Supplemental Cash Flow Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Capital Expenditures Incurred But Not Yet Paid	$ 160,600,000	$ 168,500,000	$ 127,900,000
Interest Paid, Net of Amounts Capitalized	356,500,000	256,300,000	258,300,000
Income Taxes Paid	(12,800,000)	(76,600,000)	84,500,000
The Connecticut Light And Power Company [Member]
Capital Expenditures Incurred But Not Yet Paid	42,800,000	32,700,000	46,200,000
Interest Paid, Net of Amounts Capitalized	129,400,000	136,600,000	142,200,000
Income Taxes Paid	(42,000,000)	(27,500,000)	71,500,000
Public Service Company Of New Hampshire [Member]
Capital Expenditures Incurred But Not Yet Paid	16,800,000	51,100,000	35,800,000
Interest Paid, Net of Amounts Capitalized	49,800,000	49,300,000	51,400,000
Income Taxes Paid	14,700,000	(29,000,000)	1,600,000
Western Massachusetts Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	30,000,000	61,300,000	21,200,000
Interest Paid, Net of Amounts Capitalized	25,800,000	22,100,000	20,200,000
Income Taxes Paid	(8,400,000)	(4,900,000)	5,000,000
NSTAR Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	50,000,000	34,300,000	16,700,000
Interest Paid, Net of Amounts Capitalized	94,600,000	96,100,000	95,800,000
Income Taxes Paid	$ 88,100,000	$ (62,200,000)	$ 147,600,000