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Stock-Based Compensation - Vesting and Deferred Compensation Plan (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock options      
Number of Shares      
Outstanding at the beginning of the period (in shares) 1,263,665    
Stock options granted (in shares) 364,675    
Options exercised (in shares) (165,959)    
Options forfeited (in shares) (31,069)    
Outstanding at the end of the period (in shares) 1,431,312 1,263,665  
Vested and exercisable at the end of the period (in shares) 732,408    
Weighted Average Price      
Outstanding at the beginning of the period (in dollars per share) $ 70.95    
Options granted (in dollars per share) 107.63    
Options exercised (in dollars per share) 56.61    
Options forfeited (in dollars per share) 92.49    
Outstanding at the end of the period (in dollars per share) 81.49 $ 70.95  
Vested and exercisable at the end of the period (in dollars per share) $ 66.24    
Weighted average contractual term 6 years 9 months 18 days    
The weighted average contractual term of options exercisable 5 years 2 months 12 days    
Aggregate intrinsic value of options outstanding $ 46.2    
Aggregate intrinsic value of options vested and exercisable 34.7    
Stock-based compensation expense      
Unrecognized compensation cost related to awards other than options $ 5.1    
Weighted average period of recognition of unrecognized compensation cost related to restricted stock awards 1 year 10 months 13 days    
Award Activity      
Units granted (in shares) 364,675    
Performance share awards      
Vesting period of shares awarded under the Program 3 years    
Restricted stock awards      
Stock-based compensation expense      
Unrecognized compensation cost related to awards other than options $ 9.0    
Weighted average period of recognition of unrecognized compensation cost related to restricted stock awards 2 years 1 month 6 days    
Award Activity      
Outstanding balance at the beginning of the period (in shares) 225,265    
Units granted (in shares) 91,098    
Units vested and issued (in shares) (105,282)    
Units forfeited (in shares) (2,760)    
Outstanding balance at the end of the period (in shares) 208,321 225,265  
Performance share awards      
Units or shares outstanding (in dollars per share) $ 95.63 $ 82.59  
Units or shares granted (in dollars per share) 106.78 $ 84.73 $ 90.54
Units or shares vested and issued (in dollars per share) 77.41    
Units or shares forfeited (in dollars per share) $ 94.21    
Vesting period of shares awarded under the Program 3 years    
Fair value of shares vested during year ended December 31 $ 11.4    
Performance share awards      
Stock-based compensation expense      
Unrecognized compensation cost related to awards other than options $ 6.7    
Weighted average period of recognition of unrecognized compensation cost related to restricted stock awards 1 year 7 months 28 days    
Award Activity      
Outstanding balance at the beginning of the period (in shares) 219,559    
Units granted (in shares) 47,285    
Units converted (withheld) (in shares) (8,786)    
Units vested and issued (in shares) (86,619)    
Units vested and deferred (in shares) (17,413)    
Units forfeited (in shares) (2,760)    
Outstanding balance at the end of the period (in shares) 151,266 219,559  
Performance share awards      
Units or shares outstanding (in dollars per share) $ 123.99    
Units or shares granted (in dollars per share) 141.83 $ 119.08 $ 112.39
Units converted (withheld) (in dollars per share) 112.39    
Units or shares vested and issued (in dollars per share) 95.72    
Units vested and deferred (in dollars per share) 95.72    
Units or shares forfeited (in dollars per share) $ 126.66    
Vesting period of shares awarded under the Program 3 years    
Fair value of shares vested during year ended December 31 $ 11.6    
Measurement period 3 years    
Performance share awards | 2016 Awards      
Award Activity      
Units granted (in shares) 47,285    
Restricted Stock Units      
Award Activity      
Units granted (in shares) 13,399 14,359 12,157
Performance share awards      
Units or shares granted (in dollars per share) $ 107.87 $ 83.31 $ 90.54
Deferred Compensation Equity      
Performance share awards      
Number of common stock deferred (in shares) 280,982 294,574