Environmental Matters (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Site Contingency [Line Items]
Environmental Site Quantity	61	59
Range Of Potential Loss On Enivornmental Sites	MGP sites are sites that were operated several decades ago and produced manufacturing gas from coal, which resulted in certain byproducts in the environment that may pose a risk to human health and the environment.
Site Contingency [Abstract]
Cumulative Payments Environmental Loss Contingencies	$ 17.2
Cumulative Environmental Remediation Expense	19.5
Accrual For Environmental Loss Contingencies Component Amount	2.3
Accrual For Environmental Loss Contingencies	31.7	31.7
MGP Site accrual [Member]
Site Contingency [Abstract]
Accrual For Environmental Loss Contingencies Component Amount	28.6	28.9
The Connecticut Light And Power Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	19	18
Site Contingency [Abstract]
Accrual For Environmental Loss Contingencies	3.3	2.9
The Connecticut Light And Power Company [Member] | MGP Site accrual [Member]
Site Contingency [Abstract]
Accrual For Environmental Loss Contingencies Component Amount	1.5	1.5
Public Service Company Of New Hampshire [Member]
Site Contingency [Line Items]
Environmental Site Quantity	19	18
Site Contingency [Abstract]
Accrual For Environmental Loss Contingencies	6.8	6.6
Public Service Company Of New Hampshire [Member] | MGP Site accrual [Member]
Site Contingency [Abstract]
Accrual For Environmental Loss Contingencies Component Amount	5.8	5.8
Western Massachusetts Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	10	10
Site Contingency [Abstract]
Accrual For Environmental Loss Contingencies	0.3	0.3
Western Massachusetts Electric Company [Member] | MGP Site accrual [Member]
Site Contingency [Abstract]
Accrual For Environmental Loss Contingencies Component Amount	$ 0.1	$ 0.1