SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 346.5	$ 427.2
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	113.8	82.3
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	117.1	172.8
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 39.1	$ 58.2