COMMITMENTS AND CONTINGENCIES - Schedule of Total Costs Incurred (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Long-term Purchase Commitment [Line Items]
Renewable Energy	$ 609.2	$ 584.2	$ 320.8
Natural Gas Procurement	712.7	453.4	448.5
Purchased Power and Capacity	56.4	62.7	62.1
Peaker CfDs	24.3	22.7	13.0
Transmission Support Commitments	15.4	22.1	21.8
The Connecticut Light and Power Company
Long-term Purchase Commitment [Line Items]
Renewable Energy	457.1	426.3	160.6
Purchased Power and Capacity	53.1	59.3	50.4
Peaker CfDs	24.3	22.7	13.0
Transmission Support Commitments	6.1	8.7	8.6
NSTAR Electric Company
Long-term Purchase Commitment [Line Items]
Renewable Energy	84.7	88.8	89.9
Purchased Power and Capacity	3.0	3.1	5.1
Peaker CfDs	0.0	0.0	0.0
Transmission Support Commitments	6.0	8.7	8.6
Public Service Company of New Hampshire
Long-term Purchase Commitment [Line Items]
Renewable Energy	67.4	69.1	70.3
Purchased Power and Capacity	0.3	0.3	6.6
Peaker CfDs	0.0	0.0	0.0
Transmission Support Commitments	$ 3.3	$ 4.7	$ 4.6