SEGMENT INFORMATION (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Information Line Items
Operating Revenues	$ 1,068,000,000	$ 1,114,900,000	$ 1,047,500,000	$ 1,235,300,000	$ 1,204,100,000	$ 1,243,300,000	$ 1,111,400,000	$ 1,339,400,000	$ 4,465,657,000	$ 4,898,167,000	$ 5,439,430,000
Depreciation And Amortization									(469,200,000)	(629,200,000)	(540,800,000)
Operating Expenses									(3,202,300,000)	(3,469,100,000)	(4,147,200,000)
Operating Income	184,900,000	203,800,000	178,100,000	227,400,000	195,300,000	199,600,000	178,300,000	226,700,000	794,176,000	799,891,000	751,375,000
Total Interest Expense									(250,425,000)	(237,291,000)	(273,637,000)
Other Income/(Loss), Net									27,715,000	41,916,000	37,801,000
Income Tax Expense									(170,953,000)	(210,409,000)	(179,947,000)
Net Income	114,800,000	91,400,000	78,700,000	115,600,000	131,300,000	101,900,000	73,300,000	87,600,000	400,513,000	394,107,000	335,592,000
Net Income Attributable to Noncontrolling Interests									(5,820,000)	(6,158,000)	(5,559,000)
Net Income Attributable to Controlling Interests	113,200,000	90,000,000	77,300,000	114,200,000	129,300,000	100,500,000	71,900,000	86,200,000	394,693,000	387,949,000	330,033,000
Total Assets	15,647,066,000				14,472,601,000				15,647,066,000	14,472,601,000
Investments in Property, Plant and Equipment									1,076,730,000	954,472,000	908,146,000
The Connecticut Light And Power Company [Member]
Segment Reporting Information Line Items
Operating Revenues	593,000,000	673,700,000	608,000,000	673,700,000	707,000,000	789,200,000	707,900,000	795,000,000	2,548,387,000	2,999,102,000	3,424,538,000
Depreciation And Amortization									(222,900,000)	(423,100,000)	(388,700,000)
Operating Expenses									(1,862,300,000)	(2,088,400,000)	(2,570,700,000)
Operating Income	84,700,000	137,700,000	114,800,000	126,000,000	124,600,000	131,400,000	106,200,000	125,500,000	463,183,000	487,650,000	465,106,000
Total Interest Expense									(132,727,000)	(137,738,000)	(155,817,000)
Other Income/(Loss), Net									9,741,000	26,669,000	25,874,000
Income Tax Expense									(90,033,000)	(132,438,000)	(118,847,000)
Net Income	66,800,000	66,500,000	52,600,000	64,300,000	82,600,000	69,000,000	44,100,000	48,400,000	250,164,000	244,143,000	216,316,000
Total Assets	8,791,396,000				8,255,192,000				8,791,396,000	8,255,192,000
Investments in Property, Plant and Equipment									424,865,000	380,304,000	435,723,000
Public Service Company Of New Hampshire [Member]
Segment Reporting Information Line Items
Operating Revenues	243,700,000	259,600,000	240,200,000	269,500,000	259,500,000	277,000,000	238,300,000	258,600,000	1,013,003,000	1,033,439,000	1,109,591,000
Depreciation And Amortization									(154,900,000)	(128,800,000)	(79,800,000)
Operating Expenses									(678,000,000)	(728,400,000)	(895,200,000)
Operating Income	46,800,000	48,500,000	37,900,000	46,900,000	43,100,000	49,800,000	43,400,000	39,900,000	180,104,000	176,186,000	134,587,000
Total Interest Expense									(44,147,000)	(47,067,000)	(46,489,000)
Other Income/(Loss), Net									14,255,000	11,749,000	9,462,000
Income Tax Expense									(49,945,000)	(50,801,000)	(31,990,000)
Net Income	25,500,000	25,600,000	21,700,000	27,500,000	23,900,000	28,800,000	21,600,000	15,800,000	100,267,000	90,067,000	65,570,000
Total Assets	3,116,541,000				2,879,121,000				3,116,541,000	2,879,121,000
Investments in Property, Plant and Equipment									241,772,000	296,335,000	266,440,000
Western Massachusetts Electric Company [Member]
Segment Reporting Information Line Items
Operating Revenues	107,700,000	104,500,000	98,400,000	106,700,000	98,800,000	103,700,000	92,500,000	100,200,000	417,315,000	395,161,000	402,413,000
Depreciation And Amortization									(49,300,000)	(41,500,000)	(34,000,000)
