Income Taxes (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2017
Income Taxes [Line Items]
Weighted average rate reversal of deferred taxes to originate the deferred tax liability				35.00%
Income tax expense (benefit) from discontinued operations		$ 7	$ 31
Scenario Forecast
Income Taxes [Line Items]
Deferred tax benefit	$ 20
Scenario Forecast | Virginia Electric and Power Company
Income Taxes [Line Items]
Deferred tax benefit	$ 15