Other Income, Net (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	$ 6,800,000	$ 22,500,000	$ 16,700,000
Other Income/(Loss), Net	19,742,000	27,715,000	41,916,000
The Connecticut Light And Power Company [Member]
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	1,900,000	6,000,000	4,900,000
Other Income/(Loss), Net	10,300,000	9,741,000	26,669,000
Public Service Company Of New Hampshire [Member]
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	1,900,000	13,200,000	10,400,000
Other Income/(Loss), Net	3,008,000	14,255,000	11,749,000
Western Massachusetts Electric Company [Member]
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	1,000,000	1,000,000	600,000
Other Income/(Loss), Net	$ 2,503,000	$ 1,489,000	$ 2,597,000