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Stock-based Compensation (Details) - Restricted Stock Units (RSUs) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2017
Mar. 31, 2017
Jun. 30, 2016
Mar. 31, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Restricted stock unit activity [Roll Forward]                
Unvested at beginning of period (in shares) 785,700 750,900 820,400 706,000 750,900 706,000 706,000  
Granted (in shares) 11,100   49,700   151,900 250,600    
Vested (in shares) (3,300)   (9,700)   (106,400) (94,600)    
Forfeited (in shares) (17,600)   (43,100)   (20,500) (44,700)    
Unvested at end of period (in shares) 775,900 785,700 817,300 820,400 775,900 817,300 750,900 706,000
Unvested shares expected to vest (in shares) 775,900   796,100   775,900 796,100    
Restricted stock unit activity, additional disclosures [Abstract]                
Weighted average grant date fair value, beginning of period (in dollars per share) $ 117.62 $ 113.97 $ 112.83 $ 111.78 $ 113.97 $ 111.78 $ 111.78  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value 118.00   108.75   116.54 107.45    
Weighted average grant date fair value, vested (in dollars per share) 117.81   92.60   90.32 91.22    
Weighted average grant date fair value, forfeited (in dollars per share) 112.71   117.52   113.16 116.50    
Weighted average grant date fair value, end of period (in dollars per share) 117.74 $ 117.62 112.58 $ 112.83 117.74 112.58 $ 113.97 $ 111.78
Weighted average grant date fair value, unvested shares expected to vest (in dollars per share) $ 117.74   $ 112.69   $ 117.74 $ 112.69    
Weighted average remaining contractual life, unvested shares outstanding 1 year 7 months 10 days 1 year 9 months 26 days 2 years 2 years 2 months 14 days 1 year 7 months 10 days 1 year 11 months 27 days 1 year 8 months 16 days 2 years 10 days
Weighted average remaining contractual life, unvested shares outstanding, expected to vest 1 year 7 months 11 days   2 years   1 year 7 months 11 days 1 year 11 months 29 days    
Unamortized deferred compensation cost $ 30.7       $ 30.7