SEGMENT INFORMATION (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Segment Reporting Information [Line Items]
Operating Revenues	$ 1,099,623,000	$ 1,235,251,000
Depreciation And Amortization	(105,400,000)	(125,600,000)
Operating Expenses	(779,800,000)	(882,300,000)
Operating Income	214,393,000	227,357,000
Total Interest Expense	(66,447,000)	(58,549,000)
Other Income/(Loss), Net	8,773,000	10,313,000
Income Tax Expense	(55,964,000)	(63,537,000)
Net Income	100,755,000	115,584,000
Net Income Attributable to Noncontrolling Interests	(1,493,000)	(1,429,000)
Net Income Attributable to Controlling Interests	99,262,000	114,155,000
Total Assets	15,978,354,000	14,418,500,000	15,647,066,000
Investments in Property, Plant and Equipment	304,294,000	236,689,000
The Connecticut Light And Power Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	591,965,000	673,682,000
Depreciation And Amortization	(49,400,000)	(58,800,000)
Operating Expenses	(430,700,000)	(488,900,000)
Operating Income	111,865,000	125,983,000
Total Interest Expense	(33,508,000)	(29,753,000)
Other Income/(Loss), Net	5,300,000	4,606,000
Income Tax Expense	(29,672,000)	(36,499,000)
Net Income	53,985,000	64,337,000
Total Assets	8,800,653,000	8,186,500,000	8,791,396,000
Investments in Property, Plant and Equipment	108,842,000	106,829,000
Public Service Company Of New Hampshire [Member]
Segment Reporting Information [Line Items]
Operating Revenues	242,997,000	269,470,000
Depreciation And Amortization	(32,500,000)	(46,600,000)
Operating Expenses	(165,100,000)	(176,000,000)
Operating Income	45,386,000	46,936,000
Total Interest Expense	(12,813,000)	(10,457,000)
Other Income/(Loss), Net	2,042,000	4,459,000
Income Tax Expense	(13,353,000)	(13,474,000)
Net Income	21,262,000	27,464,000
Total Assets	3,039,120,000	2,856,400,000	3,116,541,000
Investments in Property, Plant and Equipment	67,059,000	57,718,000
Western Massachusetts Electric Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	114,025,000	106,684,000
Depreciation And Amortization	(12,200,000)	(9,900,000)
Operating Expenses	(73,100,000)	(75,700,000)
Operating Income	28,660,000	21,052,000
Total Interest Expense	(6,395,000)	(5,574,000)
Other Income/(Loss), Net	1,092,000	739,000
Income Tax Expense	(9,171,000)	(6,251,000)
Net Income	14,186,000	9,966,000
Total Assets	1,575,200,000	1,216,500,000	1,502,893,000
Investments in Property, Plant and Equipment	85,011,000	33,037,000
Electric Distribution Member
Segment Reporting Information [Line Items]
Operating Revenues	786,000,000	891,600,000
Depreciation And Amortization	(73,000,000)	(91,900,000)
Operating Expenses	(621,300,000)	(692,200,000)
Operating Income	91,700,000	107,500,000
Total Interest Expense	(33,000,000)	(29,500,000)
Interest Income	1,100,000	1,100,000
Other Income/(Loss), Net	4,400,000	3,700,000
Income Tax Expense	(21,400,000)	(26,300,000)
Net Income	42,800,000	56,500,000
Net Income Attributable to Noncontrolling Interests	(800,000)	(800,000)
Net Income Attributable to Controlling Interests	42,000,000	55,700,000
Total Assets	9,553,300,000	8,796,100,000
Investments in Property, Plant and Equipment	130,700,000	138,700,000
Electric Distribution Member | The Connecticut Light And Power Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	474,700,000	549,900,000
Depreciation And Amortization	(33,400,000)	(41,500,000)
Operating Expenses	(396,200,000)	(451,800,000)
Operating Income	45,100,000	56,600,000
Total Interest Expense	(18,800,000)	(16,500,000)
Interest Income	600,000	600,000
Other Income/(Loss), Net	2,700,000	1,400,000
Income Tax Expense	(8,000,000)	(12,800,000)
Net Income	21,600,000	29,300,000
Total Assets	6,141,600,000	5,563,700,000
Investments in Property, Plant and Equipment	70,900,000	80,300,000
Electric Distribution Member | Public Service Company Of New Hampshire [Member]
Segment Reporting Information [Line Items]
Operating Revenues	219,500,000	247,900,000
Depreciation And Amortization	(29,300,000)	(43,800,000)
Operating Expenses	(156,700,000)	(168,000,000)
Operating Income	33,500,000	36,100,000
Total Interest Expense	(10,600,000)	(8,700,000)
Interest Income	300,000	500,000
Other Income/(Loss), Net	1,200,000	3,500,000
Income Tax Expense	(9,300,000)	(9,900,000)
Net Income	15,100,000	21,500,000
Total Assets	2,473,200,000	2,363,700,000
Investments in Property, Plant and Equipment	47,600,000	46,100,000
Electric Distribution Member | Western Massachusetts Electric Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	91,900,000	93,900,000
Depreciation And Amortization	(10,300,000)	(6,600,000)
Operating Expenses	(68,500,000)	(72,600,000)
Operating Income	13,100,000	14,700,000
Total Interest Expense	(3,600,000)	(4,500,000)
Interest Income	100,000	100,000
Other Income/(Loss), Net	600,000	(1,000,000)
Income Tax Expense	(4,100,000)	(3,600,000)
Net Income	6,100,000	5,700,000
Total Assets	944,200,000	871,800,000
Investments in Property, Plant and Equipment	12,200,000	12,200,000
Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Operating Revenues	139,000,000	180,200,000
