Pension And OPEB (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Benefit Costs	$ 1	$ 14	$ 3	$ 42
Postretirement Healthcare Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions			14
Total Benefit Costs	21	18	64	53
Long Island Electric Utility Servco LLC Pension and OPEB [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions			35
Total Benefit Costs	18	$ 16		$ 28
Long Island Electric Utility Servco LLC Pension and OPEB [Member] | Postretirement Healthcare Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Benefit Costs	1		3
Cost Reduction related to change in estimate [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Benefit Costs	$ 12		$ 36