Environmental Matters (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Site Contingency [Line Items]
Environmental Site Quantity	64	65
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 43.3
Ending Balance	46.9	$ 43.3
MGP Site accrual [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	38.8
Ending Balance	$ 41.0	$ 38.8
The Connecticut Light And Power Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	14	16
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 3.8
Ending Balance	$ 4.8	$ 3.8
NSTAR Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	14	13
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 1.1
Ending Balance	$ 2.2	$ 1.1
Public Service Company Of New Hampshire [Member]
Site Contingency [Line Items]
Environmental Site Quantity	12	13
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 5.2
Ending Balance	$ 3.4	$ 5.2
Western Massachusetts Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	4	4
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 0.5
Ending Balance	$ 0.6	$ 0.5