DERIVATIVE INSTRUMENTS - Schedule of Gross Fair Value of Contracts (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Level 2 | Current Derivative Assets
Current and Long-Term Derivative Assets
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
Level 2 | Current Derivative Liabilities
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(5.2)	0.0
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(5.2)	0.0
Level 2 | Long-Term Derivative Liabilities:
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(0.1)	0.0
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(0.1)	0.0
CL&P | Level 3 | Current Derivative Assets
Current and Long-Term Derivative Assets
Commodity Supply and Price Risk Management	12.2	9.6
Netting	(0.4)	(3.4)
Net Amount Recorded as a Derivative	11.8	6.2
CL&P | Level 3 | Long-Term Derivative Assets
Current and Long-Term Derivative Assets
Commodity Supply and Price Risk Management	67.5	74.2
Netting	(2.1)	(2.3)
Net Amount Recorded as a Derivative	65.4	71.9
CL&P | Level 3 | Current Derivative Liabilities
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(67.8)	(55.1)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(67.8)	(55.1)
CL&P | Level 3 | Long-Term Derivative Liabilities:
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(338.6)	(379.5)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	$ (338.6)	$ (379.5)