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EMPLOYEE BENEFITS - Schedule of Defined Contribution Plans (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost recognized | $ 67.3 | $ 59.9 | $ 55.5
CL&P |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost recognized | 9.0 | 7.7 | 7.0
NSTAR Electric |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost recognized | 13.7 | 12.8 | 12.2
PSNH |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost recognized | $ 5.4 | $ 4.8 | $ 4.3

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