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EMPLOYEE BENEFITS - Schedule of Other Retirement Benefits (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Actuarially-Determined Liability $ 32.6 $ 43.4 $ 42.8
Other Retirement Benefits Expense 2.6 10.9 2.2
Retirement benefit payable 9.2    
CL&P      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Actuarially-Determined Liability 0.2 0.2 0.2
Other Retirement Benefits Expense 0.8 4.0 0.7
NSTAR Electric      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Actuarially-Determined Liability 0.0 0.1 0.1
Other Retirement Benefits Expense 0.8 3.7 0.7
PSNH      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Actuarially-Determined Liability 1.1 1.3 1.5
Other Retirement Benefits Expense $ 0.4 $ 1.3 $ 0.3