SEGMENT INFORMATION (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Segment Reporting Information [Line Items]
Operating Revenues	$ 1,628,684,000	$ 1,047,481,000	$ 2,728,307,000	$ 2,282,732,000
Depreciation And Amortization	(210,800,000)	(107,700,000)	(315,400,000)	(232,400,000)
Operating Expenses	(1,258,400,000)	(761,700,000)	(2,039,000,000)	(1,644,800,000)
Operating Income	159,515,000	178,134,000	373,908,000	405,491,000
Total Interest Expense	(89,047,000)	(62,234,000)	(155,495,000)	(120,783,000)
Other Income/(Loss), Net	1,806,000	7,334,000	10,580,000	17,647,000
Income Tax Expense	(26,055,000)	(44,515,000)	(82,019,000)	(108,052,000)
Net Income	46,219,000	78,719,000	146,974,000	194,303,000
Net Income Attributable to Noncontrolling Interests	(1,880,000)	(1,441,000)	(3,373,000)	(2,870,000)
Net Income Attributable to Controlling Interests	44,339,000	77,278,000	143,601,000	191,433,000
Total Assets	27,506,300,000		27,506,300,000		15,647,066,000
Investments in Property, Plant and Equipment			690,376,000	468,526,000
The Connecticut Light And Power Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	562,141,000	608,013,000	1,154,106,000	1,281,695,000
Operating Income	40,381,000	114,799,000	152,245,000	240,782,000
Total Interest Expense	(33,771,000)	(34,298,000)	(67,278,000)	(64,050,000)
Other Income/(Loss), Net	447,000	2,058,000	5,747,000	6,663,000
Income Tax Expense	(124,000)	(29,924,000)	(29,796,000)	(66,423,000)
Net Income	6,933,000	52,635,000	60,918,000	116,972,000
Total Assets	8,835,371,000		8,835,371,000		8,791,396,000
Investments in Property, Plant and Equipment			220,712,000	201,966,000
Public Service Company Of New Hampshire [Member]
Segment Reporting Information [Line Items]
Operating Revenues	255,105,000	240,191,000	498,102,000	509,661,000
Operating Income	47,046,000	37,939,000	92,432,000	84,874,000
Total Interest Expense	(12,785,000)	(10,401,000)	(25,597,000)	(20,857,000)
Other Income/(Loss), Net	549,000	4,361,000	2,590,000	8,820,000
Income Tax Expense	(13,578,000)	(10,234,000)	(26,931,000)	(23,708,000)
Net Income	21,232,000	21,665,000	42,494,000	49,129,000
Total Assets	3,036,797,000		3,036,797,000		3,116,541,000
Investments in Property, Plant and Equipment			120,792,000	111,459,000
Western Massachusetts Electric Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	106,836,000	98,390,000	220,861,000	205,074,000
Operating Income	25,056,000	18,477,000	53,716,000	39,529,000
Total Interest Expense	(6,869,000)	(5,460,000)	(13,264,000)	(11,034,000)
Other Income/(Loss), Net	188,000	242,000	1,280,000	981,000
Income Tax Expense	(7,237,000)	(5,088,000)	(16,408,000)	(11,339,000)
Net Income	11,138,000	8,171,000	25,324,000	18,137,000
Total Assets	1,633,513,000		1,633,513,000		1,502,893,000
Investments in Property, Plant and Equipment			152,687,000	76,898,000
Electric Distribution Member
Segment Reporting Information [Line Items]
Operating Revenues	1,229,900,000	794,400,000	2,016,000,000	1,686,000,000
Depreciation And Amortization	(153,300,000)	(76,500,000)	(225,400,000)	(167,500,000)
Operating Expenses	(1,004,800,000)	(633,200,000)	(1,627,000,000)	(1,326,300,000)
Operating Income	71,800,000	84,700,000	163,600,000	192,200,000
Total Interest Expense	(44,800,000)	(31,000,000)	(77,900,000)	(60,600,000)
Interest Income	1,000,000	600,000	2,100,000	1,900,000
Other Income/(Loss), Net	(200,000)	2,800,000	4,200,000	6,600,000
Income Tax Expense	(6,900,000)	(17,200,000)	(28,300,000)	(43,600,000)
Net Income	20,900,000	39,900,000	63,700,000	96,500,000
Net Income Attributable to Noncontrolling Interests	(1,200,000)	(800,000)	(2,000,000)	(1,700,000)
Net Income Attributable to Controlling Interests	19,700,000	39,100,000	61,700,000	94,800,000
Total Assets	15,161,300,000		15,161,300,000		9,653,100,000
Investments in Property, Plant and Equipment			305,700,000	251,100,000
Electric Distribution Member | The Connecticut Light And Power Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	442,200,000	489,900,000	916,900,000	1,039,800,000
Net Income	(27,200,000)	19,800,000	(5,500,000)	49,200,000
Total Assets	6,137,500,000		6,137,500,000		6,161,000,000
Electric Distribution Member | Public Service Company Of New Hampshire [Member]
Segment Reporting Information [Line Items]
Operating Revenues	229,400,000	219,100,000	448,900,000	467,000,000
Net Income	14,700,000	16,000,000	29,900,000	37,500,000
Total Assets	2,451,500,000		2,451,500,000		2,551,300,000
Electric Distribution Member | Western Massachusetts Electric Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	81,600,000	85,400,000	173,600,000	179,300,000
Net Income	1,400,000	4,000,000	7,500,000	9,700,000
Total Assets	943,300,000		943,300,000		942,600,000
Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Operating Revenues	133,500,000	78,400,000	272,500,000	258,600,000
Depreciation And Amortization	(12,400,000)	(6,300,000)	(20,100,000)	(13,100,000)
Operating Expenses	(115,800,000)	(65,700,000)	(218,000,000)	(199,200,000)
Operating Income	5,300,000	6,400,000	34,400,000	46,300,000
Total Interest Expense	(8,800,000)	(5,200,000)	(14,300,000)	(10,400,000)
Interest Income	0	0	0	0
Other Income/(Loss), Net	100,000	400,000	100,000	800,000
Income Tax Expense	1,300,000	(400,000)	(7,600,000)	(13,000,000)
Net Income	(2,100,000)	1,200,000	12,600,000	23,700,000
Net Income Attributable to Noncontrolling Interests	0	0	0	0
Net Income Attributable to Controlling Interests	(2,100,000)	1,200,000	12,600,000	23,700,000
Total Assets	2,432,700,000		2,432,700,000		1,511,300,000
Investments in Property, Plant and Equipment			59,700,000	45,400,000
Transmission [Member]
Segment Reporting Information [Line Items]
Operating Revenues	228,700,000	152,100,000	391,600,000	310,300,000
Depreciation And Amortization	(28,600,000)	(21,400,000)	(49,800,000)	(44,800,000)
Operating Expenses	(65,500,000)	(44,200,000)	(113,200,000)	(92,500,000)
Operating Income	134,600,000	86,500,000	228,600,000	173,000,000
Total Interest Expense	(26,200,000)	(19,100,000)	(45,800,000)	(35,400,000)
Interest Income	100,000	100,000	300,000	300,000
Other Income/(Loss), Net	700,000	3,300,000	3,900,000	8,100,000
Income Tax Expense	(44,800,000)	(28,000,000)	(75,600,000)	(57,900,000)
Net Income	64,400,000	42,800,000	111,400,000	88,100,000
Net Income Attributable to Noncontrolling Interests	(700,000)	(600,000)	(1,400,000)	(1,200,000)
Net Income Attributable to Controlling Interests	63,700,000	42,200,000	110,000,000	86,900,000
Total Assets	5,327,700,000		5,327,700,000		3,792,900,000
Investments in Property, Plant and Equipment			297,200,000	146,000,000
Transmission [Member] | The Connecticut Light And Power Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	119,900,000	118,100,000	237,200,000	241,900,000
Net Income	34,100,000	32,800,000	66,400,000	67,800,000
Total Assets	2,697,900,000		2,697,900,000		2,630,400,000
Transmission [Member] | Public Service Company Of New Hampshire [Member]
Segment Reporting Information [Line Items]
Operating Revenues	25,700,000	21,100,000	49,200,000	42,700,000
Net Income	6,500,000	5,700,000	12,600,000	11,600,000
Total Assets	585,300,000		585,300,000		565,200,000
Transmission [Member] | Western Massachusetts Electric Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	25,200,000	13,000,000	47,300,000	25,800,000
Net Income	9,700,000	4,200,000	17,800,000	8,400,000
Total Assets	690,200,000		690,200,000		560,300,000
Other Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	230,200,000	130,800,000	363,400,000	261,200,000
Depreciation And Amortization	(17,700,000)	(3,900,000)	(21,600,000)	(8,300,000)
Operating Expenses	(263,000,000)	(126,800,000)	(397,800,000)	(261,700,000)
Operating Income	(50,500,000)	100,000	(56,000,000)	(8,800,000)
Total Interest Expense	(11,100,000)	(8,500,000)	(20,400,000)	(17,100,000)
Interest Income	1,400,000	1,500,000	2,600,000	2,700,000
Other Income/(Loss), Net	117,900,000	85,400,000	240,500,000	234,900,000
Income Tax Expense	25,000,000	2,000,000	31,000,000	7,800,000
Net Income	82,700,000	80,500,000	197,700,000	219,500,000
Net Income Attributable to Noncontrolling Interests	0	0	0	0
Net Income Attributable to Controlling Interests	82,700,000	80,500,000	197,700,000	219,500,000
Total Assets	20,614,100,000		20,614,100,000		6,618,000,000
Investments in Property, Plant and Equipment			27,800,000	26,000,000
Eliminations Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	(193,600,000)	(108,200,000)	(315,200,000)	(233,400,000)
Depreciation And Amortization	1,200,000	400,000	1,500,000	1,300,000
Operating Expenses	190,700,000	108,200,000	317,000,000	234,900,000
Operating Income	(1,700,000)	400,000	3,300,000	2,800,000
Total Interest Expense	1,900,000	1,600,000	2,900,000	2,700,000
Interest Income	(1,400,000)	(1,500,000)	(2,700,000)	(2,800,000)
Other Income/(Loss), Net	(117,800,000)	(85,300,000)	(240,400,000)	(234,800,000)
Income Tax Expense	(700,000)	(900,000)	(1,500,000)	(1,400,000)
Net Income	(119,700,000)	(85,700,000)	(238,400,000)	(233,500,000)
Net Income Attributable to Noncontrolling Interests	0	0	0	0
Net Income Attributable to Controlling Interests	(119,700,000)	(85,700,000)	(238,400,000)	(233,500,000)
Total Assets	(16,029,500,000)		(16,029,500,000)		(5,928,200,000)
Investments in Property, Plant and Equipment			0	0
Total [Member]
Segment Reporting Information [Line Items]
Interest Income	1,100,000	700,000	2,300,000	2,100,000
Other Income/(Loss), Net	$ 700,000	$ 6,600,000	$ 8,300,000	$ 15,600,000