SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 357.4	$ 317.8
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	97.8	107.6
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	92.6	79.0
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 50.0	$ 35.8