Pension And OPEB (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Benefit Costs	$ (7)	$ 12
Postretirement Healthcare Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Benefit Costs	(19)	(15)
Long Island Electric Utility Servco LLC Pension and OPEB [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	30
Total Benefit Costs	8	7
Long Island Electric Utility Servco LLC Pension and OPEB [Member] | Postretirement Healthcare Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Benefit Costs	$ 2	$ 1