Income Taxes (Details) - USD ($)		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019		Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018
Income Tax Examination [Line Items]
Present value of minimum obligation						$ 529,000,000
Tax Credit Valuation Allowance		$ (8,000,000)	$ (8,000,000)	[1]
Income tax expense (benefit)		45,000,000	80,000,000		$ 113,000,000
Deferred tax expense (benefit) excluding selected items		46,000,000	132,000,000		75,000,000
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities		(43,000,000)	(35,000,000)		(51,000,000)
Income Tax Expense (Benefit), Intraperiod Tax Allocation		(1,000,000)	$ 0		$ 0
Deferred Tax Assets, Tax Deferred Expense						2,000,000		$ 97,000,000			$ 24,000,000
Tax Credit Carryforward, Amount						143,000,000		83,000,000
Potential Tax Adjustments		$ 0
Federal detriment						5,000,000		5,000,000
Federal Benefit						2,000,000		2,000,000
Deferred Tax Assets, Operating Loss Carryforwards, State and Local						190,000,000		388,000,000
state tax credit carryforward, net of federal detirment	[2]					18,000,000		18,000,000
valuation allowances for state credit carryforwards, net of federal benefit	[3]					(8,000,000)		(8,000,000)
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%	21.00%		21.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent		3.60%	3.60%		3.70%
Effective Income Tax Rate Reconciliation, Tax Credit, Percent		10.30%	7.50%		0.60%
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent	[4]	(5.00%)	(3.30%)		(4.50%)
Effective Income Tax Rate Reconciliation,Other Reconciling Items, Percent		(1.10%)	(1.30%)		(0.60%)
Amortization of excess nonplant deferred taxes		(0.20%)	(0.20%)		(1.40%)
Effective Income Tax Rate Reconcilation, Change in unrecognized tax benefits, percent		(0.20%)	0.30%		0.10%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent		(0.70%)	(0.40%)		(0.70%)
Effective Income Tax Rate Reconciliation, Percent		7.10%	12.20%		17.00%
Tax Audits [Abstract]
Tax Credit Carryforward, Amount						143,000,000		83,000,000
Federal detriment						5,000,000		5,000,000
Potential Federal Tax Adjustments		$ 0
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Allowance for Doubtful Accounts						8,000,000		5,000,000	[1]
Payable for interest related to unrecognized tax benefits						0	$ (1,000,000)	(1,000,000)		$ (1,000,000)	(1,000,000)	$ 0
Unrecognized Tax Benefits, Interest on Income Taxes Expense		(1,000,000)	$ 0		$ (1,000,000)
Unrecognized tax benefit — Permanent tax positions						7,000,000		7,000,000
Unrecognized Tax Benefits That Would Not Impact Effective Tax Rate						2,000,000		5,000,000
Unrecognized Tax Benefits		12,000,000	10,000,000		10,000,000	9,000,000	12,000,000	12,000,000		10,000,000	10,000,000	10,000,000
Unrecognized Tax Benefits, Income Tax Penalties Expense		0	0		0
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions		2,000,000	1,000,000		1,000,000
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions		6,000,000	1,000,000		0
Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions		11,000,000	0		0
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		0	0		1,000,000
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions						7,000,000		7,000,000
Unrecognized tax benefit — Temporary tax positions						2,000,000		5,000,000
Total unrecognized tax benefit		12,000,000	10,000,000		10,000,000	9,000,000	$ 12,000,000	12,000,000		$ 10,000,000	10,000,000	$ 10,000,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at Jan. 1		12,000,000	10,000,000
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions		6,000,000	1,000,000		0
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions		(11,000,000)	0		0
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		0	0		(1,000,000)
Balance at Dec. 31		$ 9,000,000	$ 12,000,000		$ 10,000,000
Tax Benefits Associated With Nol And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards						(8,000,000)		(8,000,000)
Net Deferred Tax Liability associated with the Unrecognized Tax Benefit Amounts and Related NOLs and Tax Credit Carryforwards						(6,000,000)		(5,000,000)
Deferred Tax Assets, Operating Loss Carryforwards, State and Local						190,000,000		388,000,000
state tax credit carryforward, net of federal detirment	[2]					18,000,000		18,000,000
valuation allowances for state credit carryforwards, net of federal benefit	[3]					(8,000,000)		(8,000,000)
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Allowance for Doubtful Accounts						8,000,000		5,000,000	[1]
Upper bound of decrease in unrecognized tax benefit that is reasonably possible						3,000,000
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%	21.00%		21.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent		3.60%	3.60%		3.70%
Effective Income Tax Rate Reconciliation, Tax Credit, Percent		(10.30%)	(7.50%)		(0.60%)
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent	[4]	5.00%	3.30%		4.50%
Amortization of excess nonplant deferred taxes		(0.20%)	(0.20%)		(1.40%)
Effective Income Tax Rate Reconciliation,Other Reconciling Items, Percent		1.10%	1.30%		0.60%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent		(0.70%)	(0.40%)		(0.70%)
