DERIVATIVE INSTRUMENTS - Gross Fair Value of Contracts (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Prepaid Expenses and Other Current 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	$ 2.6	$ 0.0
Netting	(0.2)	0.0
Net Amount Recorded as a Derivative	2.4	0.0
Prepaid Expenses and Other Current Assets [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	15.7	16.7
Netting	(10.4)	(10.9)
Net Amount Recorded as a Derivative	5.3	5.8
Prepaid Expenses and Other Current Assets [Member] | 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	15.3	16.7
Netting	(10.4)	(10.9)
Net Amount Recorded as a Derivative	4.9	5.8
Prepaid Expenses and Other Current Assets [Member] | NSTAR Electric 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	0.4	0.0
Netting	0.0	0.0
Net Amount Recorded as a Derivative	0.4	0.0
Other Noncurrent 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	0.1
Netting	0.0	0.0
Net Amount Recorded as a Derivative	0.0	0.1
Other Noncurrent Assets [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	79.3	62.0
Netting	(13.8)	(19.3)
Net Amount Recorded as a Derivative	65.5	42.7
Other Noncurrent Assets [Member] | 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	79.3	60.7
Netting	(13.8)	(19.3)
Net Amount Recorded as a Derivative	65.5	41.4
Other Noncurrent Assets [Member] | NSTAR Electric 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	0.0	1.3
Netting	0.0	0.0
Net Amount Recorded as a Derivative	0.0	1.3
Other Current Liabilities [Member] | Fair Value, Inputs, Level 2 [Member]
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	0.0	(5.8)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	0.0	(5.8)
Other Current Liabilities [Member] | Fair Value, Inputs, Level 3 [Member]
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(87.1)	(92.3)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(87.1)	(92.3)
Other Current 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	(85.3)	(91.8)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(85.3)	(91.8)
Other Current Liabilities [Member] | NSTAR Electric Company [Member] | Fair Value, Inputs, Level 3 [Member]
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(1.8)	(0.5)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(1.8)	(0.5)
Derivative Liabilities Non Current [Member] | Fair Value, Inputs, Level 3 [Member]
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(427.4)	(337.1)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(427.4)	(337.1)
Derivative Liabilities Non Current [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	(426.1)	(336.2)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(426.1)	(336.2)
Derivative Liabilities Non Current [Member] | NSTAR Electric Company [Member] | Fair Value, Inputs, Level 3 [Member]
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(1.3)	(0.9)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	$ (1.3)	$ (0.9)