DERIVATIVE INSTRUMENTS - Gross Fair Value of Contracts (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Current Derivative Assets | Level 2
Derivative Assets (Liabilities), at Fair Value, Net, by Balance Sheet Classification [Abstract]
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
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	10.4	13.9
Netting	(7.7)	(9.4)
Net Amount Recorded as a Derivative	2.7	4.5
Long-term Derivative Assets | Level 2
Derivative Assets (Liabilities), at Fair Value, Net, by Balance Sheet Classification [Abstract]
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
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	74.3	77.3
Netting	(6.9)	(11.7)
Net Amount Recorded as a Derivative	67.4	65.6
Current Derivative Liabilities | Level 2
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(1.5)	0.0
Netting	0.4	0.0
Net Amount Recorded as a Derivative	(1.1)	0.0
Current Derivative Liabilities | Level 3
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(62.2)	(79.7)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(62.2)	(79.7)
Current Derivative Liabilities | The Connecticut Light And Power Company | Level 3
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(59.9)	(77.8)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(59.9)	(77.8)
Current Derivative Liabilities | NSTAR Electric Company | Level 3
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(2.3)	(1.9)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(2.3)	(1.9)
Long-Term Derivative Liabilities | Level 3
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(391.9)	(413.7)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(391.9)	(413.7)
Long-Term Derivative Liabilities | The Connecticut Light And Power Company | Level 3
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(391.8)	(412.8)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(391.8)	(412.8)
Long-Term Derivative Liabilities | NSTAR Electric Company | Level 3
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(0.1)	(0.9)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	$ (0.1)	$ (0.9)