Other Income, Net (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	$ 2,800,000	$ 5,500,000
Other Income/(Loss), Net	8,773,000	10,313,000
The Connecticut Light And Power Company [Member]
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	1,200,000	1,500,000
Other Income/(Loss), Net	5,300,000	4,606,000
Public Service Company Of New Hampshire [Member]
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	900,000	3,500,000
Other Income/(Loss), Net	2,042,000	4,459,000
Western Massachusetts Electric Company [Member]
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	300,000	100,000
Other Income/(Loss), Net	$ 1,092,000	$ 739,000