Operating Expenses									(279,600,000)	(295,100,000)	(309,800,000)
Operating Income	29,000,000	19,800,000	18,500,000	21,100,000	13,100,000	14,900,000	14,300,000	16,400,000	88,363,000	58,657,000	58,581,000
Total Interest Expense									(23,612,000)	(21,839,000)	(19,286,000)
Other Income/(Loss), Net									1,489,000	2,597,000	1,824,000
Income Tax Expense									(23,186,000)	(16,325,000)	(14,923,000)
Net Income	16,500,000	8,400,000	8,200,000	10,000,000	4,900,000	7,300,000	5,200,000	5,700,000	43,054,000	23,090,000	26,196,000
Total Assets	1,502,893,000				1,199,559,000				1,502,893,000	1,199,559,000
Investments in Property, Plant and Equipment									237,996,000	115,178,000	105,440,000
Electric Distribution Member
Segment Reporting Information Line Items
Operating Revenues									3,343,100,000	3,802,000,000	4,358,400,000
Depreciation And Amortization									(343,200,000)	(506,700,000)	(431,500,000)
Operating Expenses									(2,631,400,000)	(2,919,600,000)	(3,604,600,000)
Operating Income									368,500,000	375,700,000	322,300,000
Total Interest Expense									(123,800,000)	(133,400,000)	(149,100,000)
Interest Income									3,700,000	700,000	4,500,000
Other Income/(Loss), Net									11,600,000	24,400,000	24,000,000
Income Tax Expense									(67,600,000)	(90,300,000)	(60,200,000)
Net Income									192,400,000	177,100,000	141,500,000
Net Income Attributable to Noncontrolling Interests									(3,300,000)	(3,600,000)	(3,300,000)
Net Income Attributable to Controlling Interests									189,100,000	173,500,000	138,200,000
Total Assets	9,653,100,000				8,910,100,000				9,653,100,000	8,910,100,000
Investments in Property, Plant and Equipment									540,700,000	560,100,000	521,500,000
Electric Distribution Member | The Connecticut Light And Power Company [Member]
Segment Reporting Information Line Items
Operating Revenues									2,065,300,000	2,500,300,000	2,954,600,000
Depreciation And Amortization									(158,700,000)	(355,500,000)	(330,300,000)
Operating Expenses									(1,722,700,000)	(1,942,400,000)	(2,441,700,000)
Operating Income									183,900,000	202,400,000	182,600,000
Total Interest Expense									(71,700,000)	(77,600,000)	(93,100,000)
Interest Income									2,400,000	1,900,000	2,700,000
Other Income/(Loss), Net									2,500,000	14,600,000	16,200,000
Income Tax Expense									(21,100,000)	(43,600,000)	(31,100,000)
Net Income									96,000,000	97,700,000	77,300,000
Total Assets	6,161,000,000				5,640,000,000				6,161,000,000	5,640,000,000
Investments in Property, Plant and Equipment									303,200,000	270,200,000	270,800,000
Electric Distribution Member | Public Service Company Of New Hampshire [Member]
Segment Reporting Information Line Items
Operating Revenues									923,700,000	951,000,000	1,035,800,000
Depreciation And Amortization									(143,400,000)	(118,400,000)	(70,500,000)
Operating Expenses									(644,400,000)	(696,000,000)	(865,800,000)
Operating Income									135,900,000	136,600,000	99,500,000
Total Interest Expense									(36,200,000)	(38,600,000)	(39,800,000)
Interest Income									900,000	(1,700,000)	2,100,000
Other Income/(Loss), Net									11,200,000	11,600,000	6,000,000
Income Tax Expense									(35,600,000)	(38,600,000)	(20,200,000)
Net Income									76,200,000	69,300,000	47,600,000
Total Assets	2,551,300,000				2,388,400,000				2,551,300,000	2,388,400,000
Investments in Property, Plant and Equipment									189,000,000	252,200,000	207,800,000
Electric Distribution Member | Western Massachusetts Electric Company [Member]
Segment Reporting Information Line Items
Operating Revenues									354,400,000	350,900,000	368,200,000
Depreciation And Amortization									(41,100,000)	(32,900,000)	(30,800,000)
Operating Expenses									(264,600,000)	(281,300,000)	(297,300,000)
Operating Income									48,700,000	36,700,000	40,100,000
