SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 273.7	$ 336.3
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	85.0	121.9
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	70.2	83.2
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 37.9	$ 30.4