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Share-Based Compensation - Incentive Unit Awards (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Share-Based Compensation      
Personnel accruals $ 27.7   $ 55.9
Incentive Unit Award      
Share-Based Compensation      
Vesting period 3 years    
Number of share right to receive cash payment on vesting equal to fair market value is received per award (in shares) 1    
Unrecognized compensation expense $ 4.0    
Weighted-average period for amortization of unrecognized compensation cost 1 year 5 months    
Compensation expense (benefit) $ (0.7) $ 1.2  
Incentive Unit Award | Personnel Accruals      
Share-Based Compensation      
Personnel accruals $ 2.5   $ 3.3
Incentive Unit Award | Tranche One      
Share-Based Compensation      
Vesting rights percentage 33.33%    
Incentive Unit Award | Tranche Two      
Share-Based Compensation      
Vesting rights percentage 33.33%    
Incentive Unit Award | Tranche Three      
Share-Based Compensation      
Vesting rights percentage 33.33%