Effective Income Tax Rate Reconcilation, Change in unrecognized tax benefits, percent		(0.20%)	0.30%		0.10%
Effective Income Tax Rate Reconciliation, Percent		7.10%	12.20%		17.00%
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Income Tax Expense (Benefit)		$ 45,000,000	$ 80,000,000		$ 113,000,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred tax expense (benefit) excluding selected items		46,000,000	132,000,000		75,000,000
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities		(43,000,000)	(35,000,000)		(51,000,000)
Income Tax Expense (Benefit), Intraperiod Tax Allocation		(1,000,000)	0		0
Deferred Tax Assets, Tax Deferred Expense						2,000,000		97,000,000			24,000,000
Valuation Allowance [Line Items]
Present value of minimum obligation						529,000,000
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Allowance for Doubtful Accounts						8,000,000		5,000,000	[1]
Tax Credit Valuation Allowance		(8,000,000)	(8,000,000)	[1]
Deferred Tax Assets, Tax Deferred Expense						2,000,000		97,000,000			$ 24,000,000
Net Deferred Tax Liablility [Member]
Income Tax Examination [Line Items]
Differences between book and tax basis of property		2,132,000,000	2,039,000,000	[1]
Regulatory Assets						257,000,000		253,000,000	[1]
Operating Lease Assets						129,000,000		148,000,000	[1]
Deferred tax liability - Pension expense						19,000,000		22,000,000	[1]
Deferred Tax Assets, Tax Deferred Expense, Other						6,000,000		7,000,000	[1]
Deferred Tax Liabilities, Tax Deferred Income						2,543,000,000		2,469,000,000	[1]
Regulatory liabilities						319,000,000		327,000,000	[1]
Present value of minimum obligation						129,000,000		148,000,000	[1]
Deferred ITCs		5,000,000	6,000,000	[1]
Deferred Tax Assets, Other						25,000,000		25,000,000	[1]
Deferred Tax Liabilities, Net						1,897,000,000		1,851,000,000	[1]
Deferred Tax Assets, Tax Deferred Expense						646,000,000		618,000,000	[1]
Deferred Tax Assets, Operating Loss Carryforwards						161,000,000		101,000,000	[1]
Tax Audits [Abstract]
Operating Loss Carryforwards						7,000,000		14,000,000	[1]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Tax Assets, Tax Deferred Expense						646,000,000		618,000,000	[1]
Valuation Allowance [Line Items]
Differences between book and tax basis of property		2,132,000,000	2,039,000,000	[1]
Regulatory Assets						257,000,000		253,000,000	[1]
Operating Lease Assets						129,000,000		148,000,000	[1]
Deferred tax liability - Pension expense						19,000,000		22,000,000	[1]
Deferred Tax Assets, Tax Deferred Expense, Other						6,000,000		7,000,000	[1]
Deferred Tax Liabilities, Tax Deferred Income						2,543,000,000		2,469,000,000	[1]
Regulatory liabilities						319,000,000		327,000,000	[1]
Deferred Tax Assets, Operating Loss Carryforwards						161,000,000		101,000,000	[1]
Present value of minimum obligation						129,000,000		148,000,000	[1]
Operating Loss Carryforwards						7,000,000		14,000,000	[1]
Deferred ITCs		5,000,000	6,000,000	[1]
Deferred Tax Assets, Other						25,000,000		25,000,000	[1]
Deferred Tax Assets, Tax Deferred Expense						646,000,000		618,000,000	[1]
Deferred Tax Liabilities, Net						$ 1,897,000,000		$ 1,851,000,000	[1]
income tax expense [Member]
Income Tax Examination [Line Items]
Deferred ITCs		(2,000,000)	(2,000,000)		3,000,000
Current Federal Tax Expense (Benefit)		44,000,000	(9,000,000)		79,000,000
Current State and Local Tax Expense (Benefit)		4,000,000	(5,000,000)		14,000,000
Current Change In Unrecognized Tax Expense (Benefit)		(3,000,000)	(1,000,000)		(1,000,000)
Deferred Federal Income Tax Expense (Benefit)		(26,000,000)	61,000,000		5,000,000
Deferred State and Local Income Tax Expense (Benefit)		26,000,000	33,000,000		17,000,000
Deferred Change In Unrecognized Tax Expense (Benefit)		2,000,000	3,000,000		2,000,000
Income tax expense (benefit)		45,000,000	80,000,000		113,000,000
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)		44,000,000	(9,000,000)		79,000,000
Current State and Local Tax Expense (Benefit)		4,000,000	(5,000,000)		14,000,000
Current Change In Unrecognized Tax Expense (Benefit)		(3,000,000)	(1,000,000)		(1,000,000)
Deferred Federal Income Tax Expense (Benefit)		(26,000,000)	61,000,000		5,000,000
Deferred State and Local Income Tax Expense (Benefit)		26,000,000	33,000,000		17,000,000
Deferred Change In Unrecognized Tax Expense (Benefit)		2,000,000	3,000,000		2,000,000
Income Tax Expense (Benefit)		45,000,000	80,000,000		113,000,000
Valuation Allowance [Line Items]
Deferred ITCs		$ (2,000,000)	$ (2,000,000)		$ 3,000,000

[1] Prior periods have been reclassified to conform to current year presentation.

[2] State tax credit carryforwards are net of federal detriment of $5 million as of both Dec. 31, 2020 and 2019.

[3] Valuation allowances for state tax credit carryforwards were net of federal benefit of $2 million as of both Dec. 31, 2020 and 2019.

[4] Regulatory differences for income tax primarily relate to the credit of excess deferred taxes to customers through the average rate assumption method. Income tax benefits associated with the credit of excess deferred credits are offset by corresponding revenue reductions and additional prepaid pension asset amortization.