SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 336.3	$ 274.4
CL&P
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	121.9	117.7
NSTAR Electric
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	83.2	59.5
PSNH
Schedule of Supplemental Cash Flow [Line Items]
Plant additions included in accounts payable	$ 30.4	$ 36.0