SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - AFUDC Costs and Weighted Average AFUDC (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 64.1	$ 64.4	$ 44.6
Equity Funds	99.0	97.8	78.1
Total AFUDC	$ 163.1	$ 162.2	$ 122.7
Average AFUDC Rate (in percent)	6.40%	6.50%	5.80%
CL&P
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 14.5	$ 8.1	$ 7.7
Equity Funds	12.2	22.4	20.0
Total AFUDC	$ 26.7	$ 30.5	$ 27.7
Average AFUDC Rate (in percent)	6.00%	6.70%	6.70%
NSTAR Electric
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 28.9	$ 27.2	$ 17.2
Equity Funds	58.4	58.8	45.7
Total AFUDC	$ 87.3	$ 86.0	$ 62.9
Average AFUDC Rate (in percent)	6.70%	7.00%	5.90%
PSNH
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 5.7	$ 8.9	$ 6.1
Equity Funds	13.0	7.0	5.4
Total AFUDC	$ 18.7	$ 15.9	$ 11.5
Average AFUDC Rate (in percent)	7.30%	5.50%	5.10%