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Stock-Based Compensation Narrative (Details)
$ in Millions
6 Months Ended
Jun. 30, 2018
USD ($)
shares
Dec. 31, 2017
USD ($)
Employee Stock Option [Member]    
[Line Items]    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $ $ 4.1  
Unrecognized compensation expense, Expected weighted average period for recognition (years) 1 year 7 months 6 days  
Restricted Stock Units (RSUs) [Member]    
[Line Items]    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $ $ 48.8  
Unrecognized compensation expense, Expected weighted average period for recognition (years) 2 years 2 months 12 days  
Phantom Share Units (PSUs) [Member]    
[Line Items]    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $ $ 4.1  
Unrecognized compensation expense, Expected weighted average period for recognition (years) 2 years 29 days  
Deferred Compensation Share-based Arrangements, Liability, Current | $ $ 1.0 $ 1.7
Employee Stock Purchase Plan [Member]    
[Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant | shares 322,765  
Stock Incentive Plan [Member]    
[Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized | shares 12,623,538  
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant | shares 6,571,411  
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  
SSNI Plan [Member]    
[Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized | shares 2,880,039