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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, 2019
Mar. 31, 2019
Jun. 30, 2018
Mar. 31, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Restricted stock unit activity [Roll Forward]                
Unvested at beginning of period (in shares) 527,000.0 652,700 658,000.0 727,700 652,700 727,700 727,700  
Granted (in shares) 204,300   100,700   279,000.0 220,000.0    
Vested (in shares) (26,600)   (4,300)   (222,100) (178,100)    
Forfeited (in shares) (8,400)   (4,100)   (13,300) (19,300)    
Unvested at end of period (in shares) 696,300 527,000.0 750,300 658,000.0 696,300 750,300 652,700 727,700
Restricted stock unit activity, additional disclosures [Abstract]                
Weighted average grant date fair value, beginning of period (in dollars per share) $ 140.55 $ 131.32 $ 126.31 $ 118.96 $ 131.32 $ 118.96 $ 118.96  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value 150.03   164.04   150.78 162.33    
Weighted average grant date fair value, vested (in dollars per share) 118.02   172.95   115.17 122.42    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value 136.19   129.03   132.56 125.56    
Weighted average grant date fair value, end of period (in dollars per share) $ 144.17 $ 140.55 $ 131.09 $ 126.31 $ 144.17 $ 131.09 $ 131.32 $ 118.96
Weighted average remaining contractual life, unvested shares outstanding 2 years 3 months 2 years 1 month 24 days 2 years 4 months 9 days 2 years 2 months 12 days 2 years 3 months 2 years 4 months 9 days 2 years 7 days 1 year 2 months 26 days
Unamortized deferred compensation cost $ 57.5       $ 57.5