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Shareholders' Equity - Narrative (Details) - USD ($)
1 Months Ended 12 Months Ended
Dec. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Nov. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Preference stock, shares authorized (in shares)   15,000,000.0 15,000,000.0  
Preference stock, shares issued (in shares)   0 0  
Preference stock, shares outstanding (in shares)   0 0  
Common stock, shares authorized (in shares)   150,000,000.0 150,000,000.0  
Common stock, par value (in USD per share)   $ 0.10 $ 0.10  
Common stock, shares issued (in shares)   73,600,000 69,200,000  
Common stock, shares outstanding (in shares)   73,600,000 69,200,000  
Number of shares reserved for future issuance   20,200,000    
Share price (usd per share)   $ 31.20    
Expected volatility, Minimum   32.20%    
Expected volatility, Maximum   42.70%    
Risk-free interest rate, Minimum   1.66%    
Risk-free interest rate, Maximum   2.40%    
Common Stock        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of common shares repurchased (in shares)   3,500,000    
Purchase of treasury stock   $ 64,500,000    
Retirement Savings Plan Contributions        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Exercise stock options contractual life   7 years    
Stock Options        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Exercise stock options contractual life   10 years    
Stock outstanding (in shares)   400,000 600,000  
Weighted average grant date fair value of restricted stock granted (in USD per share)     $ 18.01 $ 23.04
Intrinsic value for options exercised $ 0 $ 2,900,000 $ 2,100,000 $ 3,900,000
SARS        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock outstanding (in shares)   1,000,000 1,000,000  
Weighted-average grant date fair value of SARS granted (in USD per share)   $ 15.41 $ 7.66  
Stock granted in period (in shares) 0 400,000   0
Intrinsic value paid $ 3,300,000 $ 4,900,000 $ 2,300,000  
Total stock-based compensation not yet recognized   $ 7,500,000    
Recognition period for compensation Cost   23 months    
SARS | Employees        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting rate   33.30%    
SARS | Director        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting rate   50.00%    
SARS | 1999 Plan | Employees        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Exercise stock options contractual life   10 years    
SARS | 1999 Plan | Director        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Exercise stock options contractual life   10 years    
SARS | 2009 Plan | Employees        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Exercise stock options contractual life   7 years    
SARS | 2009 Plan | Director        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Exercise stock options contractual life   7 years    
Performance- based common shares        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Weighted average grant date fair value of restricted stock granted (in USD per share)   $ 12.99    
Restricted Stock, Granted (in shares)   200,000    
Performance-based stock options        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Weighted-average grant date fair value of SARS granted (in USD per share)   $ 5.81    
Stock granted in period (in shares)   200,000    
Restricted Stock, Service-Based        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Recognition period for compensation Cost   6 months    
Restricted stock outstanding (in shares)   400,000 600,000  
Unrecognized compensation cost related to restricted stock   $ 5,000,000    
Intrinsic value of restricted stock outstanding   $ 11,700,000    
Weighted average grant date fair value of restricted stock granted (in USD per share)   $ 24.98 $ 17.65 $ 20.70
Restricted Stock, Granted (in shares)   100,000    
Performance-Based Restricted Stock        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Recognition period for compensation Cost   17 months    
Restricted stock outstanding (in shares)   1,000,000 1,100,000  
Unrecognized compensation cost related to restricted stock   $ 4,900,000    
Intrinsic value of restricted stock outstanding   32,700,000    
Intrinsic value of restricted stock which is expected to vest   $ 17,800,000    
Weighted average grant date fair value of restricted stock granted (in USD per share)   $ 22.35 $ 15.97 21.33
Restricted Stock, Granted (in shares)   500,000    
Performance-Based Restricted Stock | Minimum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share price (usd per share)   $ 22.00    
Performance-Based Restricted Stock | Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share price (usd per share)   $ 27.00    
Employee Stock Purchase Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Common stock purchase price, percentage of fair value on last day of respective offering period (not less than)   85.00%    
Shares issued in period (in shares) 100,000 100,000 200,000  
Average price of shares issued in period (in USD per share) $ 15.66 $ 25.43 $ 18.11 $ 20.61
Employee Stock Purchase Plan | 2009 Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share authorized for issuance (in shares)   1,500,000.0