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Long-term and Short-term Debt (Credit Facilities) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Debt Instrument [Line Items]    
Collateral financing arrangements $ 4,139 $ 4,196
General Credit Facility Three [Member]    
Debt Instrument [Line Items]    
Expiration May 30, 2019  
Line of Credit Facility, Maximum Borrowing Capacity $ 4,000  
Letters of Credit Issued 484  
Collateral financing arrangements 0  
Unused Commitments $ 3,516