SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 359.6	$ 357.4
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	75.3	97.8
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	94.2	92.6
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 32.8	$ 50.0