LEASES - Other Information Related to Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Weighted-Average Remaining Lease Term (Years):
Operating Leases	9 years	11 years
Finance Leases	13 years	15 years
Weighted-Average Discount Rate (Percentage):
Operating Leases	4.00%	3.20%
Finance Leases	3.30%	2.70%
Cash Paid for Amounts Included in the Measurement of Lease Liabilities:
Operating Cash Flows from Operating Leases	$ 10.5	$ 11.3	$ 12.1
Operating Cash Flows from Finance Leases	2.0	2.0	3.4
Financing Cash Flows from Finance Leases	3.9	3.9	4.1
Supplemental Non-Cash Information on Lease Liabilities:
Right-of-use-Assets Obtained in Exchange for New Operating Lease Liabilities	12.8	18.9	2.1
Right-of-use-Assets Obtained in Exchange for New Finance Lease Liabilities	$ 18.5	$ 3.5	2.3
CL&P
Weighted-Average Remaining Lease Term (Years):
Operating Leases	4 years	4 years
Finance Leases	9 years
Weighted-Average Discount Rate (Percentage):
Operating Leases	5.20%	3.80%
Finance Leases	5.30%	0.00%
Cash Paid for Amounts Included in the Measurement of Lease Liabilities:
Operating Cash Flows from Operating Leases	$ 0.7	$ 0.3	0.3
Operating Cash Flows from Finance Leases	0.0	0.0	0.1
Financing Cash Flows from Finance Leases	0.0	0.0	1.4
Supplemental Non-Cash Information on Lease Liabilities:
Right-of-use-Assets Obtained in Exchange for New Operating Lease Liabilities	0.6	2.4	0.0
Right-of-use-Assets Obtained in Exchange for New Finance Lease Liabilities	$ 18.3	$ 0.0	0.0
NSTAR Electric
Weighted-Average Remaining Lease Term (Years):
Operating Leases	13 years	17 years
Finance Leases	18 years	19 years
Weighted-Average Discount Rate (Percentage):
Operating Leases	4.20%	4.00%
Finance Leases	2.90%	2.90%
Cash Paid for Amounts Included in the Measurement of Lease Liabilities:
Operating Cash Flows from Operating Leases	$ 2.5	$ 2.1	2.1
Operating Cash Flows from Finance Leases	0.6	0.6	0.6
Financing Cash Flows from Finance Leases	0.0	0.0	0.0
Supplemental Non-Cash Information on Lease Liabilities:
Right-of-use-Assets Obtained in Exchange for New Operating Lease Liabilities	7.0	0.0	1.9
Right-of-use-Assets Obtained in Exchange for New Finance Lease Liabilities	$ 0.0	$ 0.0	0.0
PSNH
Weighted-Average Remaining Lease Term (Years):
Operating Leases	3 years
Weighted-Average Discount Rate (Percentage):
Operating Leases	5.20%	0.00%
Finance Leases	0.00%	0.00%
Cash Paid for Amounts Included in the Measurement of Lease Liabilities:
Operating Cash Flows from Operating Leases	$ 0.4	$ 0.1	0.1
Operating Cash Flows from Finance Leases	0.0	0.0	0.0
Financing Cash Flows from Finance Leases	0.0	0.1	0.1
Supplemental Non-Cash Information on Lease Liabilities:
Right-of-use-Assets Obtained in Exchange for New Operating Lease Liabilities	5.0	0.0	0.0
Right-of-use-Assets Obtained in Exchange for New Finance Lease Liabilities	$ 0.0	$ 0.0	$ 0.0