DERIVATIVE INSTRUMENTS - Gross Fair Value of Contracts (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
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	$ 8.5	$ 9.5
Netting	(6.5)	(7.1)
Net Amount Recorded as a Derivative	2.0	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	80.2	71.9
Netting	(4.1)	(5.3)
Net Amount Recorded as a Derivative	76.1	66.6
Current Derivative Liabilities | Level 2
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(1.3)	(4.5)
Netting	0.2	0.0
Net Amount Recorded as a Derivative	(1.1)	(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	(49.6)	(54.4)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(49.6)	(54.4)
Long-Term Derivative Liabilities | Level 2
Derivative Assets (Liabilities), at Fair Value, Net, by Balance Sheet Classification [Abstract]
Commodity Supply and Price Risk Management	(0.4)	(0.4)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(0.4)	(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	(415.0)	(376.9)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	$ (415.0)	$ (376.9)