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Stock-Based Compensation Narrative (Details)
$ in Millions
9 Months Ended
Sep. 30, 2018
USD ($)
shares
Dec. 31, 2017
USD ($)
Employee Stock Option [Member]    
[Line Items]    
Compensation cost not yet recognized | $ $ 3.5  
Unrecognized compensation expense, Expected weighted average period for recognition (years) 1 year 7 months 6 days  
Restricted Stock Units (RSUs) [Member]    
[Line Items]    
Compensation cost not yet recognized | $ $ 46.0  
Unrecognized compensation expense, Expected weighted average period for recognition (years) 2 years 1 month 6 days  
Phantom Share Units (PSUs) [Member]    
[Line Items]    
Compensation cost not yet recognized | $ $ 3.9  
Unrecognized compensation expense, Expected weighted average period for recognition (years) 2 years  
Deferred compensation share-based Arrangements, liability, current | $ $ 1.7 $ 1.7
Employee Stock Purchase Plan [Member]    
[Line Items]    
Share-based compensation arrangement by share-based payment award, number of shares available for grant (in shares) | shares 305,979  
Stock Incentive Plan [Member]    
[Line Items]    
Share-based compensation arrangement by share-based payment award, number of shares authorized (in shares) | shares 12,623,538  
Share-based compensation arrangement by share-based payment award, number of shares available for grant (in shares) | shares 6,449,209  
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 (in shares) | shares 2,880,039