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Fair Value Measurements and Credit Concentration (Net Asset and Liability Measured on Recurring Basis Level 3 Unobservable Inputs Reconciliation Rollforward) (Details) - Level 3 - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Reconciliation of Total    
Balance at beginning of period $ 60 $ 119
Total realized/unrealized gains (losses) included in earnings 60 (17)
Settlements (45) 30
Transfers into Level 3 1 4
Transfers out of Level 3 (2) 1
Balance at end of period 74 137
Auction Rate Securities    
Reconciliation of Auction Rate Securities    
Balance at beginning of period 6 8
Balance at end of period 6 8
Energy Commodities, net    
Reconciliation of Energy Commodities, net    
Balance at beginning of period 54 111
Total realized/unrealized gains (losses) included in earnings 60 (17)
Settlements (45) 30
Transfers into Level 3 1 4
Transfers out of Level 3 (2) 1
Balance at end of period $ 68 $ 129