DERIVATIVE INSTRUMENTS - Schedule of Gross Fair Value of Contracts (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Inputs, Level 3 [Member] | Prepaid Expenses and Other Current Assets [Member]
Current and Long-Term Derivative Assets
Commodity Supply and Price Risk Management	$ 13.9	$ 16.7
Netting	(9.4)	(10.9)
Net Amount Recorded as a Derivative	4.5	5.8
Fair Value, Inputs, Level 3 [Member] | Other Noncurrent Assets [Member]
Current and Long-Term Derivative Assets
Commodity Supply and Price Risk Management	77.3	62.0
Netting	(11.7)	(19.3)
Net Amount Recorded as a Derivative	65.6	42.7
Fair Value, Inputs, Level 3 [Member] | Other Current Liabilities [Member]
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(79.7)	(92.3)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(79.7)	(92.3)
Fair Value, Inputs, Level 3 [Member] | Derivative Liabilities Non Current [Member]
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(413.7)	(337.1)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(413.7)	(337.1)
Fair Value, Inputs, Level 2 [Member] | Prepaid Expenses and Other Current Assets [Member]
Current and Long-Term Derivative Assets
Commodity Supply and Price Risk Management	6.0	0.0
Netting	0.0	0.0
Net Amount Recorded as a Derivative	6.0	0.0
Fair Value, Inputs, Level 2 [Member] | Other Noncurrent Assets [Member]
Current and Long-Term Derivative Assets
Commodity Supply and Price Risk Management	0.3	0.1
Netting	(0.1)	0.0
Net Amount Recorded as a Derivative	0.2	0.1
Fair Value, Inputs, Level 2 [Member] | Other Current Liabilities [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)
The Connecticut Light And Power Company [Member] | Fair Value, Inputs, Level 3 [Member] | Other Noncurrent Assets [Member]
Current and Long-Term Derivative Assets
Commodity Supply and Price Risk Management	77.3	60.7
Netting	(11.7)	(19.3)
Net Amount Recorded as a Derivative	65.6	41.4
The Connecticut Light And Power Company [Member] | Fair Value, Inputs, Level 3 [Member] | Other Current Liabilities [Member]
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(77.8)	(91.8)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(77.8)	(91.8)
The Connecticut Light And Power Company [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Liabilities Non Current [Member]
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(412.8)	(336.2)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(412.8)	(336.2)
NSTAR Electric Company [Member] | Fair Value, Inputs, Level 3 [Member] | Other Noncurrent Assets [Member]
Current and Long-Term Derivative Assets
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
NSTAR Electric Company [Member] | Fair Value, Inputs, Level 3 [Member] | Other Current Liabilities [Member]
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(1.9)	(0.5)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	(1.9)	(0.5)
NSTAR Electric Company [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Liabilities Non Current [Member]
Current and Long-Term Derivative Liabilities
Commodity Supply and Price Risk Management	(0.9)	(0.9)
Netting	0.0	0.0
Net Amount Recorded as a Derivative	$ (0.9)	$ (0.9)