SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 262.9	$ 273.7
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	70.9	85.0
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	68.2	70.2
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 23.5	$ 37.9