Total Interest Expense									(15,900,000)	(17,100,000)	(16,100,000)
Interest Income									400,000	400,000	(300,000)
Other Income/(Loss), Net									(2,100,000)	(1,800,000)	1,800,000
Income Tax Expense									(10,900,000)	(8,100,000)	(8,800,000)
Net Income									20,200,000	10,100,000	16,700,000
Total Assets	942,600,000				884,200,000				942,600,000	884,200,000
Investments in Property, Plant and Equipment									48,500,000	37,600,000	42,900,000
Natural Gas Distribution [Member]
Segment Reporting Information Line Items
Operating Revenues									430,800,000	434,300,000	449,600,000
Depreciation And Amortization									(27,700,000)	(23,800,000)	(26,800,000)
Operating Expenses									(333,500,000)	(340,000,000)	(368,100,000)
Operating Income									69,600,000	70,500,000	54,700,000
Total Interest Expense									(21,000,000)	(17,900,000)	(22,100,000)
Interest Income									0	0	0
Other Income/(Loss), Net									1,300,000	800,000	300,000
Income Tax Expense									(18,200,000)	(20,700,000)	(11,900,000)
Net Income									31,700,000	32,700,000	21,000,000
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income Attributable to Controlling Interests									31,700,000	32,700,000	21,000,000
Total Assets	1,511,300,000				1,447,200,000				1,511,300,000	1,447,200,000
Investments in Property, Plant and Equipment									98,200,000	82,500,000	54,800,000
Transmission [Member]
Segment Reporting Information Line Items
Operating Revenues									635,400,000	625,600,000	577,900,000
Depreciation And Amortization									(84,000,000)	(86,700,000)	(71,000,000)
Operating Expenses									(188,200,000)	(192,100,000)	(170,900,000)
Operating Income									363,200,000	346,800,000	336,000,000
Total Interest Expense									(76,700,000)	(73,200,000)	(72,500,000)
Interest Income									500,000	1,800,000	1,000,000
Other Income/(Loss), Net									10,700,000	14,300,000	7,600,000
Income Tax Expense									(95,600,000)	(109,300,000)	(105,500,000)
Net Income									202,100,000	180,400,000	166,600,000
Net Income Attributable to Noncontrolling Interests									(2,500,000)	(2,600,000)	(2,300,000)
Net Income Attributable to Controlling Interests									199,600,000	177,800,000	164,300,000
Total Assets	3,792,900,000				3,434,000,000				3,792,900,000	3,434,000,000
Investments in Property, Plant and Equipment									388,900,000	239,200,000	286,000,000
Transmission [Member] | The Connecticut Light And Power Company [Member]
Segment Reporting Information Line Items
Operating Revenues									483,100,000	498,800,000	469,900,000
Depreciation And Amortization									(64,200,000)	(67,600,000)	(58,400,000)
Operating Expenses									(139,600,000)	(146,000,000)	(129,000,000)
Operating Income									279,300,000	285,200,000	282,500,000
Total Interest Expense									(61,000,000)	(60,100,000)	(62,700,000)
Interest Income									400,000	1,500,000	800,000
Other Income/(Loss), Net									4,400,000	8,600,000	6,100,000
Income Tax Expense									(68,900,000)	(88,800,000)	(87,700,000)
Net Income									154,200,000	146,400,000	139,000,000
Total Assets	2,630,400,000				2,615,200,000				2,630,400,000	2,615,200,000
Investments in Property, Plant and Equipment									121,700,000	110,100,000	164,900,000
Transmission [Member] | Public Service Company Of New Hampshire [Member]
Segment Reporting Information Line Items
Operating Revenues									89,300,000	82,400,000	73,800,000
Depreciation And Amortization									(11,500,000)	(10,400,000)	(9,300,000)
Operating Expenses									(33,600,000)	(32,400,000)	(29,400,000)
Operating Income									44,200,000	39,600,000	35,100,000
Total Interest Expense									(7,900,000)	(8,500,000)	(6,700,000)
Interest Income									100,000	200,000	100,000
Other Income/(Loss), Net									2,000,000	1,700,000	1,300,000
