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Equity-Based Compensation - Class A Units (Details)
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
$ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Conversion ratio 1.25  
Class A Common Units    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number of units granted in the period (in dollars per share)   $ 18.58
Conversion ratio 0.80  
Minimum | Class A Common Units    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Service-based vesting period   2 years
Maximum | Class A Common Units    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Service-based vesting period   4 years