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ASSET RETIREMENT OBLIGATIONS - Schedule of Reconciliation of Beginning and Ending Balance (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Balance as of Beginning of Year $ 505.8 $ 502.7
Liabilities Settled During the Year (24.6) (24.9)
Accretion 29.5 29.2
Revisions in Estimated Cash Flows 80.2 (1.2)
Balance as of End of Year 590.9 505.8
CL&P    
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Balance as of Beginning of Year 39.9 37.4
Liabilities Settled During the Year 0.0 0.0
Accretion 2.6 2.5
Revisions in Estimated Cash Flows 0.0 0.0
Balance as of End of Year 42.5 39.9
NSTAR Electric    
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Balance as of Beginning of Year 104.8 101.3
Liabilities Settled During the Year 0.0 0.0
Accretion 4.5 4.3
Revisions in Estimated Cash Flows 0.0 (0.8)
Balance as of End of Year 109.3 104.8
PSNH    
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Balance as of Beginning of Year 5.2 4.9
Liabilities Settled During the Year 0.0 0.0
Accretion 0.4 0.3
Revisions in Estimated Cash Flows 0.0 0.0
Balance as of End of Year $ 5.6 $ 5.2