Income Taxes (Narrative) (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Mar. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Taxes [Line Items]
Changes in excess deferred income tax amortization associated with the gain on the sale of assets				$ 0	$ 7
Effective tax rate, percent				18.20%	5.70%
Income tax expense		$ 15	$ 40	$ 59	$ (63)
Adjusted taxable income for certain businesses	50.00%
Prior Years | Federal and State
Income Taxes [Line Items]
Income tax expense					59
Unrecognized tax benefits increase (decrease)					70
SEMI
Income Taxes [Line Items]
Changes in excess deferred income tax amortization associated with the gain on the sale of assets				$ 7
SCANA Combination
Income Taxes [Line Items]
Income tax expense					$ 198