SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of AFUDC and Weighted Average AFUDC (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 25.6	$ 19.7	$ 12.5
Equity Funds	45.0	44.0	34.4
Total AFUDC	$ 70.6	$ 63.7	$ 46.9
Average AFUDC Rate	5.40%	4.90%	5.10%
CL&P
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 7.1	$ 6.3	$ 5.1
Equity Funds	13.2	12.2	12.1
Total AFUDC	$ 20.3	$ 18.5	$ 17.2
Average AFUDC Rate	6.30%	5.80%	6.20%
NSTAR Electric
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 10.4	$ 7.8	$ 4.8
Equity Funds	19.8	15.6	10.2
Total AFUDC	$ 30.2	$ 23.4	$ 15.0
Average AFUDC Rate	5.70%	5.00%	5.00%
PSNH
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 2.8	$ 1.3	$ 0.7
Equity Funds	3.4	0.0	0.0
Total AFUDC	$ 6.2	$ 1.3	$ 0.7
Average AFUDC Rate	4.60%	0.70%	0.70%