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Benefit Plans (Narrative) (Details) - USD ($)
$ in Millions
1 Months Ended 3 Months Ended 9 Months Ended
Sep. 30, 2016
Mar. 31, 2016
Sep. 30, 2016
Sep. 30, 2015
Defined Benefit Plan Disclosure [Line Items]        
Increase to liabilities     $ (56) $ 78
Pension [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Increase to liabilities   $ 250    
Voluntary employer contributions $ 500      
Additional employer contributions     20  
Pension [Member] | Maximum [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Anticipated additional employer contributions     $ 15  
Postretirement [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Increase to liabilities   $ 70