SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 323.7	$ 305.7
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	114.0	110.9
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	85.2	71.1
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 29.9	$ 46.6