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Stock-based Compensation (Details) - Restricted Stock Units (RSUs) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2016
Jun. 30, 2016
Sep. 30, 2015
Jun. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Dec. 31, 2014
Restricted stock unit activity [Roll Forward]                
Unvested at beginning of period (in shares) 817,300   805,400   706,000 745,300 745,300  
Granted (in shares) 27,000   27,600   277,600 158,000    
Vested (in shares) (96,200)   (129,900)   (190,800) (186,100)    
Forfeited (in shares) (2,400)   (4,100)   (47,100) (18,200)    
Unvested at end of period (in shares) 745,700 817,300 699,000 805,400 745,700 699,000 706,000 745,300
Unvested shares expected to vest (in shares) 726,000   679,200   726,000 679,200    
Restricted stock unit activity, additional disclosures [Abstract]                
Weighted average grant date fair value, beginning of period (in dollars per share) $ 112.58   $ 102.39   $ 111.78 $ 90.43 $ 90.43  
Weighted average grant date fair value, granted (in dollars per share) 112.75   149.94   107.96 159.00    
Weighted average grant date fair value, vested (in dollars per share) 101.71   75.79   96.51 77.65    
Weighted average grant date fair value, forfeited (in dollars per share) 107.79   96.94   116.07 93.82    
Weighted average grant date fair value, end of period (in dollars per share) 114.00 $ 112.58 109.24 $ 102.39 114.00 109.24 $ 111.78 $ 90.43
Weighted average grant date fair value, unvested shares expected to vest (in dollars per share) $ 114.10   $ 109.42   $ 114.10 $ 109.42    
Weighted average remaining contractual life, unvested shares outstanding 1 year 10 months 29 days 2 years 2 years 2 months 25 days 2 years 1 month 22 days 1 year 10 months 29 days 2 years 2 months 27 days 2 years 12 days 2 years 4 months 18 days
Weighted average remaining contractual life, unvested shares outstanding, expected to vest 1 year 11 months   2 years 2 months 25 days   1 year 11 months 2 years 2 months 27 days    
Unamortized deferred compensation cost $ 32.8       $ 32.8      
Fair value of shares vested 9.8   $ 9.8   18.4 $ 14.4    
Share Based Compensation Arrangement By ShareBased Payment Award, Equity Instruments Other Than Options, Grants In Period, Weighted Average Grant Date Fair Value $ 3.0   $ 4.1   $ 30.0 $ 25.1