Environmental Matters (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Site Contingency [Line Items]
Environmental Site Quantity	65	68
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 35.4
Ending Balance	34.7	35.4
MGP Site accrual [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Ending Balance	29.8	31.4
The Connecticut Light And Power Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	16	18
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	3.4
Ending Balance	4.0	3.4
NSTAR Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	13	12
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	1.2
Ending Balance	1.1	1.2
Public Service Company Of New Hampshire [Member]
Site Contingency [Line Items]
Environmental Site Quantity	13	15
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	5.4
Ending Balance	5.3	5.4
Western Massachusetts Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	4	5
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	0.4
Ending Balance	$ 0.5	$ 0.4