SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of AFUDC and Weighted Average AFUDC (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 19.7	$ 12.5	$ 10.8
Equity Funds	44.0	34.4	26.2
Total AFUDC	$ 63.7	$ 46.9	$ 37.0
Average AFUDC Rate	4.90%	5.10%	4.40%
The Connecticut Light And Power Company
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 6.3	$ 5.1	$ 3.3
Total AFUDC	$ 18.5	$ 17.2	$ 9.6
Average AFUDC Rate	5.80%	6.20%	4.70%
NSTAR Electric Company
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 7.8	$ 4.8	$ 5.3
Total AFUDC	$ 23.4	$ 15.0	$ 15.5
Average AFUDC Rate	5.00%	5.00%	3.20%
Public Service Company Of New Hampshire
Components of Other Operating Costs and Expense [Line Items]
Borrowed Funds	$ 1.3	$ 0.7	$ 0.8
Total AFUDC	$ 1.3	$ 0.7	$ 1.1
Average AFUDC Rate	0.70%	0.70%	1.00%