Environmental Matters (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Site Contingency [Line Items]
Environmental Site Quantity	68	77
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 39.4
Ending Balance	36.5	39.4
MGP Site accrual [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Ending Balance	32.4	34.5
The Connecticut Light And Power Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	18	19
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	3.7
Ending Balance	3.5	3.7
NSTAR Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	12	16
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	1.7
Ending Balance	1.2	1.7
Public Service Company Of New Hampshire [Member]
Site Contingency [Line Items]
Environmental Site Quantity	15	16
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	4.9
Ending Balance	5.6	4.9
Western Massachusetts Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	5	6
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	0.6
Ending Balance	$ 0.4	$ 0.6