COMMITMENTS AND CONTINGENCIES - Schedule of Total Costs Incurred (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Long-term Purchase Commitment [Line Items]
Purchased Power and Capacity	$ 72.0	$ 103.9	$ 152.5
Renewable Energy	218.5	235.5	210.9
Peaker CfDs	20.9	38.7	47.7
Natural Gas Procurement	432.4	377.0	323.9
Coal, Wood and Other	0.0	47.7	55.7
Transmission Support Commitments	23.4	19.8	15.9
The Connecticut Light And Power Company
Long-term Purchase Commitment [Line Items]
Purchased Power and Capacity	49.4	81.0	132.7
Renewable Energy	63.2	51.0	42.1
Peaker CfDs	20.9	38.7	47.7
Coal, Wood and Other	0.0	0.0	0.0
Transmission Support Commitments	9.2	7.8	6.3
NSTAR Electric Company
Long-term Purchase Commitment [Line Items]
Purchased Power and Capacity	4.4	4.0	0.7
Renewable Energy	89.8	123.7	101.1
Peaker CfDs	0.0	0.0	0.0
Coal, Wood and Other	0.0	0.0	0.0
Transmission Support Commitments	9.2	7.8	6.2
Public Service Company Of New Hampshire
Long-term Purchase Commitment [Line Items]
Purchased Power and Capacity	18.2	18.9	19.1
Renewable Energy	65.5	60.8	67.7
Peaker CfDs	0.0	0.0	0.0
Coal, Wood and Other	0.0	47.7	55.7
Transmission Support Commitments	$ 5.0	$ 4.2	$ 3.4