SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 457.5	$ 357.2
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	104.8	96.7
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	113.4	75.1
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 59.6	$ 49.6