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Incentive Plan (Market Performance-Based Awards) (Details)
$ in Millions
1 Months Ended 12 Months Ended
Mar. 09, 2018
Mar. 01, 2018
Feb. 27, 2018
Jan. 01, 2018
Feb. 28, 2017
Feb. 27, 2017
Jan. 01, 2017
Feb. 16, 2016
Jan. 01, 2016
Jan. 31, 2018
shares
Jan. 31, 2017
shares
Dec. 31, 2018
USD ($)
simulation
shares
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2016
USD ($)
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Units granted (in shares)                       128,986 73,449 74,246
Market performance-based awards                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Performance period                       3 years    
Number of trials run | simulation                       100,000    
Expected volatility rate 22.00% 22.00%   22.00% 23.00%   25.00% 26.00% 22.00%          
Risk free interest rate 2.42% 2.34%   1.98% 1.43%   1.49% 0.89% 1.32%          
Fair value of awards | $                       $ 21.8 $ 19.5 $ 21.2
Expected service period                       4 years    
Market performance-based awards | 2014 Performance Grant                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Performance period                       3 years    
Vesting percentage           50.00%                
Market performance-based awards | 2014 Performance Grant | Tranche Five                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Vesting percentage     50.00%                      
Market performance-based awards | 2015 Performance Grant                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Performance period                       3 years    
Vesting percentage     50.00%                      
Market performance-based awards | 2015 Performance Grant | Below Threshold Level (less than)                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Vesting percentage                       0.00%   0.00%
Performance threshold                           (3.00%)
Market performance-based awards | 2015 Performance Grant | Threshold Level                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Vesting percentage                       25.00%   25.00%
Performance threshold                           (3.00%)
Market performance-based awards | 2015 Performance Grant | Target Level                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Vesting percentage                       50.00%   50.00%
Performance threshold                           1.00%
Market performance-based awards | 2015 Performance Grant | High Level (greater than)                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Vesting percentage                       100.00%   100.00%
Performance threshold                           5.00%
Market performance-based awards | 2015 Performance Grant | Tranche Five                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Vesting percentage                       50.00%    
Market performance-based awards | 2016 Performance Grant                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Vesting percentage                       50.00%    
Market performance-based awards | 2016 Performance Grant | Below Threshold Level (less than)                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Vesting percentage                         0.00%  
Performance threshold                         (3.00%)  
Market performance-based awards | 2016 Performance Grant | Threshold Level                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Vesting percentage                         25.00%  
Performance threshold                         (3.00%)  
Market performance-based awards | 2016 Performance Grant | Target Level                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Vesting percentage                         50.00%  
Performance threshold                         1.00%  
Market performance-based awards | 2016 Performance Grant | High Level (greater than)                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Vesting percentage                         100.00%  
Performance threshold                         5.00%  
Market performance-based awards | 2016 Performance Grant | Tranche Five                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Vesting percentage                       50.00%    
Market performance-based awards | 2017 Performance Grant | Tranche Five                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Vesting percentage                       50.00%    
Market performance-based awards | 2018 Performance Grant | Tranche Five                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Vesting percentage                       50.00%    
Class D Units                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Shares vested (in shares)                   363,193 399,250 339,317    
Units granted (in shares)                       2,165,692    
Distribution Equivalent Unit                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Shares vested (in shares)                   36,246 44,702 31,009    
Restricted Stock Units (RSUs)                            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                            
Shares vested (in shares)                   49,707 138,567 56,778    
Units granted (in shares)                       590,569