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Share-based payment arrangements - WARs (Details 2) - WARs - USD ($)
shares in Millions, $ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award      
Vesting period 3 years    
Compensation cost for cash-settled WARs     $ 26
Income (expense) related to the cash-settled call options $ (12) $ (11) 16
Aggregate fair value of outstanding WARs $ 13 $ 33  
Summary of the activity related to share-based compensation arrangements.      
Beginning of period (in number of shares) 61.2    
Granted (in number of WARs) 10.5    
Exercised (in number of WARs) (15.9)    
Forfeited (in number of WARs) (0.6)    
End of period (in number of shares) 55.2 61.2  
Exercisable (in number of WARs) 20.2    
Aggregate fair value at date of grant of WARs     13
Share-based liabilities paid upon exercise of WARs $ 9   $ 9