DERIVATIVE INSTRUMENTS - Gross Fair Value of Contracts (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Current Derivative Assets [Member] | Fair Value, Inputs, Level 2 [Member]
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 [Member] | Eversource and The Connecticut Light And Power Company [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative Assets (Liabilities), at Fair Value, Net, by Balance Sheet Classification [Abstract]
Commodity Supply and Price Risk Management	11.2	13.9
Netting	(7.7)	(9.4)
Net Amount Recorded as a Derivative	3.5	4.5
Long-term Derivative Assets [Member] | Fair Value, Inputs, Level 2 [Member]
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 [Member] | Eversource and The Connecticut Light And Power Company [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative Assets (Liabilities), at Fair Value, Net, by Balance Sheet Classification [Abstract]
Commodity Supply and Price Risk Management	76.0	77.3
Netting	(8.0)	(11.7)
Net Amount Recorded as a Derivative	68.0	65.6
Current Derivative Liabilities [Member] | Fair Value, Inputs, Level 2 [Member]
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(1.1)	0.0
Netting	0.1	0.0
Net Amount Recorded as a Derivative	(1.0)	0.0
Current Derivative Liabilities [Member] | Fair Value, Inputs, Level 3 [Member]
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(66.5)	(79.7)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(66.5)	(79.7)
Current Derivative Liabilities [Member] | The Connecticut Light And Power Company [Member] | Fair Value, Inputs, Level 3 [Member]
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(64.3)	(77.8)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(64.3)	(77.8)
Current Derivative Liabilities [Member] | NSTAR Electric Company [Member] | Fair Value, Inputs, Level 3 [Member]
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(2.2)	(1.9)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(2.2)	(1.9)
Long-Term Derivative Liabilities [Member] | Fair Value, Inputs, Level 3 [Member]
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(402.1)	(413.7)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(402.1)	(413.7)
Long-Term Derivative Liabilities [Member] | The Connecticut Light And Power Company [Member] | Fair Value, Inputs, Level 3 [Member]
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(402.0)	(412.8)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(402.0)	(412.8)
Long-Term Derivative Liabilities [Member] | NSTAR Electric Company [Member] | Fair Value, Inputs, Level 3 [Member]
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)