Qualifying Cash Flow Hedges (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Financial Impact of Qualifying Cash Flow Hedges on Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax, Beginning Balance	$ (24)		$ (26)		$ (27)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	1		2		1
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax, Ending Balance	(23)		(24)		(26)
Impact of Derivative Activity [Abstract]
Fair Value Hedges, Net	0		0		0
Not Designated as Hedging Instrument
Impact of Derivative Activity [Abstract]
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax	0		0		0
Derivative Instruments Gain (Loss) Reclassified To Regulatory Assets And Liabilities Net	(10)		(5)		8
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	0		0		0
Pre-tax gains (losses) reclassified into income during the period from regulatory assets and (liabilities)	(8)		(1)		(4)
Derivative, Gain (Loss) on Derivative, Net	(5)		(1)		0
Not Designated as Hedging Instrument | Commodity Trading Contract
Impact of Derivative Activity [Abstract]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	0		0		0
Pre-tax gains (losses) reclassified into income during the period from regulatory assets and (liabilities)	0		0		0
Derivative, Gain (Loss) on Derivative, Net	3	[1]	3	[2]	3	[3]
Not Designated as Hedging Instrument | Natural Gas Commodity Contract
Impact of Derivative Activity [Abstract]
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax	0		0		0
Derivative Instruments Gain (Loss) Reclassified To Regulatory Assets And Liabilities Net	(10)		(5)		8
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	0		0		0
Pre-tax gains (losses) reclassified into income during the period from regulatory assets and (liabilities)	(8)	[4]	(1)	[5]	4	[6]
Derivative, Gain (Loss) on Derivative, Net	(8)	[4]	(4)	[5]	(3)	[6]
Not Designated as Hedging Instrument | Natural Gas Commodity for Electric Generation [Member]
Impact of Derivative Activity [Abstract]
Derivative Instruments Gain (Loss) Reclassified To Regulatory Assets And Liabilities Net	0		0		1
Designated as Hedging Instrument | Cash Flow Hedges
Impact of Derivative Activity [Abstract]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(1)		(2)		(1)
Pre-tax gains (losses) reclassified into income during the period from regulatory assets and (liabilities)	0		0		0
Derivative, Gain (Loss) on Derivative, Net	0		0		0
Designated as Hedging Instrument | Cash Flow Hedges | Interest Rate Contract
Impact of Derivative Activity [Abstract]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(1)	[7]	(2)	[8]	(1)	[9]
Pre-tax gains (losses) reclassified into income during the period from regulatory assets and (liabilities)	0		0		0
Derivative, Gain (Loss) on Derivative, Net	$ 0		$ 0		$ 0

[1] Recorded to electric operating revenues. Portions of these gains and losses are subject to sharing with electric customers through margin-sharing mechanisms and deducted from gross revenue, as appropriate.

[2] Recorded to electric operating revenues. Portions of these gains and losses are subject to sharing with electric customers through margin-sharing mechanisms and deducted from gross revenue, as appropriate.

[3] Recorded to electric operating revenues. Portions of these gains and losses are subject to sharing with electric customers through margin-sharing mechanisms and deducted from gross revenue, as appropriate.

[4] Amounts for the year ended Dec. 31, 2020, included no settlement gains or losses on derivatives entered to mitigate natural gas price risk for electric generation recorded to electric fuel and purchased power, subject to cost-recovery mechanisms and reclassified to a regulatory asset or liability, as appropriate. Remaining settlement losses for the years ended Dec. 31, 2020, relate to natural gas operations and are recorded to cost of natural gas sold and transported. These losses are subject to cost-recovery mechanisms and reclassified out of income to a regulatory asset or liability, as appropriate.

[5] Amounts for the year ended Dec. 31, 2019, included no settlement gains or losses on derivatives entered to mitigate natural gas price risk for electric generation recorded to electric fuel and purchased power, subject to cost-recovery mechanisms and reclassified to a regulatory asset or liability, as appropriate. Remaining settlement losses for the years ended Dec. 31, 2019, relate to natural gas operations and are recorded to cost of natural gas sold and transported. These losses are subject to cost-recovery mechanisms and reclassified out of income to a regulatory asset or liability, as appropriate.

[6] Amounts for the year ended Dec. 31, 2018, included $1 million of settlement losses on derivatives entered to mitigate natural gas price risk for electric generation recorded to electric fuel and purchased power, subject to cost-recovery mechanisms and reclassified to a regulatory asset or liability, as appropriate. Remaining settlement losses for the years ended Dec. 31, 2018, relate to natural gas operations and are recorded to cost of natural gas sold and transported. These losses are subject to cost-recovery mechanisms and reclassified out of income to a regulatory asset or liability, as appropriate

[7] Recorded to interest charges.

[8] Recorded to interest charges.

[9] Recorded to interest charges.