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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, 2018
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Restricted stock unit activity [Roll Forward]        
Unvested at beginning of period (in shares) 727,700 750,900 750,900  
Granted (in shares) 119,300 140,900    
Vested (in shares) (173,800) (103,200)    
Forfeited (in shares) (15,200) (2,900)    
Unvested at end of period (in shares) 658,000 785,700 727,700 750,900
Restricted stock unit activity, additional disclosures [Abstract]        
Weighted average grant date fair value, beginning of period (in dollars per share) $ 118.96 $ 113.97 $ 113.97  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value 160.86 116.42    
Weighted average grant date fair value, vested (in dollars per share) 121.15 89.44    
Weighted average grant date fair value, forfeited (in dollars per share) 124.62 115.93    
Weighted average grant date fair value, end of period (in dollars per share) $ 126.31 $ 117.62 $ 118.96 $ 113.97
Weighted average remaining contractual life, unvested shares outstanding 2 years 2 months 13 days 1 year 9 months 26 days 1 year 2 months 28 days 1 year 8 months 17 days
Unamortized deferred compensation cost $ 33.6