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