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Postretirement Plans (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]      
Amount by which fair value of plan assets exceeds market-related value of plan assets (MRVA) $ 2,731    
Derivative net notional amount for fixed income as percentage of total plan assets 13.10% 3.50%  
Derivative net notional amount for global equity, currency overlay and commodities as a percentage of total plan assets 4.00% 2.00%  
Expense for defined contribution plans $ 768 $ 764 $ 742
Pension Plans [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements and Curtailments (290) (461) $ (104)
Accumulated benefit obligation (ABO) for all pension plans $ 72,330 $ 75,655  
Defined benefit plan, actual plan asset allocations 100.00% 100.00%  
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year $ 100    
Pension Plans [Member] | Fixed Income Securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, actual plan asset allocations 48.00% 48.00%  
Pension Plans [Member] | Equity Securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, actual plan asset allocations 28.00% 29.00%  
Pension Plans [Member] | Level 3 [Member] | Fixed Income Securities [Member] | Mortgage backed and asset backed      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Actual Return on Plan Assets Still Held $ (10) $ 8  
Other Postretirement Benefits Plans [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements and Curtailments $ (10) $ (1)
Other Postretirement Benefits Plans [Member] | Fixed Income Securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, actual plan asset allocations 40.00%    
Other Postretirement Benefits Plans [Member] | Equity Securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, actual plan asset allocations 60.00%