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Stock-Based Compensation - Restricted Stock Units (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Restricted Stock Units (RSUs)      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Fair value of awards vested $ 9,900,000 $ 22,600,000 $ 17,400,000
Unamortized stock compensation expense $ 9,500,000    
Compensation cost not yet recognized, period for recognition (in years) 1 year 9 months    
Restricted Stock Units (RSUs) | General and administrative expenses      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation expense $ 12,800,000 15,600,000 26,100,000
Restricted Stock Units (RSUs) | LTIP      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted average grant date fair value $ 12,100,000 11,600,000 14,600,000
Restricted Stock Units (RSUs) | LTIP | Primary Vesting Category | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting (in years) 1 year    
Restricted Stock Units (RSUs) | LTIP | Primary Vesting Category | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting (in years) 3 years    
PRSUs      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting (in years) 3 years    
Stock-based compensation expense $ 600,000.0 $ 800,000.0 $ 0
Fair value of awards vested 200,000    
Unamortized stock compensation expense $ 2,200,000    
Compensation cost not yet recognized, period for recognition (in years) 2 years 3 months 14 days