SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of AFUDC and Weighted Average AFUDC (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 12.5	$ 10.8	$ 7.2
Equity Funds	34.4	26.2	18.8
Total AFUDC	$ 46.9	$ 37.0	$ 26.0
Average AFUDC Rate	5.10%	4.40%	3.90%
The Connecticut Light And Power Company
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 5.1	$ 3.3	$ 2.6
Equity Funds	12.1	6.3	5.2
Total AFUDC	$ 17.2	$ 9.6	$ 7.8
Average AFUDC Rate	6.20%	4.70%	5.50%
NSTAR Electric Company
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 4.8	$ 5.3	$ 3.0
Equity Funds	10.2	10.2	6.0
Total AFUDC	$ 15.0	$ 15.5	$ 9.0
Average AFUDC Rate	5.00%	3.20%	3.50%
Public Service Company Of New Hampshire
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 0.7	$ 0.8	$ 1.0
Equity Funds	0.0	0.3	1.2
Total AFUDC	$ 0.7	$ 1.1	$ 2.2
Average AFUDC Rate	0.70%	1.00%	1.80%