COMMITMENTS AND CONTINGENCIES - Schedule of Reconciliation of Activity in Environmental Reserve (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	$ 115.4	$ 102.4
Additions	12.6	23.4
Payments/Reductions	(5.4)	(10.4)
Accrual for Environmental Loss Contingencies, Ending Balance	$ 122.6	$ 115.4
Environmental Loss Contingency, Statement of Financial Position [Extensible Enumeration]	Other Current Liabilities	Other Current Liabilities	Other Current Liabilities
CL&P
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	$ 13.9	$ 12.3
Additions	0.9	4.4
Payments/Reductions	(0.9)	(2.8)
Accrual for Environmental Loss Contingencies, Ending Balance	13.9	13.9
NSTAR Electric
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	3.3	4.7
Additions	0.4	0.0
Payments/Reductions	(0.3)	(1.4)
Accrual for Environmental Loss Contingencies, Ending Balance	3.4	3.3
PSNH
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual for Environmental Loss Contingencies, Beginning Balance	6.3	7.1
Additions	0.5	0.0
Payments/Reductions	(0.7)	(0.8)
Accrual for Environmental Loss Contingencies, Ending Balance	$ 6.1	$ 6.3