Supplemental Cash Flow Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Capital Expenditures Incurred But Not Yet Paid	$ 193,100,000	$ 160,600,000	$ 168,500,000
Interest Paid, Net of Amounts Capitalized	343,300,000	356,500,000	256,300,000
Income Taxes Paid	50,000,000	(12,800,000)	(76,600,000)
The Connecticut Light And Power Company [Member]
Capital Expenditures Incurred But Not Yet Paid	51,400,000	42,800,000	32,700,000
Interest Paid, Net of Amounts Capitalized	131,600,000	129,400,000	136,600,000
Income Taxes Paid	55,000,000	(42,000,000)	(27,500,000)
NSTAR Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	57,000,000	50,000,000	34,300,000
Interest Paid, Net of Amounts Capitalized	75,800,000	94,600,000	96,100,000
Income Taxes Paid	163,400,000	88,100,000	(62,200,000)
Public Service Company Of New Hampshire [Member]
Capital Expenditures Incurred But Not Yet Paid	34,900,000	16,800,000	51,100,000
Interest Paid, Net of Amounts Capitalized	43,300,000	49,800,000	49,300,000
Income Taxes Paid	(30,100,000)	14,700,000	(29,000,000)
Western Massachusetts Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	19,500,000	30,000,000	61,300,000
Interest Paid, Net of Amounts Capitalized	25,800,000	25,800,000	22,100,000
Income Taxes Paid	$ (69,000,000)	$ (8,400,000)	$ (4,900,000)