SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of AFUDC and Weighted Average AFUDC (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 44.6	$ 21.8	$ 18.4
Equity Funds	78.1	47.3	37.3
Total AFUDC	$ 122.7	$ 69.1	$ 55.7
Average AFUDC Rate (in percent)	5.80%	4.70%	4.20%
CL&P
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 7.7	$ 4.8	$ 2.9
Equity Funds	20.0	13.6	7.7
Total AFUDC	$ 27.7	$ 18.4	$ 10.6
Average AFUDC Rate (in percent)	6.70%	6.60%	5.00%
NSTAR Electric
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 17.2	$ 10.7	$ 9.0
Equity Funds	45.7	24.6	20.4
Total AFUDC	$ 62.9	$ 35.3	$ 29.4
Average AFUDC Rate (in percent)	5.90%	5.40%	4.90%
PSNH
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 6.1	$ 1.4	$ 0.8
Equity Funds	5.4	2.5	1.6
Total AFUDC	$ 11.5	$ 3.9	$ 2.4
Average AFUDC Rate (in percent)	5.10%	2.60%	2.50%