Supplemental Cash Flow Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Capital Expenditures Incurred But Not Yet Paid	$ 168,500,000	$ 127,900,000	$ 125,500,000
Interest Paid, Net of Amounts Capitalized	256,300,000	258,300,000	263,800,000
Income Taxes Paid	(76,600,000)	84,500,000	35,100,000
The Connecticut Light And Power Company [Member]
Capital Expenditures Incurred But Not Yet Paid	32,700,000	46,200,000	48,200,000
Interest Paid, Net of Amounts Capitalized	136,600,000	142,200,000	146,700,000
Income Taxes Paid	(27,500,000)	71,500,000	42,400,000
Public Service Company Of New Hampshire [Member]
Capital Expenditures Incurred But Not Yet Paid	51,100,000	35,800,000	46,500,000
Interest Paid, Net of Amounts Capitalized	49,300,000	51,400,000	49,000,000
Income Taxes Paid	(29,000,000)	1,600,000	12,800,000
Western Massachusetts Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	61,300,000	21,200,000	10,300,000
Interest Paid, Net of Amounts Capitalized	22,100,000	20,200,000	19,400,000
Income Taxes Paid	$ (4,900,000)	$ 5,000,000	$ (9,100,000)