SEGMENT INFORMATION (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Operating Revenues	$ 1,684,000,000	$ 1,861,500,000	$ 1,628,700,000	$ 1,099,600,000	$ 1,068,000,000	$ 1,114,900,000	$ 1,047,500,000	$ 1,235,300,000	$ 6,273,787,000	$ 4,465,657,000	$ 4,898,167,000
Depreciation And Amortization									(740,800,000)	(463,200,000)	(623,700,000)
Operating Expenses									(4,414,800,000)	(3,208,300,000)	(3,474,600,000)
Operating Income	331,400,000	412,900,000	159,500,000	214,400,000	184,900,000	203,800,000	178,100,000	227,400,000	1,118,206,000	794,176,000	799,891,000
Total Interest Expense									(329,945,000)	(250,425,000)	(237,291,000)
Other Income/(Loss), Net									19,742,000	27,715,000	41,916,000
Income Tax Expense									(274,926,000)	(170,953,000)	(210,409,000)
Net Income	176,600,000	209,500,000	46,200,000	100,800,000	114,800,000	91,400,000	78,700,000	115,600,000	533,077,000	400,513,000	394,107,000
Net Income Attributable to Noncontrolling Interests									(7,132,000)	(5,820,000)	(6,158,000)
Net Income Attributable to Controlling Interests	174,700,000	207,600,000	44,300,000	99,300,000	113,200,000	90,000,000	77,300,000	114,200,000	525,945,000	394,693,000	387,949,000
Total Assets	28,302,824,000				15,647,066,000				28,302,824,000	15,647,066,000
Investments in Property, Plant and Equipment									1,472,272,000	1,076,730,000	954,472,000
The Connecticut Light And Power Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	595,200,000	658,100,000	562,100,000	592,000,000	593,000,000	673,700,000	608,000,000	673,700,000	2,407,449,000	2,548,387,000	2,999,102,000
Operating Income	135,000,000	139,700,000	40,400,000	111,900,000	84,700,000	137,700,000	114,800,000	126,000,000	426,989,000	463,183,000	487,650,000
Total Interest Expense									(133,127,000)	(132,727,000)	(137,738,000)
Other Income/(Loss), Net									10,300,000	9,741,000	26,669,000
Income Tax Expense									(94,437,000)	(90,033,000)	(132,438,000)
Net Income	73,900,000	74,900,000	6,900,000	54,000,000	66,800,000	66,500,000	52,600,000	64,300,000	209,725,000	250,164,000	244,143,000
Total Assets	9,142,088,000				8,791,396,000				9,142,088,000	8,791,396,000
Investments in Property, Plant and Equipment									449,137,000	424,865,000	380,304,000
Public Service Company Of New Hampshire [Member]
Segment Reporting Information [Line Items]
Operating Revenues	233,000,000	256,900,000	255,100,000	243,000,000	243,700,000	259,600,000	240,200,000	269,500,000	988,013,000	1,013,003,000	1,033,439,000
Operating Income	51,400,000	61,300,000	47,000,000	45,400,000	46,800,000	48,500,000	37,900,000	46,900,000	205,095,000	180,104,000	176,186,000
Total Interest Expense									(50,228,000)	(44,147,000)	(47,067,000)
Other Income/(Loss), Net									3,008,000	14,255,000	11,749,000
Income Tax Expense									(60,993,000)	(49,945,000)	(50,801,000)
Net Income	27,200,000	27,200,000	21,200,000	21,300,000	25,500,000	25,600,000	21,700,000	27,500,000	96,882,000	100,267,000	90,067,000
Total Assets	3,114,597,000				3,116,541,000				3,114,597,000	3,116,541,000
Investments in Property, Plant and Equipment									203,902,000	241,772,000	296,335,000
Western Massachusetts Electric Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	107,900,000	112,500,000	106,800,000	114,000,000	107,700,000	104,500,000	98,400,000	106,700,000	441,164,000	417,315,000	395,161,000
Operating Income	28,900,000	28,100,000	25,100,000	28,700,000	29,000,000	19,800,000	18,500,000	21,100,000	110,774,000	88,363,000	58,657,000
Total Interest Expense									(26,634,000)	(23,612,000)	(21,839,000)
Other Income/(Loss), Net									2,503,000	1,489,000	2,597,000
Income Tax Expense									(32,140,000)	(23,186,000)	(16,325,000)
Net Income	15,100,000	14,100,000	11,100,000	14,200,000	16,500,000	8,400,000	8,200,000	10,000,000	54,503,000	43,054,000	23,090,000
Total Assets	1,723,069,000				1,502,893,000				1,723,069,000	1,502,893,000
Investments in Property, Plant and Equipment									264,175,000	237,996,000	115,178,000
Electric Distribution Member
Segment Reporting Information [Line Items]
Operating Revenues									4,716,500,000	3,343,100,000	3,802,000,000
Depreciation And Amortization									(530,300,000)	(337,200,000)	(501,200,000)
Operating Expenses									(3,585,400,000)	(2,637,400,000)	(2,925,100,000)
Operating Income									600,800,000	368,500,000	375,700,000
Total Interest Expense									(165,600,000)	(123,800,000)	(133,400,000)
Interest Income									2,800,000	3,700,000	700,000
Other Income/(Loss), Net									8,900,000	11,600,000	24,400,000
Income Tax Expense									(150,200,000)	(67,600,000)	(90,300,000)
Net Income									296,700,000	192,400,000	177,100,000
Net Income Attributable to Noncontrolling Interests									(4,400,000)	(3,300,000)	(3,600,000)
Net Income Attributable to Controlling Interests									292,300,000	189,100,000	173,500,000
