Assets and Liabilities with Recurring Fair Value Measurements (Textuals) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Fair Value Measurement [Domain]
Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract]
Cash and Cash Equivalents, at Carrying Value		$ 20		$ 21
Restricted Cash and Cash Equivalents		111		110
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs		197	$ 416
Included in operating revenues	[1]	(57)	113
Fair Value, Assets Measured with Unobservable Inputs on Recurring Basis, Gain (Loss) Included In Fuel And Purchased Energy Expense	[2]	(2)	13
Amount of Change in Collateral of Financial Instruments Classified as Derivative Asset (Liability)		(2)	(9)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		4	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		(14)	(26)
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount		0	0
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount		0	0
Transfers into level 3	[3],[4]	6	(7)
Transfers out of Level 3	[4],[5]	3	150
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs		129	350
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)		(59)	$ 126
Cash and Cash Equivalents, at Carrying Value		215		284
Restricted Cash and Cash Equivalents		156		159
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		104		44
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		117		18
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		$ (2)		$ 0

[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] We had $6 million in gains and $(7) million in losses transferred out of level 2 into level 3 for the three months ended March 31, 2018 and 2017, 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 months ended March 31, 2018 and 2017.

[5] We had $3 million and $150 million in gains transferred out of level 3 into level 2 for the three months ended March 31, 2018 and 2017, respectively, due to changes in market liquidity in various power markets.