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Stock-based Compensation (Details) - Restricted Stock Units (RSUs) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Dec. 31, 2015
Restricted stock unit activity [Roll Forward]        
Unvested at beginning of period (in shares) 750,900 706,000 706,000  
Granted (in shares) 140,900 200,800    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value $ 116.42 $ 107.12    
Vested (in shares) (103,200) (84,900)    
Forfeited (in shares) (2,900) (1,500)    
Unvested at end of period (in shares) 785,700 820,400 750,900 706,000
Unvested shares expected to vest (in shares) 785,700 799,000    
Restricted stock unit activity, additional disclosures [Abstract]        
Weighted average grant date fair value, beginning of period (in dollars per share) $ 113.97 $ 111.78 $ 111.78  
Weighted average grant date fair value, vested (in dollars per share) 89.44 91.06    
Weighted average grant date fair value, forfeited (in dollars per share) 115.93 87.86    
Weighted average grant date fair value, end of period (in dollars per share) 117.62 112.83 $ 113.97 $ 111.78
Weighted average grant date fair value, unvested shares expected to vest (in dollars per share) $ 117.62 $ 112.97    
Weighted average remaining contractual life, unvested shares outstanding 1 year 9 months 26 days 2 years 2 months 17 days 1 year 8 months 16 days 2 years 10 days
Weighted average remaining contractual life, unvested shares outstanding, expected to vest 1 year 9 months 26 days 2 years 2 months 21 days    
Unamortized deferred compensation cost $ 33.5