Environmental Matters (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Site Contingency [Line Items]
Environmental Site Quantity	65	65
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 43.3
Ending Balance	43.6	43.3
MGP Site accrual [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Ending Balance	35.4	38.8
The Connecticut Light And Power Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	16	16
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	3.8
Ending Balance	5.0	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	1.7	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