ASSET RETIREMENT OBLIGATIONS - Schedule of Reconciliation of Beginning and Ending Balance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	$ 502.7	$ 500.1
Liabilities Settled During the Year	(24.9)	(22.3)
Accretion	29.2	28.9
Revisions in Estimated Cash Flows	(1.2)	(4.0)
Balance as of End of Year	505.8	502.7
CL&P
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	37.4	35.0
Liabilities Settled During the Year	0.0	0.0
Accretion	2.5	2.4
Revisions in Estimated Cash Flows	0.0	0.0
Balance as of End of Year	39.9	37.4
NSTAR Electric
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	101.3	97.5
Liabilities Settled During the Year	0.0	0.0
Accretion	4.3	4.1
Revisions in Estimated Cash Flows	(0.8)	(0.3)
Balance as of End of Year	104.8	101.3
PSNH
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of Beginning of Year	4.9	4.7
Liabilities Settled During the Year	0.0	0.0
Accretion	0.3	0.2
Revisions in Estimated Cash Flows	0.0	0.0
Balance as of End of Year	$ 5.2	$ 4.9