Pension And OPEB (Narrative) (Detail) - USD ($)
$ in Millions |
3 Months Ended |
Mar. 31, 2018 |
Mar. 31, 2017 |
| Pension Plan [Member] |
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
| Total Benefit Costs |
$ (9)
|
$ 1
|
| Postretirement Healthcare Plans [Member] |
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
| Employer contributions |
14
|
|
| Total Benefit Costs |
26
|
22
|
| Long Island Electric Utility Servco LLC Pension and OPEB [Member] | Pension Plan [Member] |
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
| Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year |
40
|
|
| Total Benefit Costs |
10
|
9
|
| Long Island Electric Utility Servco LLC Pension and OPEB [Member] | Postretirement Healthcare Plans [Member] |
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
| Total Benefit Costs |
$ 1
|
$ 1
|