Pension, OPEB and Savings Plans (Estimated Future Benefit Payments) (Details) $ in Millions	Dec. 31, 2017 USD ($)
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Payments Expected Next Twelve Months	$ 337
Payments Expected Year Two	331
Payments Expected Year Three	341
Payments Expected Year Four	352
Payments Expected Year Five	364
Payments Expected Thereafter	1,954
Total Estimated Future Benefit Payments	3,679
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Payments Expected Next Twelve Months	88
Payments Expected Year Two	92
Payments Expected Year Three	96
Payments Expected Year Four	101
Payments Expected Year Five	105
Payments Expected Thereafter	560
Total Estimated Future Benefit Payments	1,042
Long Island Electric Utility Servco LLC Pension and OPEB [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Payments Expected Next Twelve Months	3
Payments Expected Year Two	4
Payments Expected Year Three	5
Payments Expected Year Four	7
Payments Expected Year Five	9
Payments Expected Thereafter	78
Total Estimated Future Benefit Payments	106
Long Island Electric Utility Servco LLC Pension and OPEB [Member] | Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Payments Expected Next Twelve Months	4
Payments Expected Year Two	6
Payments Expected Year Three	8
Payments Expected Year Four	9
Payments Expected Year Five	12
Payments Expected Thereafter	87
Total Estimated Future Benefit Payments	$ 126