SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 394.1	$ 359.6
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	96.7	75.3
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	94.5	94.2
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 49.7	$ 32.8