SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of AFUDC and Weighted Average AFUDC (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 18.4	$ 23.7	$ 25.6
Equity Funds	37.3	42.0	45.0
Total AFUDC	$ 55.7	$ 65.7	$ 70.6
Average AFUDC Rate	4.20%	5.00%	5.40%
The Connecticut Light and Power Company
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 2.9	$ 6.6	$ 7.1
Equity Funds	7.7	13.8	13.2
Total AFUDC	$ 10.6	$ 20.4	$ 20.3
Average AFUDC Rate	5.00%	5.90%	6.30%
NSTAR Electric Company
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 9.0	$ 9.1	$ 10.4
Equity Funds	20.4	21.5	19.8
Total AFUDC	$ 29.4	$ 30.6	$ 30.2
Average AFUDC Rate	4.90%	5.70%	5.70%
Public Service Company of New Hampshire
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 0.8	$ 2.1	$ 2.8
Equity Funds	1.6	4.2	3.4
Total AFUDC	$ 2.4	$ 6.3	$ 6.2
Average AFUDC Rate	2.50%	4.70%	4.60%