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Incentive Plans (Narrative) (Details) - USD ($)
shares in Millions, $ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Compensation cost related to unvested restricted performace shares, executive continuity awards and restricted stock attributable to future performance, net of estimated forfeitures $ 66    
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 6 days    
Restricted performance share plan      
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%    
Share-based Payment Arrangement [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares available for grant 8.5 9.8 11.9