Pension And OPEB (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 21
Total Benefit Costs	14	$ 19
Postretirement Healthcare Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	14
Total Benefit Costs	18	$ 22
Long Island Electric Utility Servco LLC Pension and OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year	28
Total Benefit Costs	6
Cost Reduction related to change in estimate [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Benefit Costs	9
Cost Reduction related to change in estimate [Member] | Postretirement Healthcare Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Benefit Costs	$ 3