Supplemental Cash Flow Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Capital Expenditures Incurred But Not Yet Paid	$ 181,900,000	$ 193,100,000	$ 160,600,000
Interest Paid, Net of Amounts Capitalized	349,600,000	343,300,000	356,500,000
Income Taxes Paid	334,200,000	50,000,000	(12,800,000)
The Connecticut Light And Power Company [Member]
Capital Expenditures Incurred But Not Yet Paid	63,500,000	51,400,000	42,800,000
Interest Paid, Net of Amounts Capitalized	144,100,000	131,600,000	129,400,000
Income Taxes Paid	135,400,000	55,000,000	(42,000,000)
NSTAR Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	34,600,000	57,000,000	50,000,000
Interest Paid, Net of Amounts Capitalized	75,300,000	75,800,000	94,600,000
Income Taxes Paid	217,100,000	163,400,000	88,100,000
Public Service Company Of New Hampshire [Member]
Capital Expenditures Incurred But Not Yet Paid	39,300,000	34,900,000	16,800,000
Interest Paid, Net of Amounts Capitalized	41,100,000	43,300,000	49,800,000
Income Taxes Paid	2,300,000	(30,100,000)	14,700,000
Western Massachusetts Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	14,200,000	19,500,000	30,000,000
Interest Paid, Net of Amounts Capitalized	25,900,000	25,800,000	25,800,000
Income Taxes Paid	$ 25,100,000	$ (69,000,000)	$ (8,400,000)