DERIVATIVE INSTRUMENTS - Gross Fair Values and Net Amounts of Contracts (Details) - USD ($) $ in Millions	Sep. 30, 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	$ 13.5	$ 12.2
Netting	(0.4)	(0.4)
Net Amount Recorded as a Derivative	13.1	11.8
Current Derivative Assets | Other | Level 2
Derivative Assets (Liabilities), at Fair Value, Net, by Balance Sheet Classification [Abstract]
Commodity Supply and Price Risk Management	3.4	0.0
Netting	(0.7)	0.0
Net Amount Recorded as a Derivative	2.7	0.0
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	59.1	67.5
Netting	(1.9)	(2.1)
Net Amount Recorded as a Derivative	57.2	65.4
Current Derivative Liabilities | CL&P | Level 3
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(69.2)	(67.8)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(69.2)	(67.8)
Current Derivative Liabilities | Other | Level 2
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	0.0	(5.2)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	0.0	(5.2)
Long-Term Derivative Liabilities | CL&P | Level 3
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(312.9)	(338.6)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(312.9)	(338.6)
Long-Term Derivative Liabilities | Other | Level 2
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	0.0	(0.1)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	$ 0.0	$ (0.1)