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Share-Based Compensation - Incentive Unit Awards (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Share-Based Compensation          
Personnel accruals $ 48.5   $ 48.5   $ 45.7
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 3.5   $ 3.5    
Weighted-average period for amortization of unrecognized compensation cost     1 year    
Compensation expense (benefit) 0.7 $ 1.0 $ 3.2 $ 1.1  
Incentive Unit Award | Personnel Accruals          
Share-Based Compensation          
Personnel accruals $ 4.9   $ 4.9   $ 1.9
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%