ASSETS HELD FOR SALE - Generation Assets (Details) - USD ($) $ in Thousands |
Jun. 30, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Regulatory Assets [Line Items] | ||
| Accumulated Depreciation | $ (7,363,100) | $ (7,133,800) |
| Net Plant | 24,476,890 | 23,617,463 |
| Other Assets | 706,400 | 693,700 |
| Public Service Company of New Hampshire | ||
| Regulatory Assets [Line Items] | ||
| Accumulated Depreciation | (754,500) | (751,800) |
| Net Plant | 2,763,610 | 2,642,274 |
| Electricity Generation Plant, Non-Nuclear | Public Service Company of New Hampshire | ||
| Regulatory Assets [Line Items] | ||
| Thermal Gross Plant | 0 | 1,091,400 |
| Hydroelectric Gross Plant | 86,800 | 83,000 |
| Accumulated Depreciation | (27,500) | (575,400) |
| Net Plant | 59,300 | 599,000 |
| Fuel and Inventory | 0 | 87,700 |
| Materials and Supplies | 100 | 27,300 |
| Emissions Allowances | 0 | 19,100 |
| Other Assets | 0 | 2,600 |
| Deferred Costs from Thermal Generation Asset Sale | 0 | (516,100) |
| Total Generation Assets Held for Sale | $ 59,400 | $ 219,600 |