Income Tax Expense									(14,300,000)	(12,200,000)	(11,800,000)
Net Income									24,100,000	20,800,000	18,000,000
Total Assets	565,200,000				490,700,000				565,200,000	490,700,000
Investments in Property, Plant and Equipment									52,800,000	44,100,000	58,600,000
Transmission [Member] | Western Massachusetts Electric Company [Member]
Segment Reporting Information Line Items
Operating Revenues									62,900,000	44,300,000	34,200,000
Depreciation And Amortization									(8,200,000)	(8,600,000)	(3,200,000)
Operating Expenses									(15,000,000)	(13,800,000)	(12,500,000)
Operating Income									39,700,000	21,900,000	18,500,000
Total Interest Expense									(7,700,000)	(4,700,000)	(3,200,000)
Interest Income									0	200,000	0
Other Income/(Loss), Net									3,200,000	3,800,000	300,000
Income Tax Expense									(12,300,000)	(8,200,000)	(6,100,000)
Net Income									22,900,000	13,000,000	9,500,000
Total Assets	560,300,000				315,400,000				560,300,000	315,400,000
Investments in Property, Plant and Equipment									189,500,000	77,600,000	62,500,000
Other Segment [Member]
Segment Reporting Information Line Items
Operating Revenues									541,300,000	521,600,000	482,100,000
Depreciation And Amortization									(16,800,000)	(15,800,000)	(13,400,000)
Operating Expenses									(534,100,000)	(505,400,000)	(435,900,000)
Operating Income									(9,600,000)	400,000	32,800,000
Total Interest Expense									(33,700,000)	(17,400,000)	(36,200,000)
Interest Income									5,300,000	5,300,000	7,700,000
Other Income/(Loss), Net									455,200,000	436,400,000	371,600,000
Income Tax Expense									14,300,000	11,000,000	100,000
Net Income									431,500,000	435,700,000	376,000,000
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income Attributable to Controlling Interests									431,500,000	435,700,000	376,000,000
Total Assets	6,618,000,000				6,283,000,000				6,618,000,000	6,283,000,000
Investments in Property, Plant and Equipment									48,900,000	72,700,000	0
Eliminations Segment [Member]
Segment Reporting Information Line Items
Operating Revenues									(484,900,000)	(485,300,000)	(428,600,000)
Depreciation And Amortization									2,500,000	3,800,000	1,900,000
Operating Expenses									484,900,000	488,000,000	432,300,000
Operating Income									2,500,000	6,500,000	5,600,000
Total Interest Expense									4,800,000	4,600,000	6,300,000
Interest Income									(5,300,000)	(6,300,000)	(7,600,000)
Other Income/(Loss), Net									(455,300,000)	(435,500,000)	(371,400,000)
Income Tax Expense									(3,900,000)	(1,100,000)	(2,400,000)
Net Income									(457,200,000)	(431,800,000)	(369,500,000)
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income Attributable to Controlling Interests									(457,200,000)	(431,800,000)	(369,500,000)
Total Assets	(5,928,200,000)				(5,601,700,000)				(5,928,200,000)	(5,601,700,000)
Investments in Property, Plant and Equipment									0	0	45,800,000
Total [Member]
Segment Reporting Information Line Items
Interest Income									4,200,000	1,500,000	5,600,000
Other Income/(Loss), Net									23,500,000	40,400,000	32,100,000
Total [Member] | The Connecticut Light And Power Company [Member]
Segment Reporting Information Line Items
Interest Income									2,800,000	3,400,000	3,500,000
Other Income/(Loss), Net									6,900,000	23,200,000	22,300,000
Total [Member] | Public Service Company Of New Hampshire [Member]
Segment Reporting Information Line Items
Interest Income									1,000,000	(1,500,000)	2,200,000
Other Income/(Loss), Net									13,200,000	13,300,000	7,300,000
Total [Member] | Western Massachusetts Electric Company [Member]
Segment Reporting Information Line Items
Interest Income									400,000	600,000	(300,000)
Other Income/(Loss), Net									$ 1,100,000	$ 2,000,000	$ 2,100,000