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Share-Based Incentive Compenstaion (details) - Cost Recognition - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based compensation      
Total compensation cost recognized in earnings for all share-based incentive compensation awards $ 136 $ 155 $ 141
Tax benefit recognized in earnings related to compensation costs 45 52 47
Total unrecognized compensation cost related to all nonvested share-based incentive compensation awards $ 143    
Weighted-average period of recognition for unrecognized compensation cost (in years) 1 year 9 months 18 days    
Cash received from the exercise of employee stock options under share-based compensation plans $ 173 332 183
Tax benefit for tax deductions from employee stock options exercised $ 31 58  
Minimum [Member]      
Share-based compensation      
Estimated annual forfeiture rate reflected in share-based compensation cost (percent) 3.00%    
Maxiumum [Member]      
Share-based compensation      
Estimated annual forfeiture rate reflected in share-based compensation cost (percent) 4.00%    
Performance shares [Member]      
Share-based compensation      
Estimated attainment of performance shares at grant date (percent) 100.00%    
Compensation cost adjustments for the updated estimate of performance shares due to attaining certain performance levels $ 3 $ 11 $ 8