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EMPLOYEE BENEFITS - Pension Plan Narrative (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Defined Contribution Plan Disclosure [Line Items] |  |  |
Expected long-term rate of return | 7.94% | 7.00% |
Pension and SERP |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Decrease in liability as a result of a change in discount rate |  | $ 1,480.0 |
Increase of liability as a result of change in discount rate | $ 98.9 |  |
Expected long-term rate of return | 8.25% | 8.25% | 8.25%
Employer Contributions | $ 5.0 | $ 80.0 |
Estimated future employer contributions in next fiscal year | 5.0 |  |
Pension and SERP | CL&P |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Employer Contributions | 0.0 | 0.0 |
Pension and SERP | PSNH |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Employer Contributions | 0.0 | 0.0 |
PBOP |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Decrease in liability as a result of a change in discount rate | 17.0 | $ 180.1 |
Increase of liability as a result of change in discount rate | $ 12.0 |  |
Expected long-term rate of return | 8.25% | 8.25% | 8.25%
Employer Contributions | $ 1.9 | $ 3.1 |
Estimated future employer contributions in next fiscal year | $ 2.4 |  |
PBOP | Aquarion |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Expected long-term rate of return | 7.00% | 7.00% |
PBOP | CL&P |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Employer Contributions | $ 0.0 | $ 0.0 |
PBOP | PSNH |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Employer Contributions | $ 0.0 | $ 0.0 |
Pre-65 Plan | PBOP |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Health care trend rate | 6.75% |  |
Ultimate rate | 5.00% |  |
Pre-65 Plan | Minimum | PBOP | Aquarion |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Health care trend rate | 3.50% | 3.50% |
Pre-65 Plan | Maximum | PBOP | Aquarion |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Health care trend rate | 7.00% | 6.50% |
Post-65 Plan | PBOP |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Ultimate rate | 3.50% |  |
Eversource Service Pension and PBOP Plans |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Assumed rate of return | 8.25% |  |
Aquarion Plans |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Assumed rate of return | 7.00% |  |

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