COMMITMENTS AND CONTINGENCIES - Schedule of Total Costs Incurred (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Long-term Purchase Commitment [Line Items]
Renewable Energy Purchase Contracts	$ 581.4	$ 678.1	$ 609.2
Natural Gas Procurement	695.8	1,042.8	712.7
Purchased Power and Capacity	69.0	61.6	56.4
Peaker CfDs	20.1	13.4	24.3
Transmission Support Commitments	14.2	12.7	15.4
CL&P
Long-term Purchase Commitment [Line Items]
Renewable Energy Purchase Contracts	474.1	513.2	457.1
Purchased Power and Capacity	65.5	57.7	53.1
Peaker CfDs	20.1	13.4	24.3
Transmission Support Commitments	5.6	5.0	6.1
NSTAR Electric
Long-term Purchase Commitment [Line Items]
Renewable Energy Purchase Contracts	60.0	90.8	84.7
Purchased Power and Capacity	2.9	3.0	3.0
Peaker CfDs	0.0	0.0	0.0
Transmission Support Commitments	5.6	5.0	6.0
PSNH
Long-term Purchase Commitment [Line Items]
Renewable Energy Purchase Contracts	47.3	74.1	67.4
Purchased Power and Capacity	0.6	0.9	0.3
Peaker CfDs	0.0	0.0	0.0
Transmission Support Commitments	$ 3.0	$ 2.7	$ 3.3