COMMITMENTS AND CONTINGENCIES - Schedule of Total Costs Incurred (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Long-term Purchase Commitment [Line Items]
Renewable Energy	$ 320.8	$ 218.5	$ 235.5
Purchased Power and Capacity	62.1	72.0	103.9
Peaker CfDs		20.9	38.7
Natural Gas Procurement	448.5	432.4	377.0
Transmission Support Commitments	21.8	23.4	19.8
Coal, Wood and Other	0.0	0.0	47.7
CL&P
Long-term Purchase Commitment [Line Items]
Renewable Energy	160.6	63.2	51.0
Purchased Power and Capacity	50.4	49.4	81.0
Peaker CfDs		20.9	38.7
Transmission Support Commitments	8.6	9.2	7.8
Coal, Wood and Other	0.0	0.0	0.0
NSTAR Electric
Long-term Purchase Commitment [Line Items]
Renewable Energy	89.9	89.8	123.7
Purchased Power and Capacity	5.1	4.4	4.0
Peaker CfDs	0.0	0.0	0.0
Transmission Support Commitments	8.6	9.2	7.8
Coal, Wood and Other	0.0	0.0	0.0
PSNH
Long-term Purchase Commitment [Line Items]
Renewable Energy	70.3	65.5	60.8
Purchased Power and Capacity	6.6	18.2	18.9
Peaker CfDs	0.0	0.0	0.0
Transmission Support Commitments	4.6	5.0	4.2
Coal, Wood and Other	$ 0.0	$ 0.0	$ 47.7