Environmental Matters (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Site Contingency [Line Items]
Environmental Site Quantity	64	64
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 51.1
Ending Balance	51.7	$ 51.1
MGP Site accrual [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	45.5
Ending Balance	$ 46.1	$ 45.5
The Connecticut Light And Power Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	14	14
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 4.6
Ending Balance	$ 4.6	$ 4.6
NSTAR Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	15	15
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 2.4
Ending Balance	$ 2.3	$ 2.4
Public Service Company Of New Hampshire [Member]
Site Contingency [Line Items]
Environmental Site Quantity	12	12
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 4.5
Ending Balance	$ 4.4	$ 4.5
Western Massachusetts Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	4	4
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 0.6
Ending Balance	$ 0.6	$ 0.6