Other Income, Net (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Public Utilities Allowance For Funds Used During Construction Additions	$ 6,300,000	$ 3,800,000	$ 11,900,000	$ 6,900,000
Other Income/(Loss), Net	7,334,000	1,552,000	17,647,000	9,608,000
The Connecticut Light and Power Company [Member]
Public Utilities Allowance For Funds Used During Construction Additions	1,200,000	1,200,000	2,700,000	2,500,000
Other Income/(Loss), Net	2,058,000	745,000	6,663,000	5,678,000
Public Service Company of New Hampshire [Member]
Public Utilities Allowance For Funds Used During Construction Additions	4,400,000	2,400,000	7,800,000	4,200,000
Other Income/(Loss), Net	4,361,000	(197,000)	8,820,000	2,215,000
Western Massachusetts Electric Company [Member]
Public Utilities Allowance For Funds Used During Construction Additions	100,000	200,000	200,000	200,000
Other Income/(Loss), Net	$ 242,000	$ 161,000	$ 981,000	$ 765,000