DERIVATIVE INSTRUMENTS - Schedule of Gross Fair Value of Contracts (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Inputs, Level 2 | Current Derivative Assets
Current and Long-Term Derivative Assets
Commodity Supply and Price Risk Management	$ 1.5	$ 0.0
Netting	(0.9)	0.0
Net Amount Recorded as a Derivative	0.6	0.0
Fair Value, Inputs, Level 2 | Current Derivative Liabilities
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	0.0	(4.5)
Netting (1)	0.0	0.0
Net Amount Recorded as a Derivative	0.0	(4.5)
Fair Value, Inputs, Level 2 | Long-Term Derivative Liabilities:
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	0.0	(0.4)
Netting (1)	0.0	0.0
Net Amount Recorded as a Derivative	0.0	(0.4)
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	9.6	9.5
Netting	(3.4)	(7.1)
Net Amount Recorded as a Derivative	6.2	2.4
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	74.2	71.9
Netting	(2.3)	(5.3)
Net Amount Recorded as a Derivative	71.9	66.6
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	(55.1)	(54.4)
Netting (1)	0.0	0.0
Net Amount Recorded as a Derivative	(55.1)	(54.4)
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	(379.5)	(376.9)
Netting (1)	0.0	0.0
Net Amount Recorded as a Derivative	$ (379.5)	$ (376.9)