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Stock-Based Compensation, Restricted Stock Units (Details) - Restricted Stock [Member] - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 3 years    
Weighted average grant date fair value (in dollars per share) $ 169.23 $ 116.72 $ 128.59
Length of service after grant date 1 year    
Retirement eligibility, employee age 60 years    
Service period, minimum 5 years    
Retirement eligibility, employee age with no service requirement 65 years    
Unrecognized compensation costs related to stock awards $ 34    
Unrecognized compensation costs related to equity awards after income taxes $ 26    
Weighted average remaining period for recognition of unrecognized costs related to equity awards 1 year 1 month 6 days    
Fair value of restricted stock unit awards vested $ 53 $ 57 $ 82
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Beginning Balance, Nonvested (in shares) 977,900    
Granted (in shares) 305,600    
Vested (in shares) (318,400)    
Forfeited (in shares) (62,400)    
Ending Balance, Nonvested (in shares) 902,700 977,900  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]      
Beginning Balance, Nonvested (in dollars per share) $ 119.55    
Granted (in dollars per share) 169.23 $ 116.72 $ 128.59
Vested (in dollars per share) 114.46    
Forfeited (in dollars per share) 134.56    
Ending Balance, Nonvested (in dollars per share) $ 137.12 $ 119.55  
Directors [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period 1 year