COMMITMENTS AND CONTINGENCIES - Schedule of Total Costs Incurred (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Long-term Purchase Commitment [Line Items]
Renewable Energy	$ 584.2	$ 320.8	$ 218.5
Natural Gas Procurement	453.4	448.5	432.4
Purchased Power and Capacity	62.7	62.1	72.0
Peaker CfDs	22.7	13.0	20.9
Transmission Support Commitments	22.1	21.8	23.4
The Connecticut Light and Power Company
Long-term Purchase Commitment [Line Items]
Renewable Energy	426.3	160.6	63.2
Purchased Power and Capacity	59.3	50.4	49.4
Peaker CfDs	22.7	13.0	20.9
Transmission Support Commitments	8.7	8.6	9.2
NSTAR Electric Company
Long-term Purchase Commitment [Line Items]
Renewable Energy	88.8	89.9	89.8
Purchased Power and Capacity	3.1	5.1	4.4
Peaker CfDs	0.0	0.0	0.0
Transmission Support Commitments	8.7	8.6	9.2
Public Service Company of New Hampshire
Long-term Purchase Commitment [Line Items]
Renewable Energy	69.1	70.3	65.5
Purchased Power and Capacity	0.3	6.6	18.2
Peaker CfDs	0.0	0.0	0.0
Transmission Support Commitments	$ 4.7	$ 4.6	$ 5.0