SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 317.8	$ 303.7
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	107.6	103.0
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	79.0	62.5
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 35.8	$ 48.3