Income Taxes (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2017
Effective Income Tax Computation [Line Items]
Federal statutory income tax rate			21.00%	21.00%	35.00%
Income tax benefit associated with remeasurement of state deferred taxes			$ 45
Income tax expense attributable to noncontrolling interest			55
Income tax expense (benefit)	$ (110)	$ (84)	(123)	$ 161
Discontinued Operations
Effective Income Tax Computation [Line Items]
Income tax expense (benefit)			(572)	$ 47
Discontinued Operations | Atlantic Coast Pipeline | Supply Header Project
Effective Income Tax Computation [Line Items]
Income tax charge due to write off of tax-related regulatory assets			81
SCANA
Effective Income Tax Computation [Line Items]
Deferred income tax expense			$ 198