Pension And OPEB (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Benefit Costs	$ (20)	$ (7)
Postretirement Healthcare Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Benefit Costs	(24)	(19)
Long Island Electric Utility Servco LLC Pension and OPEB [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Benefit Costs	9	8
Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year	37
Long Island Electric Utility Servco LLC Pension and OPEB [Member] | Postretirement Healthcare Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Benefit Costs	$ 3	$ 2