Stock-Based Compensation - Options (Details) - 2018 Plan - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 17, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Unrecognized compensation expense
Unrecognized compensation expense		$ 7,500		$ 7,500
Selling, general and administrative expense
Unrecognized compensation expense
Total compensation expense		$ 1,400	$ 1,600	$ 6,900	$ 4,500
Stock options
Stock-based compensation
Minimum fair market value per share of underlying stock used to determine option grant price, as a percent	100.00%
Expiration of stock options	10 years
Outstanding
Balance at beginning of period (in shares)				3,570,300
Granted (in shares)				2,050,600
Forfeited (in shares)				(618,200)
Balance at the end of period (in shares)		5,002,700		5,002,700		3,570,300
Weighted Average Exercise Price
Balance at the beginning of period				$ 6.06
Balance at the end of period (in shares)		$ 4.13		$ 4.13		$ 6.06
Additional information
Average Remaining Vesting Period				2 years 8 months 27 days		2 years 11 months 1 day
Aggregate Intrinsic Value		$ 9,864		$ 9,864		$ 9,590
Stock options | Minimum
Stock-based compensation
Vesting period					2 years
Stock options | Maximum
Stock-based compensation
Vesting period					4 years