SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 453.6	$ 400.8
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	112.8	101.9
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	132.7	117.4
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 75.2	$ 54.6