Assets and Liabilities with Recurring Fair Value Measurements (Textuals) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value Measurement [Domain]
Fair Value Disclosures [Abstract]
Cash and Cash Equivalents, at Carrying Value		$ 573	$ 23
Cash Equivalents Included In Restricted Cash, Fair Value Disclosure		211	145
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs		(8)	197	$ 416
Fair Value, Net Derivative Asset (Liability), Recurring Basis, Still Held, Unrealized Gain (Loss)		150	(133)	82
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount		0	0	0
Included in operating revenues	[1]	171	(88)	32
Included in fuel and purchased energy expense	[2]	(21)	(45)	50
Amount of Change in Collateral of Financial Instruments Classified as Derivative Asset (Liability)		0	0	(17)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		5	18	4
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Issues		(3)	(2)	(1)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Settlements		56	(86)	(179)
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount		0	0	0
Transfers into level 3	[3],[4]	(1)	0	2
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3	[4],[5]	30	2	106
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs		171	(8)	197
Cash and Cash Equivalents, at Carrying Value		1,131	205
Cash Equivalents Included In Restricted Cash, Fair Value Disclosure		345	201
Fair Value, Inputs, Level 1 [Member]
Fair Value Disclosures [Abstract]
Derivative, Collateral, Right to Reclaim Cash, Net		112	(1)
Transfer to Level 2 [Member]
Purchases, issuances and settlements:
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3		30	2	104
Fair Value Disclosures [Abstract]
Derivative, Collateral, Right to Reclaim Cash, Net		2	48
Fair Value, Inputs, Level 3 [Member]
Fair Value Disclosures [Abstract]
Derivative, Collateral, Right to Reclaim Cash, Net		$ 0	$ 0
Other Assets [Member]
Purchases, issuances and settlements:
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3				$ 2

[1] For power contracts and other power-related products, included on our Consolidated Statements of Operations.

[2] For natural gas and power contracts, swaps and options, included on our Consolidated Statements of Operations.

[3] We had $1 million in gains, nil and $(2) million in losses transferred out of level 2 into level 3 for the years ended December 31, 2019, 2018 and 2017, respectively.

[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 during the years ended December 31, 2019, 2018 and 2017.

[5] We had $30 million, $2 million and $104 million in gains transferred out of level 3 into level 2 during the years ended December 31, 2019, 2018 and 2017, respectively, due to changes in market liquidity in various power markets and $2 million in gains transferred out of level 3 during the years ended December 31, 2017, to other assets following the election of the normal purchase normal sales exemption and the discontinuance of derivative accounting treatment as of the date of this election for certain commodity contracts.