)	$ 10.75