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Postretirement Plans (Narrative) (Details) - USD ($)
$ in Millions
1 Months Ended 12 Months Ended
Aug. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]        
Amount by which fair value of plan assets exceeds market-related value of plan assets (MRVA)   $ 2,260    
Derivative net notional amount for fixed income as percentage of total plan assets   5.00% 6.00%  
Derivative net notional amount for global equity, currency overlay and commodities as a percentage of total plan assets   6.90% 1.20%  
Company contribution   $ 3,500    
Expense for defined contribution plans   1,522 $ 1,413 $ 768
Pension Plans [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Accumulated benefit obligation (ABO) for all pension plans   77,414 74,240  
Company contribution   $ 4,025 $ 113  
Defined benefit plan, actual plan asset allocations   100.00% 100.00%  
Pension Plans [Member] | Fixed Income Securities [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined benefit plan, actual plan asset allocations   46.00% 48.00%  
Pension Plans [Member] | Equity Securities [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined benefit plan, actual plan asset allocations   31.00% 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, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held   $ 6 $ 4  
Other Postretirement Benefits Plans [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Company contribution   $ 6 $ 6  
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%    
Cash Contribution [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Company contribution $ 500      
Non-Cash Contribution [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Company contribution $ 3,500