COMMITMENTS AND CONTINGENCIES - Schedule of Total Costs Incurred (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Long-term Purchase Commitment [Line Items]
Supply and Stranded Cost	$ 152.5	$ 147.6	$ 99.2
Renewable Energy	210.9	144.3	114.4
Peaker CfDs	47.7	42.7	18.1
Natural Gas Procurement	323.9	428.6	482.5
Coal, Wood and Other	55.7	95.9	120.5
Transmission Support Commitments	15.9	25.3	25.0
The Connecticut Light And Power Company [Member]
Long-term Purchase Commitment [Line Items]
Supply and Stranded Cost	132.7	120.3	63.0
Renewable Energy	42.1	20.0	0.7
Peaker CfDs	47.7	42.7	18.1
Coal, Wood and Other	0.0	0.0	0.0
Transmission Support Commitments	6.3	10.0	9.9
NSTAR Electric Company [Member]
Long-term Purchase Commitment [Line Items]
Supply and Stranded Cost	0.7	6.5	7.0
Renewable Energy	93.6	86.7	87.4
Peaker CfDs	0.0	0.0	0.0
Coal, Wood and Other	0.0	0.0	0.0
Transmission Support Commitments	4.9	7.8	7.7
Public Service Company Of New Hampshire [Member]
Long-term Purchase Commitment [Line Items]
Supply and Stranded Cost	19.1	20.8	26.0
Renewable Energy	67.7	37.2	26.3
Peaker CfDs	0.0	0.0	0.0
Coal, Wood and Other	55.7	95.9	120.5
Transmission Support Commitments	3.4	5.4	5.3
Western Massachusetts Electric Company [Member]
Long-term Purchase Commitment [Line Items]
Supply and Stranded Cost	0.0	0.0	3.2
Renewable Energy	7.5	0.4	0.0
Peaker CfDs	0.0	0.0	0.0
Coal, Wood and Other	0.0	0.0	0.0
Transmission Support Commitments	$ 1.3	$ 2.1	$ 2.1