Equity-Based Compensation - Information Related to Stock Options (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value		$ 0	$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate			31.80%
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value [Table Text Block]		$ 1.84	$ 5.00
Weighted Average Exercise Price per Stock Option
Stock options vested and exercisable (in dollars per share)		$ 14.16
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Remaining Contractual Term		9 years 22 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Aggregate Intrinsic Value		$ 63
Aggregate Intrinsic Value
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross		2,085
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price		$ 6.97
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Number		7,352
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate		2.80%	1.90%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term		5 years 10 months 21 days	5 years
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate		1.70%	0.60%
Common Unit Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate				39.80%
Aggregate Intrinsic Value
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate			1.40%	1.50%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term			2 years	3 years 8 months 12 days
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate			0.60%	0.90%
2017 Incentive Award Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value		$ 176	$ 28,073
Number of Stock Options (in thousands)
Stock options outstanding (in shares)	5,725	5,725
Stock options forfeited (in shares)	(458)
Stock options outstanding (in shares)	7,352		5,725
Stock options vested and exercisable (in shares)	5,201
Weighted Average Exercise Price per Stock Option
Stock options outstanding (in dollars per share)	$ 17.00	$ 17.00
Stock options outstanding (in dollars per share)	14.16		$ 17.00
Stock options vested and exercisable (in dollars per share)	$ 17.00
Weighted Average Remaining Contractual Life
Stock options outstanding	9 years 22 days
Stock options vested and exercisable	8 years 8 months 12 days
Aggregate Intrinsic Value
Stock options outstanding	$ 63
Stock options vested and exercisable	$ 0
Share-based Compensation Arrangements by Share-based Payment Award, Options, Expirations in Period, Weighted Average Exercise Price	$ 17.00
RSUs | 2017 Incentive Award Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)		3,091
Aggregate Intrinsic Value
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition	3 years 26 days
Granted (in dollars per share)		$ 15.03	18.01
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period		190
Vested (in dollars per share)		$ 15.53
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period		143
Forfeited (in dollars per share)		$ 16.95
Unvested (In shares)		$ 14.93	$ 18.01
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options		$ 3,800
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate		28.20%
Aggregate Intrinsic Value
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition		3 years 1 month 15 days
Common Unit Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate			29.30%
Aggregate Intrinsic Value
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition	2 years 9 months 12 days
Granted (in dollars per share)		$ 0	$ 0	$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period		797
Vested (in dollars per share)		$ 11.11
Unvested (In shares)		$ 11.11