Depreciation And Amortization	(7,700,000)	(6,800,000)
Operating Expenses	(102,200,000)	(133,500,000)
Operating Income	29,100,000	39,900,000
Total Interest Expense	(5,400,000)	(5,200,000)
Interest Income	0	0
Other Income/(Loss), Net	0	400,000
Income Tax Expense	(9,000,000)	(12,600,000)
Net Income	14,700,000	22,500,000
Net Income Attributable to Noncontrolling Interests	0	0
Net Income Attributable to Controlling Interests	14,700,000	22,500,000
Total Assets	1,498,200,000	1,437,300,000
Investments in Property, Plant and Equipment	20,500,000	21,800,000
Transmission [Member]
Segment Reporting Information [Line Items]
Operating Revenues	162,800,000	158,200,000
Depreciation And Amortization	(21,100,000)	(23,400,000)
Operating Expenses	(47,600,000)	(48,300,000)
Operating Income	94,100,000	86,500,000
Total Interest Expense	(19,700,000)	(16,300,000)
Interest Income	100,000	200,000
Other Income/(Loss), Net	3,300,000	4,800,000
Income Tax Expense	(30,800,000)	(29,900,000)
Net Income	47,000,000	45,300,000
Net Income Attributable to Noncontrolling Interests	(700,000)	(600,000)
Net Income Attributable to Controlling Interests	46,300,000	44,700,000
Total Assets	3,900,700,000	3,476,900,000
Investments in Property, Plant and Equipment	135,900,000	61,800,000
Transmission [Member] | The Connecticut Light And Power Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	117,300,000	123,800,000
Depreciation And Amortization	(16,000,000)	(17,300,000)
Operating Expenses	(34,500,000)	(37,100,000)
Operating Income	66,800,000	69,400,000
Total Interest Expense	(14,700,000)	(13,300,000)
Interest Income	100,000	100,000
Other Income/(Loss), Net	1,900,000	2,500,000
Income Tax Expense	(21,700,000)	(23,700,000)
Net Income	32,400,000	35,000,000
Total Assets	2,659,100,000	2,622,800,000
Investments in Property, Plant and Equipment	37,900,000	26,500,000
Transmission [Member] | Public Service Company Of New Hampshire [Member]
Segment Reporting Information [Line Items]
Operating Revenues	23,500,000	21,600,000
Depreciation And Amortization	(3,200,000)	(2,800,000)
Operating Expenses	(8,400,000)	(8,000,000)
Operating Income	11,900,000	10,800,000
Total Interest Expense	(2,200,000)	(1,800,000)
Interest Income	0	100,000
Other Income/(Loss), Net	600,000	500,000
Income Tax Expense	(4,100,000)	(3,600,000)
Net Income	6,200,000	6,000,000
Total Assets	565,900,000	492,700,000
Investments in Property, Plant and Equipment	19,500,000	11,600,000
Transmission [Member] | Western Massachusetts Electric Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	22,100,000	12,800,000
Depreciation And Amortization	(1,900,000)	(3,300,000)
Operating Expenses	(4,600,000)	(3,100,000)
Operating Income	15,600,000	6,400,000
Total Interest Expense	(2,800,000)	(1,100,000)
Interest Income	0	0
Other Income/(Loss), Net	400,000	1,700,000
Income Tax Expense	(5,100,000)	(2,700,000)
Net Income	8,100,000	4,300,000
Total Assets	631,000,000	344,700,000
Investments in Property, Plant and Equipment	72,800,000	20,800,000
Other Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	133,300,000	130,400,000
Depreciation And Amortization	(3,900,000)	(4,300,000)
Operating Expenses	(134,900,000)	(135,000,000)
Operating Income	(5,500,000)	(8,900,000)
Total Interest Expense	(9,400,000)	(8,600,000)
Interest Income	1,300,000	1,300,000
Other Income/(Loss), Net	122,600,000	149,500,000
Income Tax Expense	6,000,000	5,700,000
Net Income	115,000,000	139,000,000
Net Income Attributable to Noncontrolling Interests	0	0
Net Income Attributable to Controlling Interests	115,000,000	139,000,000
Total Assets	7,261,700,000	6,275,000,000
Investments in Property, Plant and Equipment	17,200,000	14,400,000
Eliminations Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	(121,500,000)	(125,100,000)
Depreciation And Amortization	300,000	800,000
Operating Expenses	126,200,000	126,700,000
Operating Income	5,000,000	2,400,000
Total Interest Expense	1,100,000	1,100,000
Interest Income	(1,300,000)	(1,300,000)
Other Income/(Loss), Net	(122,700,000)	(149,500,000)
Income Tax Expense	(800,000)	(400,000)
Net Income	(118,700,000)	(147,700,000)
Net Income Attributable to Noncontrolling Interests	0	0
Net Income Attributable to Controlling Interests	(118,700,000)	(147,700,000)
Total Assets	(6,235,500,000)	(5,566,800,000)
Investments in Property, Plant and Equipment	0	0
Total [Member]
Segment Reporting Information [Line Items]
Interest Income	1,200,000	1,300,000
Other Income/(Loss), Net	7,600,000	8,900,000
Total [Member] | The Connecticut Light And Power Company [Member]
Segment Reporting Information [Line Items]
Interest Income	700,000	700,000
Other Income/(Loss), Net	4,600,000	3,900,000
Total [Member] | Public Service Company Of New Hampshire [Member]
Segment Reporting Information [Line Items]
Interest Income	300,000	600,000
Other Income/(Loss), Net	1,800,000	4,000,000
Total [Member] | Western Massachusetts Electric Company [Member]
Segment Reporting Information [Line Items]
Interest Income	100,000	100,000
Other Income/(Loss), Net	$ 1,000,000	$ 700,000