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Equity-Based Compensation - Common Unit Award Activity (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, Fair Value Assumptions, Expected Volatility Rate     31.80%  
Weighted Average Grant Date Fair Value per Common Unit        
Share-Based Compensation Arrangement By Share-Based Payment Award, Non-Option Equity Instruments, Converted To Common Units, Total Aggregate Intrinsic Value   $ 0 $ 318,033 $ 0
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%  
Incentive Units        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Granted (in dollars per share)   $ 0.00 $ 10.06 $ 2.04
Stock Options        
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   28.20%    
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%  
Granted (in dollars per share)   $ 0 $ 0 $ 0
Number of Unvested Common Units Outstanding (in thousands)        
Vested (in shares)   (797)    
Weighted Average Grant Date Fair Value per Common Unit        
Vested (in dollars per share)   $ 11.11    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value   $ 10,659 $ 1,115 $ 835
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options   23,600    
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%
Weighted Average Grant Date Fair Value per Common Unit        
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%
2005 Common Membership Unit Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Intrinsic value   $ 1,180 $ 869 $ 601
Chief Executive Officer [Member] | Common Unit Awards        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Granted (in shares) 7,500      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Converted into Common Units   7,500    
President [Member] | Common Unit Awards        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Granted (in shares) 1,500      
Share-based Compensation Arrangement By Share-based Payment Award, Non-Option Equity Instruments, Grants In Period, Weighted Average Exercise Price $ 11.69      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Converted into Common Units   472