COMMITMENTS AND CONTINGENCIES - Schedule of Total Costs Incurred (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Long-term Purchase Commitment [Line Items]
Supply and Stranded Cost	$ 103.9	$ 152.5	$ 147.6
Renewable Energy	235.5	210.9	144.3
Peaker CfDs	38.7	47.7	42.7
Natural Gas Procurement	377.0	323.9	428.6
Coal, Wood and Other	47.7	55.7	95.9
Transmission Support Commitments	19.8	15.9	25.3
The Connecticut Light And Power Company
Long-term Purchase Commitment [Line Items]
Supply and Stranded Cost	81.0	132.7	120.3
Renewable Energy	51.0	42.1	20.0
Peaker CfDs	38.7	47.7	42.7
Coal, Wood and Other	0.0	0.0	0.0
Transmission Support Commitments	7.8	6.3	10.0
NSTAR Electric Company
Long-term Purchase Commitment [Line Items]
Supply and Stranded Cost	4.0	0.7	6.5
Renewable Energy	123.7	101.1	87.1
Peaker CfDs	0.0	0.0	0.0
Coal, Wood and Other	0.0	0.0	0.0
Transmission Support Commitments	7.8	6.2	9.9
Public Service Company Of New Hampshire
Long-term Purchase Commitment [Line Items]
Supply and Stranded Cost	18.9	19.1	20.8
Renewable Energy	60.8	67.7	37.2
Peaker CfDs	0.0	0.0	0.0
Coal, Wood and Other	47.7	55.7	95.9
Transmission Support Commitments	$ 4.2	$ 3.4	$ 5.4