DERIVATIVE INSTRUMENTS - Gross Fair Values and Net Amounts of Contracts (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
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	$ 11.6	$ 9.6
Netting	(0.3)	(3.4)
Net Amount Recorded as a Derivative	11.3	6.2
Current Derivative Assets | Other | Level 2
Derivative Assets (Liabilities), at Fair Value, Net, by Balance Sheet Classification [Abstract]
Commodity Supply and Price Risk Management	0.0	1.5
Netting	0.0	(0.9)
Net Amount Recorded as a Derivative	0.0	0.6
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	70.9	74.2
Netting	(2.2)	(2.3)
Net Amount Recorded as a Derivative	68.7	71.9
Current Derivative Liabilities | CL&P | Level 3
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(64.6)	(55.1)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(64.6)	(55.1)
Current Derivative Liabilities | Other | Level 2
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(3.5)	0.0
Netting	0.2	0.0
Net Amount Recorded as a Derivative	(3.3)	0.0
Long-Term Derivative Liabilities | CL&P | Level 3
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(357.7)	(379.5)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(357.7)	(379.5)
Long-Term Derivative Liabilities | Other | Level 2
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(0.2)	0.0
Netting	0.0	0.0
Net Amount Recorded as a Derivative	$ (0.2)	$ 0.0