Environmental Matters (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Site Contingency [Line Items]
Environmental Site Quantity	72	77
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$ 39.4
Ending Balance	40.7	39.4
Accrual For Environmental Loss Contingencies Component Amount	2.0
Site Contingency [Abstract]
Cumulative Payments Environmental Loss Contingencies	17.5
Cumulative Environmental Remediation Expense	19.5
The Connecticut Light And Power Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	19	19
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	3.7
Ending Balance	3.6	3.7
NSTAR Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	14	16
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	1.7
Ending Balance	1.5	1.7
Public Service Company Of New Hampshire [Member]
Site Contingency [Line Items]
Environmental Site Quantity	16	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	$ 0.5	$ 0.6