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Incentive Plans Incentive Plans (Stock-based compensation) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Deferred Compensation Arrangement with Individual, Compensation Expense $ 26 $ 27 $ 87 $ 89  
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized 131   $ 131    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period     3 years    
Restricted Stock Liability Awards [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized 21   $ 21    
Accounts Payable, Current $ 11   $ 11   $ 23