DERIVATIVE INSTRUMENTS - Schedule of Gross Fair Value of Contracts (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Level 2 | Current Derivative Liabilities
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	$ (3.3)	$ (5.2)
Netting	0.1	0.0
Net Amount Recorded as a Derivative	(3.2)	(5.2)
Level 2 | Long-Term Derivative Liabilities
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)
The Connecticut Light and Power Company | Level 3 | Current Derivative Assets
Current and Long-Term Derivative Assets
Commodity Supply and Price Risk Management	13.7	12.2
Netting	(0.4)	(0.4)
Net Amount Recorded as a Derivative	13.3	11.8
The Connecticut Light and Power Company | Level 3 | Long-Term Derivative Assets
Current and Long-Term Derivative Assets
Commodity Supply and Price Risk Management	58.7	67.5
Netting	(1.8)	(2.1)
Net Amount Recorded as a Derivative	56.9	65.4
The Connecticut Light and Power Company | Level 3 | Current Derivative Liabilities
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(68.8)	(67.8)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(68.8)	(67.8)
The Connecticut Light and Power Company | Level 3 | Long-Term Derivative Liabilities
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(294.5)	(338.6)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	$ (294.5)	$ (338.6)