SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 477.9	$ 457.5
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	89.2	104.8
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	142.2	113.4
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 86.5	$ 59.6