SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 458.6	$ 462.4
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	73.0	86.3
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	164.8	135.9
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 67.5	$ 71.9