Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2020	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Line Items]
Refund of AMT credit carryforwards			$ 572	$ 573
Deferred tax liabilities			$ 117
Reclassification of AMT credits from noncurrent deferred tax liabilities to a current federal income tax receivable				286
AMT credit carryforwards
Income Tax Disclosure [Line Items]
Refund of AMT credit carryforwards	$ 572			$ 573
Income tax interest income	$ 19
Analysis of existing regulatory liability
Income Tax Disclosure [Line Items]
Measurement period adjustment					$ 23
Bonus depreciation
Income Tax Disclosure [Line Items]
Measurement period adjustment					10
Nondeductible long-term incentives
Income Tax Disclosure [Line Items]
Measurement period adjustment					7
Refundable AMT credits
Income Tax Disclosure [Line Items]
Measurement period adjustment		$ 76
Regulatory liability
Income Tax Disclosure [Line Items]
Measurement period adjustment					83
Reclassified AMT credit carryforwards
Income Tax Disclosure [Line Items]
Measurement period adjustment					$ 573