SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of AFUDC and Weighted Average AFUDC (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 23.7	$ 25.6	$ 19.7
Equity Funds	42.0	45.0	44.0
Total AFUDC	$ 65.7	$ 70.6	$ 63.7
Average AFUDC Rate	5.00%	5.40%	4.90%
The Connecticut Light and Power Company
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 6.6	$ 7.1	$ 6.3
Equity Funds	13.8	13.2	12.2
Total AFUDC	$ 20.4	$ 20.3	$ 18.5
Average AFUDC Rate	5.90%	6.30%	5.80%
NSTAR Electric Company
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 9.1	$ 10.4	$ 7.8
Equity Funds	21.5	19.8	15.6
Total AFUDC	$ 30.6	$ 30.2	$ 23.4
Average AFUDC Rate	5.70%	5.70%	5.00%
Public Service Company of New Hampshire
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 2.1	$ 2.8	$ 1.3
Equity Funds	4.2	3.4	0.0
Total AFUDC	$ 6.3	$ 6.2	$ 1.3
Average AFUDC Rate	4.70%	4.60%	0.70%