COMMITMENTS AND CONTINGENCIES - Schedule of Total Costs Incurred (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Long-term Purchase Commitment [Line Items]
Renewable Energy Purchase Contracts	$ 591.4	$ 581.4	$ 678.1
Natural Gas Procurement	695.0	695.8	1,042.8
Capacity and Purchased Power	70.5	69.0	61.6
Peaker CfDs	23.1	20.1	13.4
Transmission Support Commitments	16.7	14.2	12.7
CL&P
Long-term Purchase Commitment [Line Items]
Renewable Energy Purchase Contracts	529.0	474.1	513.2
Capacity and Purchased Power	67.6	65.5	57.7
Peaker CfDs	23.1	20.1	13.4
Transmission Support Commitments	6.6	5.6	5.0
NSTAR Electric
Long-term Purchase Commitment [Line Items]
Renewable Energy Purchase Contracts	62.4	60.0	90.8
Capacity and Purchased Power	2.9	2.9	3.0
Peaker CfDs	0.0	0.0	0.0
Transmission Support Commitments	6.6	5.6	5.0
PSNH
Long-term Purchase Commitment [Line Items]
Renewable Energy Purchase Contracts	0.0	47.3	74.1
Capacity and Purchased Power	0.0	0.6	0.9
Peaker CfDs	0.0	0.0	0.0
Transmission Support Commitments	$ 3.5	$ 3.0	$ 2.7