Stock-based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Jan. 01, 2018
Stock Options
Stock Options, Outstanding Beginning Balance (in shares)	603	613			613	615
Stock Options, Granted (in shares)	18	16
Stock Options, Exercised (in shares)	(16)
Stock Options, Forfeited (in shares)	(12)	(18)
Stock Options, Outstanding Ending Balance (in shares)	603	613			613	615
Stock Options, Exercisable (in shares)	478	389
Grant Date Weighted- Average Exercise Price
Grant Date Weighted-Average Exercise Price, Beginning balance (in USD per share)	$ 2.86	$ 2.85			$ 2.85	$ 2.87
Grant Date Weighted Average Exercise Price, Granted (in USD per share)	2.91	2.68
Grant Date Weighted Average Exercise Price, Exercised (in USD per share)	2.59
Grant Date Weighted Average Exercise Price, Forfeited (in USD per share)	3.05	3.15
Grant Date Weighted-Average Exercise Price, Ending balance (in USD per share)	2.86	2.85			$ 2.85	$ 2.87
Grant Date Weighted Average Exercise Price, Exercisable (in USD per share)	$ 2.86	$ 2.86
Weighted- Average Remaining Contractual Life (in Years)
Weighted Average Remaining Contractual Life (Years), Beginning Balance	2 years 1 month 2 days	2 years 11 months 4 days	3 years 2 months 4 days	3 years 5 months 23 days
Weighted Average Remaining Contractual Life (Years), Granted	7 years	6 years 11 months 15 days
Weighted Average Remaining Contractual Life (Years), Ending Balance	2 years 1 month 2 days	2 years 11 months 4 days	3 years 2 months 4 days	3 years 5 months 23 days
Weighted Average Remaining Contractual Life (Years), Exercisable	1 year 11 months 19 days	2 years 9 months 29 days
Intrinsic Value
Intrinsic Value, Outstanding Beginning Balance	$ 42	$ 337			$ 0	$ 0
Intrinsic Value, Grants in Period	0	0
Intrinsic Value, Exercises in Period	0
Intrinsic Value, Forfeitures in Period	0	0
Intrinsic Value, Outstanding Ending Balance	42	337			$ 0	$ 0
Intrinsic Value, Exercisable End of Period	$ 0	$ 0