SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 400.8	$ 385.6
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	101.9	102.1
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	117.4	85.8
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 54.6	$ 49.5