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Incentive Plans (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock rights and stock units 5,857,733 7,275,934 8,117,489 8,660,855
Compensation cost related to unvested restricted performace shares, executive continuity awards and restricted stock attributable to future performance, net of estimated forfeitures $ 126      
Compensation cost related to unvested restricted performace shares, executive continuity awards and restricted stock attributable to future performance, net of estimated forfeitures, weighted-average period (in years) 1 year 7 months 12 days      
Stock Compensation Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares available for grant 16,200,000 16,300,000 17,800,000  
Performance Shares [Member] | Other Participants [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Weight of return on investment (ROI) on awards 75.00%      
Weight of total shareholder return (TSR) on awards 25.00%      
Performance Shares [Member] | Officer [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Weight of return on investment (ROI) on awards 50.00%      
Weight of total shareholder return (TSR) on awards 50.00%