Consideration of Credit Risk and Concentrations (Details)		3 Months Ended		9 Months Ended
		Sep. 30, 2020 USD ($) Counterparty	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) Counterparty	Sep. 30, 2019 USD ($)
Impact of Derivative Activity [Abstract]
Fair value hedges		$ 0	$ 0	$ 0	$ 0
Other Derivative Instruments
Impact of Derivative Activity [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss		0	0	0	0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		(1,000,000)	(3,000,000)	(4,000,000)	(1,000,000)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net		0	0	0	0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)		(3,000,000)	(1,000,000)	(1,000,000)	(1,000,000)
Pre-tax gains (losses) recognized during the period in income		2,000,000	1,000,000	(7,000,000)	1,000,000
Other Derivative Instruments | Electric Commodity
Impact of Derivative Activity [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss		0		0	0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		(3,000,000)		(3,000,000)	4,000,000
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)	[1]	(3,000,000)	(1,000,000)	(6,000,000)
Pre-tax gains (losses) recognized during the period in income		0		0
Other Derivative Instruments | Natural Gas Commodity
Impact of Derivative Activity [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss		0	0	0	0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		2,000,000	(3,000,000)	(1,000,000)	(5,000,000)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net				0	0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	[2]			(5,000,000)	(1,000,000)
Pre-tax gains (losses) recognized during the period in income	[2]			(6,000,000)	(4,000,000)
Other Derivative Instruments | Commodity Trading
Impact of Derivative Activity [Abstract]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net		0	0	0	0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)		0	0	0	0
Pre-tax gains (losses) recognized during the period in income	[3]	2,000,000	1,000,000	(1,000,000)	5,000,000
Other Derivative Instruments | Natural Gas Commodity for Electric Generation
Impact of Derivative Activity [Abstract]
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		0	0	0	0
Designated as Hedging Instrument | Cash Flow Hedges
Impact of Derivative Activity [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss			(12,000,000)	(13,000,000)	(33,000,000)
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities			0	0	0
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net		1,000,000	1,000,000	5,000,000	3,000,000
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)		0	0	0	0
Pre-tax gains (losses) recognized during the period in income		0	0	0	0
Designated as Hedging Instrument | Cash Flow Hedges | Interest Rate
Impact of Derivative Activity [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss			(12,000,000)	(13,000,000)	(33,000,000)
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities			0	0	0
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	[4]	1,000,000	1,000,000	5,000,000	3,000,000
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)		0	0	0	0
Pre-tax gains (losses) recognized during the period in income		$ 0	$ 0	$ 0	$ 0
Credit Concentration Risk
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty		10		10
Credit Concentration Risk | Municipal or Cooperative Entities or Other Utilities [Member]
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty		8		8
Credit Concentration Risk | External Credit Rating, Investment Grade [Member]
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty		6		6
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties		$ 147,000,000		$ 147,000,000
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths)		57.00%		57.00%
Credit Concentration Risk | Internal Investment Grade [Member]
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty		3		3
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties		$ 38,000,000		$ 38,000,000
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths)		15.00%		15.00%
Credit Concentration Risk | External Credit Rating, Non Investment Grade [Member]
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty		1		1
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties		$ 9,000,000		$ 9,000,000
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths)		4.00%		4.00%

[1] Recorded to electric fuel and purchased power. These derivative settlement gains and losses are shared with electric customers through fuel and purchased energy cost-recovery mechanisms, and reclassified out of income as regulatory assets or liabilities, as appropriate.

[2] Amounts for both the three and nine months ended Sept. 30, 2020 and 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, as appropriate. Remaining settlement losses for both the three and nine months ended Sept. 30, 2020 and 2019 related to natural gas operations and were recorded to cost of natural gas sold and transported. These gains and losses are subject to cost-recovery mechanisms and reclassified out of income to a regulatory asset or liability, 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] Recorded to interest charges.