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Stock-based compensation (Details Textual) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Amount by which the pool of available shares will be reduced for each stock option granted 1    
The amount of common stock available under the plan for future grants and/or issuances (in shares) 36,000,000    
Description of vesting of restricted stock units and stock options RSUs and stock options generally vest in equal amounts on the second, third and fourth anniversaries of the grant date. RSUs    
Number of common shares issued for each performance unit earned 1    
RSUs [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
The number of shares by which the pool of available shares will be reduced for other types of awards granted 1.9    
Number of shares added back for tax withholding on full value awards 1.9    
Weighted average grant date fair value, granted (in usd per share) $ 163.99 $ 156.76 $ 166.74
Total fair value of units that vested during the year $ 182,000,000 $ 193,000,000 $ 206,000,000
Total unrecognized compensation cost related to nonvested awards $ 304,000,000    
Weighted average number of years over which compensation cost related to nonvested awards is expected to be recognized 1 year 9 months 18 days    
Units outstanding (in shares) 3,400,000 3,900,000  
Weighted-average grant date fair value $ 155.11 $ 141.07  
Units granted (in shares) 1,300,000    
Units, forfeited (in shares) 300,000    
Weighted average grant date fair value, forfeited (in usd per share) $ 149.79    
Stock options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expiration period of stock options from date of grant 10 years    
Total intrinsic value of stock options exercised during the year $ 60,000,000 $ 102,000,000 150,000,000
Actual tax benefits realized from tax deductions from option exercises $ 21,000,000 $ 37,000,000 $ 55,000,000
Performance units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
The number of shares by which the pool of available shares will be reduced for other types of awards granted 1.9    
Number of shares added back for tax withholding on full value awards 1.9    
Weighted average grant date fair value, granted (in usd per share) $ 178.87 $ 170.56 $ 182.55
Total fair value of units that vested during the year $ 219,000,000 $ 347,000,000 $ 0
Total unrecognized compensation cost related to nonvested awards $ 144,000,000    
Weighted average number of years over which compensation cost related to nonvested awards is expected to be recognized 1 year    
Period over which the grants of equity instruments vest 3 years    
Units outstanding (in shares) 2,200,000 2,800,000  
Weighted-average grant date fair value $ 177.16 $ 144.43  
Units granted (in shares) 800,000    
Units, forfeited (in shares) 100,000    
Weighted average grant date fair value, forfeited (in usd per share) $ 179.58