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Certain Employee Benefit and Stock Based Compensation Plans (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based compensation expense $ 13 $ 40
Unamortized share-based compensation expense related to nonvested equity compensation arrangements $ 314  
Period over which unamortized share-based compensation expense will be amortized. 5 years  
Nonvested Cash Payable Incentive Performance Units And Restricted Share Units    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Aggregate intrinsic value $ 71