SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 423.8	$ 458.6
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	88.0	73.0
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	128.0	164.8
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 60.9	$ 67.5