Environmental Matters (Details) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Site Contingency [Line Items]
Environmental Site Quantity	83	59
Site Contingency [Abstract]
HWP Reserve Amount	$ 2,100,000
Cumulative Payments Environmental Loss Contingencies	17,400,000
Cumulative Environmental Remediation Expense	19.5
Accrual For Environmental Loss Contingencies	38,600,000	31,700,000
MGP Site accrual [Member]
Site Contingency [Abstract]
Accrual For Environmental Loss Contingencies Component Amount	34,200,000	28,900,000
The Connecticut Light And Power Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	18	18
Site Contingency [Abstract]
Accrual For Environmental Loss Contingencies	3,200,000	2,900,000
Public Service Company Of New Hampshire [Member]
Site Contingency [Line Items]
Environmental Site Quantity	19	18
Site Contingency [Abstract]
Accrual For Environmental Loss Contingencies	5,200,000	6,600,000
Western Massachusetts Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	10	10
Site Contingency [Abstract]
Accrual For Environmental Loss Contingencies	400,000	300,000
NSTAR Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	14	13
Site Contingency [Abstract]
Accrual For Environmental Loss Contingencies	$ 1,400,000	$ 1,300,000