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Stock-Based Compensation - Restricted Stock Units (Details) - Restricted Stock Units (RSUs) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Fair value of awards vested $ 6.7 $ 9.9 $ 22.6
Unamortized stock compensation expense $ 8.6    
Compensation cost not yet recognized, period for recognition 1 year 8 months 9 days    
General and administrative expenses      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation expense $ 8.9 12.8 15.6
LTIP      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted average grant date fair value $ 9.5 $ 12.1 $ 11.6
LTIP | Primary Vesting Category | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 1 year    
LTIP | Primary Vesting Category | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 3 years