Total Assets	18,047,300,000				9,653,100,000				18,047,300,000	9,653,100,000
Investments in Property, Plant and Equipment									611,700,000	540,700,000	560,100,000
Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Operating Revenues									572,900,000	430,800,000	434,300,000
Depreciation And Amortization									(49,100,000)	(27,700,000)	(23,800,000)
Operating Expenses									(445,200,000)	(333,500,000)	(340,000,000)
Operating Income									78,600,000	69,600,000	70,500,000
Total Interest Expense									(31,300,000)	(21,000,000)	(17,900,000)
Interest Income									0	0	0
Other Income/(Loss), Net									400,000	1,300,000	800,000
Income Tax Expense									(16,900,000)	(18,200,000)	(20,700,000)
Net Income									30,800,000	31,700,000	32,700,000
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income Attributable to Controlling Interests									30,800,000	31,700,000	32,700,000
Total Assets	2,717,400,000				1,511,300,000				2,717,400,000	1,511,300,000
Investments in Property, Plant and Equipment									148,700,000	98,200,000	82,500,000
Transmission [Member]
Segment Reporting Information [Line Items]
Operating Revenues									861,500,000	635,400,000	625,600,000
Depreciation And Amortization									(109,200,000)	(84,000,000)	(86,700,000)
Operating Expenses									(251,600,000)	(188,200,000)	(192,100,000)
Operating Income									500,700,000	363,200,000	346,800,000
Total Interest Expense									(96,700,000)	(76,700,000)	(73,200,000)
Interest Income									400,000	500,000	1,800,000
Other Income/(Loss), Net									7,300,000	10,700,000	14,300,000
Income Tax Expense									(159,200,000)	(95,600,000)	(109,300,000)
Net Income									252,500,000	202,100,000	180,400,000
Net Income Attributable to Noncontrolling Interests									(2,800,000)	(2,500,000)	(2,600,000)
Net Income Attributable to Controlling Interests									249,700,000	199,600,000	177,800,000
Total Assets	6,187,700,000				3,792,900,000				6,187,700,000	3,792,900,000
Investments in Property, Plant and Equipment									663,600,000	388,900,000	239,200,000
Other Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues									803,800,000	541,300,000	521,600,000
Depreciation And Amortization									(56,400,000)	(16,800,000)	(15,800,000)
Operating Expenses									(817,000,000)	(534,100,000)	(505,400,000)
Operating Income									(69,600,000)	(9,600,000)	400,000
Total Interest Expense									(43,600,000)	(33,700,000)	(17,400,000)
Interest Income									7,100,000	5,300,000	5,300,000
Other Income/(Loss), Net									795,000,000	455,200,000	436,400,000
Income Tax Expense									55,500,000	14,300,000	11,000,000
Net Income									744,400,000	431,500,000	435,700,000
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income Attributable to Controlling Interests									744,400,000	431,500,000	435,700,000
Total Assets	18,832,600,000				6,618,000,000				18,832,600,000	6,618,000,000
Investments in Property, Plant and Equipment									48,300,000	48,900,000	72,700,000
Eliminations Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues									(680,900,000)	(484,900,000)	(485,300,000)
Depreciation And Amortization									4,200,000	2,500,000	3,800,000
Operating Expenses									684,400,000	484,900,000	488,000,000
Operating Income									7,700,000	2,500,000	6,500,000
Total Interest Expense									7,300,000	4,800,000	4,600,000
Interest Income									(7,100,000)	(5,300,000)	(6,300,000)
Other Income/(Loss), Net									(795,100,000)	(455,300,000)	(435,500,000)
Income Tax Expense									(4,100,000)	(3,900,000)	(1,100,000)
Net Income									(791,300,000)	(457,200,000)	(431,800,000)
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income Attributable to Controlling Interests									(791,300,000)	(457,200,000)	(431,800,000)
Total Assets	(17,482,200,000)				(5,928,200,000)				(17,482,200,000)	(5,928,200,000)
Investments in Property, Plant and Equipment									0	0	0
Total [Member]
Segment Reporting Information [Line Items]
Operating Revenues									6,273,800,000	4,465,700,000	4,898,200,000
Operating Income									1,118,200,000	794,200,000	799,900,000
Total Interest Expense									(329,900,000)	(250,400,000)	(237,300,000)
Interest Income									3,200,000	4,200,000	1,500,000
Other Income/(Loss), Net									16,500,000	23,500,000	40,400,000
Income Tax Expense									(274,900,000)	(171,000,000)	(210,400,000)
Net Income									533,100,000	400,500,000	394,100,000
Net Income Attributable to Noncontrolling Interests									(7,200,000)	(5,800,000)	(6,200,000)
Net Income Attributable to Controlling Interests									525,900,000	394,700,000	387,900,000
Total Assets	28,302,800,000				15,647,100,000				28,302,800,000	15,647,100,000
Investments in Property, Plant and Equipment									$ 1,472,300,000	$ 1,076,700,000	$ 954,500,000