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STOCK-BASED EMPLOYEE COMPENSATION (Restricted Stock Units and Performance Share Units) (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Restricted Stock Units and Performance Share Units      
Share-Based Awards - Shares      
Unvested at December 31, 2017 524,696    
Granted (in shares) 146,112    
Vested (in shares) (146,766)    
Performance Share Units Adjustment (in shares) 6,487    
Forfeited (in shares) (6,718)    
Unvested at March 31, 2018 523,811 524,696  
Share-Based Awards - Weighted Average Grant Date Fair Value      
Unvested at December 31, 2017 $ 1,431.88    
Granted (in dollars per share) 2,033.79    
Vested (in dollars per share) 1,271.70    
Performance Share Units Adjustment (in dollars per share) 1,654.15    
Forfeited (in shares) 1,594.90    
Unvested at March 31, 2018 $ 1,645.32 $ 1,431.88  
Total unrecognized estimated compensation expense, unvested share-based awards $ 574.0    
Total future compensation cost related to unvested share-based awards, expected period of recognition 2 years 4 months    
Restricted Stock Units (RSUs)      
Share-Based Awards - Shares      
Granted (in shares) 97,006    
Share-Based Awards - Weighted Average Grant Date Fair Value      
Granted (in dollars per share) $ 2,033.79    
Vesting period (in years) 3 years    
Grant date fair value $ 197.3    
Performance Share Units 2018 Grants      
Share-Based Awards - Shares      
Granted (in shares) 49,106    
Share-Based Awards - Weighted Average Grant Date Fair Value      
Granted (in dollars per share) $ 2,033.79    
Grant date fair value $ 99.9    
Estimated number of probable shares to be issued (in shares) 49,106    
Maximum shares that could be issued (in shares) 98,212    
Minimum shares that could be issued (in shares) 37,796    
Performance Share Units 2017 Grants      
Share-Based Awards - Shares      
Granted (in shares)   73,893  
Unvested at March 31, 2018 67,946    
Share-Based Awards - Weighted Average Grant Date Fair Value      
Granted (in dollars per share)   $ 1,735.10  
Grant date fair value   $ 128.2  
Estimated number of probable shares to be issued (in shares) 85,639    
Maximum shares that could be issued (in shares) 135,892    
Minimum shares that could be issued (in shares) 54,689    
Performance Share Units 2016 Grants      
Share-Based Awards - Shares      
Granted (in shares)     85,735
Unvested at March 31, 2018 70,750    
Share-Based Awards - Weighted Average Grant Date Fair Value      
Granted (in dollars per share)     $ 1,302.25
Grant date fair value     $ 111.7
Estimated number of probable shares to be issued (in shares) 113,660    
Maximum shares that could be issued (in shares) 157,699    
Minimum shares that could be issued (in shares) 42,035