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Postretirement Plans (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]      
Amount by which fair value of plan assets exceeds market-related value of plan assets (MRVA) $ 2,352    
Derivative net notional amount for fixed income as percentage of total plan assets 6.00% 13.10%  
Derivative net notional amount for global equity, currency overlay and commodities as a percentage of total plan assets 1.20% 4.00%  
Expense for defined contribution plans $ 1,413 $ 768 $ 764
Pension Plans [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements and Curtailments (40) (290) (461)
Accumulated benefit obligation (ABO) for all pension plans $ 74,240 $ 72,330  
Defined benefit plan, actual plan asset allocations 100.00% 100.00%  
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year $ 500    
Pension Plans [Member] | Minimum [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year $ 200    
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% 28.00%  
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 $ 4 $ (10)  
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%