SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 357.2	$ 344.8
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	96.7	66.4
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	75.1	103.8
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 49.6	$ 33.9