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Assets and Liabilities with Recurring Fair Value Measurements (Textuals) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Dec. 31, 2014
Dec. 31, 2013
Fair Value Measurement [Domain]            
Assets and Liabilities with Recurring Fair Value Measurements (Textuals) [Abstract]            
Cash and Cash Equivalents, at Carrying Value $ 367   $ 367   $ 679  
Restricted Cash and Cash Equivalents 210   210   217  
Balance, beginning of period 203 $ 8 85 $ 14    
Included in operating revenues [1] 45 (7) 176 (15)    
Fair Value, Assets Measured with Unobservable Inputs on Recurring Basis, Gain (Loss) Included In Fuel And Purchased Energy Expense [2] 0 0 2 6    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 2 0 4 0    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements (10) (3) (21) (6)    
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount 0 0 0 0    
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount 0 0 0 0    
Transfers into level 3 [3],[4] 0 2 0 0    
Transfers out of Level 3 [4],[5] 3 (9) (3) (8)    
Balance, end of period 243 (9) 243 (9)    
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 45 (7) 178 (9)    
Cash and Cash Equivalents, at Carrying Value 422 $ 442 422 $ 442 717 $ 941
Restricted Cash and Cash Equivalents $ 210   $ 210   $ 244  
[1] For power contracts and other power-related products, included on our Consolidated Condensed Statements of Operations.
[2] For natural gas contracts, swaps and options, included on our Consolidated Condensed Statements of Operations.
[3] There were no transfers out of level 2 into level 3 for each of the three and six months ended June 30, 2015 and for the six months ended June 30, 2014. We had $2 million in gains transferred out of level 2 into level 3 for the three months ended June 30, 2014, due to changes in market liquidity in various power markets.
[4] We transfer amounts among levels of the fair value hierarchy as of the end of each period. There were no transfers into or out of level 1 for each of the three and six months ended June 30, 2015 and 2014.
[5] We had $3 million in losses and $(9) million in gains transferred out of level 3 into level 2 for the three months ended June 30, 2015 and 2014, respectively, and $(3) million and $(8) million in gains transferred out of level 3 into level 2 for the six months ended June 30, 2015 and 2014, respectively, due to changes in market liquidity in various power markets.