DERIVATIVE INSTRUMENTS - Gross Fair Values and Net Amounts of Contracts (Details) - CL&P - Level 2 - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Current Derivative Assets
Derivative Assets (Liabilities), at Fair Value, Net, by Balance Sheet Classification [Abstract]
Commodity Supply and Price Risk Management	$ 15.8	$ 16.4
Netting	(0.4)	(0.5)
Net Amount Recorded as a Derivative	15.4	15.9
Long-Term Derivative Assets
Derivative Assets (Liabilities), at Fair Value, Net, by Balance Sheet Classification [Abstract]
Commodity Supply and Price Risk Management	9.9	13.6
Netting	(0.4)	(0.5)
Net Amount Recorded as a Derivative	9.5	13.1
Current Derivative Liabilities
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(79.2)	(81.9)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(79.2)	(81.9)
Long-Term Derivative Liabilities
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(49.4)	(68.0)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	$ (49.4)	$ (68.0)