DERIVATIVE INSTRUMENTS - Schedule of Gross Fair Value of Contracts (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Inputs, Level 2 | Current Derivative Assets
Current and Long-Term Derivative Assets
Commodity Supply and Price Risk Management	$ 0.0	$ 6.0
Netting	0.0	0.0
Net Amount Recorded as a Derivative	0.0	6.0
Fair Value, Inputs, Level 2 | Long-Term Derivative Assets
Current and Long-Term Derivative Assets
Commodity Supply and Price Risk Management	0.0	0.3
Netting	0.0	(0.1)
Net Amount Recorded as a Derivative	0.0	0.2
Fair Value, Inputs, Level 2 | Current Derivative Liabilities
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(4.5)	0.0
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(4.5)	0.0
Fair Value, Inputs, Level 2 | Long-Term Derivative Liabilities:
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(0.4)	0.0
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(0.4)	0.0
Fair Value, Inputs, Level 3 | Current Derivative Liabilities
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(54.4)	(79.7)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(54.4)	(79.7)
Fair Value, Inputs, Level 3 | Long-Term Derivative Liabilities:
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(376.9)	(413.7)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(376.9)	(413.7)
The Connecticut Light And Power Company | Fair Value, Inputs, Level 3 | Current Derivative Assets
Current and Long-Term Derivative Assets
Commodity Supply and Price Risk Management	9.5	13.9
Netting	(7.1)	(9.4)
Net Amount Recorded as a Derivative	2.4	4.5
The Connecticut Light And Power Company | Fair Value, Inputs, Level 3 | Long-Term Derivative Assets
Current and Long-Term Derivative Assets
Commodity Supply and Price Risk Management	71.9	77.3
Netting	(5.3)	(11.7)
Net Amount Recorded as a Derivative	66.6	65.6
The Connecticut Light And Power Company | Fair Value, Inputs, Level 3 | Current Derivative Liabilities
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(54.4)	(77.8)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(54.4)	(77.8)
The Connecticut Light And Power Company | Fair Value, Inputs, Level 3 | Long-Term Derivative Liabilities:
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(376.9)	(412.8)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	$ (376.9)	$ (412.8)