DERIVATIVE INSTRUMENTS - Gross Fair Values and Net Amounts of Contracts (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Current Derivative Assets | CL&P | Level 3
Derivative Assets (Liabilities), at Fair Value, Net, by Balance Sheet Classification [Abstract]
Commodity Supply and Price Risk Management	$ 13.9	$ 13.7
Netting	(0.4)	(0.4)
Net Amount Recorded as a Derivative	13.5	13.3
Other Long-Term Assets | CL&P | Level 3
Derivative Assets (Liabilities), at Fair Value, Net, by Balance Sheet Classification [Abstract]
Commodity Supply and Price Risk Management	58.5	58.7
Netting	(1.8)	(1.8)
Net Amount Recorded as a Derivative	56.7	56.9
Current Derivative Liabilities | CL&P | Level 3
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(69.7)	(68.8)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(69.7)	(68.8)
Current Derivative Liabilities | Other | Level 2
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(0.3)	(3.3)
Netting	0.0	0.1
Net Amount Recorded as a Derivative	(0.3)	(3.2)
Long-Term Derivative Liabilities | CL&P | Level 3
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(293.6)	(294.5)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	$ (293.6)	$ (294.5)