v3.8.0.1
Assets and Liabilities with Recurring Fair Value Measurements (Textuals) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Fair Value Measurement [Domain]            
Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract]            
Cash and Cash Equivalents, at Carrying Value $ 32   $ 32   $ 26  
Restricted Cash and Cash Equivalents 141   141   127  
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs 303 $ (61) 416 $ (46)    
Included in operating revenues [1] 26 30 125 9    
Fair Value, Assets Measured with Unobservable Inputs on Recurring Basis, Gain (Loss) Included In Fuel And Purchased Energy Expense [2] (12) (31) (1) (24)    
Amount of Change in Collateral of Financial Instruments Classified as Derivative Asset (Liability) 4 (2) (4) 0    
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases 1 1 2 4    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements (40) 15 (129) (4)    
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount 0 0 0 0    
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount 0 0 0 0    
Transfers into level 3 [3],[4] 3 1 (5) 0    
Transfers out of Level 3 [4],[5] (3) 75 (122) 89    
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs 282 28 282 28    
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 14 (1) 124 (15)    
Cash and Cash Equivalents, at Carrying Value 426 $ 561 426 $ 561 418 $ 906
Restricted Cash and Cash Equivalents 222   222   188  
Fair Value, Inputs, Level 1 [Member]            
Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract]            
Derivative, Collateral, Right to Reclaim Cash, Net 51   51   28  
Fair Value, Inputs, Level 2 [Member]            
Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract]            
Derivative, Collateral, Right to Reclaim Cash, Net 17   17   27  
Fair Value, Inputs, Level 3 [Member]            
Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract]            
Derivative, Collateral, Right to Reclaim Cash, Net $ 14   $ 14   $ 17  
[1] For power contracts and other power-related products, included on our Consolidated Condensed Statements of Operations.
[2] For natural gas and power contracts, swaps and options, included on our Consolidated Condensed Statements of Operations.
[3] There were $3 million and $1 million in gains transferred out of level 2 into level 3 for the three months ended September 30, 2017 and 2016, and $(5) million and nil in losses transferred out of level 2 into level 3 for the nine months ended September 30, 2017 and 2016, respectively, 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 nine months ended September 30, 2017 and 2016.
[5] We had $3 million in gains and $(75) million in losses transferred out of level 3 into level 2 for the three months ended September 30, 2017 and 2016, respectively, and $122 million in gains and $(89) million in losses transferred out of level 3 into level 2 for the nine months ended September 30, 2017 and 2016, respectively, due to changes in market liquidity in various power markets.