DERIVATIVE INSTRUMENTS - Gross Fair Values and Net Amounts of Contracts (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
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	$ 12.8	$ 12.2
Netting	(0.4)	(0.4)
Net Amount Recorded as a Derivative	12.4	11.8
Long-term Derivative Assets | CL&P | Level 3
Derivative Assets (Liabilities), at Fair Value, Net, by Balance Sheet Classification [Abstract]
Commodity Supply and Price Risk Management	68.2	67.5
Netting	(2.1)	(2.1)
Net Amount Recorded as a Derivative	66.1	65.4
Current Derivative Liabilities | CL&P | Level 3
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(69.6)	(67.8)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(69.6)	(67.8)
Current Derivative Liabilities | Other | Level 2
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(1.4)	(5.2)
Netting	0.3	0.0
Net Amount Recorded as a Derivative	(1.1)	(5.2)
Long-Term Derivative Liabilities | CL&P | Level 3
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(342.7)	(338.6)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(342.7)	(338.6)
Long-Term Derivative Liabilities | Other | Level 2
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(0.3)	(0.1)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	$ (0.3)	$ (0.1)