DERIVATIVE INSTRUMENTS - Gross Fair Value of Contracts (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Current Derivative Assets | Level 3
Derivative Assets (Liabilities), at Fair Value, Net, by Balance Sheet Classification [Abstract]
Commodity Supply and Price Risk Management	$ 0.8	$ 0.0
Netting	(0.5)	0.0
Net Amount Recorded as a Derivative	0.3	0.0
Current Derivative Assets | The Connecticut Light and Power Company | Level 3
Derivative Assets (Liabilities), at Fair Value, Net, by Balance Sheet Classification [Abstract]
Commodity Supply and Price Risk Management	8.8	9.5
Netting	(4.8)	(7.1)
Net Amount Recorded as a Derivative	4.0	2.4
Long-term Derivative Assets | The Connecticut Light and Power Company | Level 3
Derivative Assets (Liabilities), at Fair Value, Net, by Balance Sheet Classification [Abstract]
Commodity Supply and Price Risk Management	75.0	71.9
Netting	(2.3)	(5.3)
Net Amount Recorded as a Derivative	72.7	66.6
Current Derivative Liabilities | Level 2
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	0.0	(4.5)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	0.0	(4.5)
Current Derivative Liabilities | The Connecticut Light and Power Company | Level 3
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(50.8)	(54.4)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(50.8)	(54.4)
Long-Term Derivative Liabilities | Level 2
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(0.1)	(0.4)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(0.1)	(0.4)
Long-Term Derivative Liabilities | The Connecticut Light and Power Company | Level 3
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(385.8)	(376.9)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	$ (385.8)	$ (376.9)