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Stock-Based Compensation Stock-Based Compensation Narrative (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2016
USD ($)
shares
Dec. 31, 2015
shares
Employee Stock Option [Member]      
[Line Items]      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $ $ 2.9    
Unrecognized compensation expense, Expected weighted average period for recognition 1 year 6 months    
Stock Options Expiration Period 10 years    
Award vesting rights Options generally become exercisable in three equal annual installments beginning one year from the date of grant    
Restricted Stock Units (RSUs) [Member]      
[Line Items]      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $ $ 21.7    
Unrecognized compensation expense, Expected weighted average period for recognition 1 year 7 months 6 days    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years    
Granted, Number 273,000 306,000 434,000
Phantom Share Units (PSUs) [Member]      
[Line Items]      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $ $ 2.8    
Unrecognized compensation expense, Expected weighted average period for recognition 1 year 8 months 12 days    
Deferred Compensation Share-based Arrangements, Liability, Current | $ $ 1.7 $ 1.0  
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years    
Granted, Number 32,000 63,000  
Employee Stock [Member]      
[Line Items]      
Number of shares available for future grant under the Stock Incentive Plan 340,000    
Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Purchase Date 5.00%    
ESPP, maximum percentage of employee salary eligible for participation 10.00%    
Stock Incentive Plan [Member]      
[Line Items]      
Number of shares authorized for issuance under the Stock Incentive Plan 10,473,956    
Number of shares available for future grant under the Stock Incentive Plan 4,656,327    
Reduction in stock options available for issue 1    
Authorized share reserve reduction in awards other than stock options or share appreciation rights available for issue, conversion ratio 1.7