SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 336.1	$ 323.7
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	95.0	114.0
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	75.5	85.2
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 48.3	$ 29.9