Income Taxes (Details) - USD ($)	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Federal Tax Reform [Abstract]
Tax Cuts and Jobs Act of 2017, Corporate Federal Tax Rate	21.00%
Tax Cuts and Jobs Act of 2017, Net Operating Loss Deduction Limitation, Percent of Taxable income		80.00%
Reclassification of deferred taxes to regulatory liabilities, grossed-up for taxes	$ 3,800,000,000
Regulatory liability, customer refunds, weighted average period	30 years
Provisional income tax expense for tax reform	$ 23,000,000
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions					$ 35,000,000	$ 28,000,000
Unrecognized tax benefit — Temporary tax positions					9,000,000	9,000,000
Total unrecognized tax benefit	39,000,000	$ 44,000,000	$ 39,000,000	$ 134,000,000	44,000,000	37,000,000		$ 39,000,000	$ 134,000,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at Jan. 1		37,000,000	39,000,000	134,000,000
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions		10,000,000	9,000,000	6,000,000
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions		(4,000,000)	(4,000,000)	(4,000,000)
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions		1,000,000	2,000,000	15,000,000
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions		0	(4,000,000)	(105,000,000)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		0	(5,000,000)	(7,000,000)
Balance at Dec. 31	39,000,000	44,000,000	37,000,000	39,000,000
Tax Benefits Associated With NOL And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards					(40,000,000)	(35,000,000)
Net Deferred Tax Liability associated with the Unrecognized Tax Benefit Amounts and Related NOLs and Tax Credit Carryforwards					(29,000,000)	(24,000,000)
Upper bound of decrease in unrecognized tax benefit that is reasonably possible					28,000,000
Unrecognized Tax Benefits, Income Tax Penalties Accrued					0	0		0
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued [Abstract]
Unrecognized Tax Benefits, Interest on Income Taxes Accrued					0	0		$ 0	$ (3,000,000)
Unrecognized Tax Benefits, Interest on Income Taxes Accrued		0	0	(3,000,000)
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)		(16,000,000)	(34,000,000)	1,000,000
Current State and Local Tax Expense (Benefit)		4,000,000	8,000,000	(11,000,000)
Current Change In Unrecognized Tax Expense (Benefit)		2,000,000	(6,000,000)	(83,000,000)
Deferred Federal Income Tax Expense (Benefit)		55,000,000	122,000,000	460,000,000
Deferred State and Local Income Tax Expense (Benefit)		83,000,000	85,000,000	107,000,000
Deferred Change In Unrecognized Tax Expense (Benefit)		5,000,000	11,000,000	73,000,000
Deferred investment tax credits		(5,000,000)	(5,000,000)	(5,000,000)
Income Tax Expense (Benefit)		128,000,000	181,000,000	542,000,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred tax expense (benefit) excluding selected items		344,000,000	320,000,000	(2,939,000,000)
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities		(206,000,000)	(102,000,000)	3,583,000,000
Other Comprehensive Income (Loss), Tax		5,000,000	0	(4,000,000)
Deferred Income Tax Expense (Benefit)		$ 143,000,000	$ 218,000,000	$ 640,000,000
Deferred Tax Liabilities, Gross [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment					5,474,000,000	5,082,000,000	[1]
Deferred Tax Liabilities, Operating Lease Asset					449,000,000	0	[1]
Deferred Tax Liabilities, Regulatory Assets					598,000,000	599,000,000	[1]
Pension expense					173,000,000	178,000,000	[1]
Deferred Tax Liabilities, Other					70,000,000	60,000,000	[1]
Deferred Tax Liabilities, Gross					6,764,000,000	5,919,000,000	[1]
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets Regulatory Liabilities					847,000,000	879,000,000	[1]
Deferred Tax Assets, Operating Lease Liabilities					449,000,000	0	[1]
Deferred Tax Assets Tax credit carryforward					727,000,000	642,000,000	[1]
Deferred Tax Assets, Operating Loss Carryforwards					38,000,000	51,000,000	[1]
Deferred Tax Assets, Valuation Allowance					(67,000,000)	(79,000,000)	[1]
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Benefits					128,000,000	124,000,000	[1]
Deferred Tax Assets Deferred Investment Tax Credits					14,000,000	16,000,000	[1]
Deferred Tax Assets Rate Refund					26,000,000	60,000,000	[1]
Deferred Tax Assets, Other					93,000,000	61,000,000	[1]
Deferred Tax Assets, Net of Valuation Allowance					2,255,000,000	1,754,000,000	[1]
Deferred Tax Liabilities, Net					$ 4,509,000,000	$ 4,165,000,000	[1]
Non-Plant Related Regulatory Asset [Member]
Federal Tax Reform [Abstract]
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Asset, Provisional Income Tax Expense (Benefit)	254,000,000
Plant Related Regulatory Liability [Member]
Federal Tax Reform [Abstract]
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Liability, Provisional Income Tax (Expense) Benefit	2,700,000,000
Non-Plant Related Regulated Liability [Member]
Federal Tax Reform [Abstract]
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Liability, Provisional Income Tax (Expense) Benefit	$ 174,000,000

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