Other Income, Net (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	$ 22,500,000	$ 16,700,000	$ 9,400,000
Other Income/(Loss), Net	27,715,000	41,916,000	37,801,000
The Connecticut Light And Power Company [Member]
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	6,000,000	4,900,000	5,700,000
Other Income/(Loss), Net	9,741,000	26,669,000	25,874,000
Public Service Company Of New Hampshire [Member]
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	13,200,000	10,400,000	3,600,000
Other Income/(Loss), Net	14,255,000	11,749,000	9,462,000
Western Massachusetts Electric Company [Member]
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	1,000,000	600,000	0
Other Income/(Loss), Net	$ 1,489,000	$ 2,597,000	$ 1,824,000