SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 462.4	$ 394.1
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	86.3	96.7
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	135.9	94.5
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 71.9	$ 49.7