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Share-based Compensation Performance Units (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition 2 years 22 days    
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized $ 5.8    
Performance units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 3 years    
Nonvested, shares   80  
Nonvested, Weighted Average Grant Date Fair Value $ 24.81 $ 31.84  
Granted, shares 30    
Granted, Weighted Average Grant Date Fair Value $ 38.00 $ 19.94 $ 28.25
Vested, shares (20)    
Vested, Weighted Average Grant Date Fair Value $ 93.34    
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition 1 year 230 days    
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized $ 0.8    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value $ 0.8 $ 1.8 $ 0.8
Performance units [Member] | 2015 PSU [Member] [Member] | Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Percent of Performance Share Units Earned Out Of Performance Share Units Granted 0.00%    
Performance units [Member] | 2015 PSU [Member] [Member] | Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Percent of Performance Share Units Earned Out Of Performance Share Units Granted 200.00%    
Performance units [Member] | 2016 PSU [Member] [Member] | Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Percent of Performance Share Units Earned Out Of Performance Share Units Granted 0.00%    
Performance units [Member] | 2016 PSU [Member] [Member] | Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Percent of Performance Share Units Earned Out Of Performance Share Units Granted 200.00%    
Performance units [Member] | 2017 Performance Units [Member] | Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Percent of Performance Share Units Earned Out Of Performance Share Units Granted 0.00%    
Performance units [Member] | 2017 Performance Units [Member] | Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Percent of Performance Share Units Earned Out Of Performance Share Units Granted 200.00%    
Performance Based Stock Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period (10)    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value $ 31.14    
Leveraged Restricted Share Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 3 years    
Nonvested, shares 0 10  
Nonvested, Weighted Average Grant Date Fair Value $ 0.00 $ 106.90  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period (5)    
Vested, shares (5)    
Vested, Weighted Average Grant Date Fair Value $ 106.90    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value $ 0.2 $ 1.0 $ 0.6