SEGMENT INFORMATION (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Segment Reporting Information Line Items
Operating Revenues	$ 1,047,481,000	$ 1,111,426,000	$ 2,282,732,000	$ 2,450,845,000
Depreciation And Amortization	(108,000,000)	(143,000,000)	(233,600,000)	(272,800,000)
Operating Expenses	(761,400,000)	(790,100,000)	(1,643,600,000)	(1,773,000,000)
Operating Income	178,134,000	178,344,000	405,491,000	405,007,000
Total Interest Expense	(62,234,000)	(67,197,000)	(120,783,000)	(134,458,000)
Other Income/(Loss), Net	7,334,000	1,552,000	17,647,000	9,608,000
Income Tax Expense	(44,515,000)	(39,351,000)	(108,052,000)	(119,209,000)
Net Income	78,719,000	73,348,000	194,303,000	160,948,000
Net Income Attributable to Noncontrolling Interests	(1,441,000)	(1,402,000)	(2,870,000)	(2,792,000)
Net Income Attributable to Controlling Interests	77,278,000	71,946,000	191,433,000	158,156,000
Total Assets	14,442,346,000	14,229,700,000	14,442,346,000	14,229,700,000	14,472,601,000
Investments in Property, Plant and Equipment			468,526,000	442,404,000
The Connecticut Light and Power Company [Member] | Electric Distribution Member
Segment Reporting Information Line Items
Operating Revenues	489,900,000	584,000,000	1,039,800,000	1,255,200,000
Depreciation And Amortization	(36,100,000)	(90,700,000)	(77,700,000)	(166,500,000)
Operating Expenses	(408,100,000)	(460,000,000)	(859,800,000)	(1,001,200,000)
Operating Income	45,700,000	33,300,000	102,300,000	87,500,000
Total Interest Expense	(18,500,000)	(21,500,000)	(35,000,000)	(43,900,000)
Interest Income	600,000	500,000	1,200,000	900,000
Other Income/(Loss), Net	(200,000)	(1,200,000)	1,300,000	1,100,000
Income Tax Expense	(7,800,000)	(1,900,000)	(20,600,000)	(21,300,000)
Net Income	19,800,000	9,200,000	49,200,000	24,300,000
Total Assets	5,601,300,000	5,675,000,000	5,601,300,000	5,675,000,000
Investments in Property, Plant and Equipment			141,900,000	132,400,000
Public Service Company of New Hampshire [Member] | Electric Distribution Member
Segment Reporting Information Line Items
Operating Revenues	219,100,000	218,200,000	467,000,000	457,100,000
Depreciation And Amortization	(30,800,000)	(14,000,000)	(74,600,000)	(34,000,000)
Operating Expenses	(160,800,000)	(170,000,000)	(328,700,000)	(358,500,000)
Operating Income	27,500,000	34,200,000	63,700,000	64,600,000
Total Interest Expense	(8,500,000)	(9,900,000)	(17,100,000)	(20,200,000)
Interest Income	0	(2,200,000)	400,000	(1,900,000)
Other Income/(Loss), Net	3,900,000	1,600,000	7,300,000	3,500,000
Income Tax Expense	(6,900,000)	(6,800,000)	(16,800,000)	(17,900,000)
Net Income	16,000,000	16,900,000	37,500,000	28,100,000
Total Assets	2,377,200,000	2,316,200,000	2,377,200,000	2,316,200,000
Investments in Property, Plant and Equipment			89,700,000	122,800,000
Western Massachusetts Electric Company [Member] | Electric Distribution Member
Segment Reporting Information Line Items
Operating Revenues	85,400,000	82,300,000	179,300,000	172,300,000
Depreciation And Amortization	(9,900,000)	(6,900,000)	(16,500,000)	(14,300,000)
Operating Expenses	(64,100,000)	(66,300,000)	(136,700,000)	(138,100,000)
Operating Income	11,400,000	9,100,000	26,100,000	19,900,000
Total Interest Expense	(4,100,000)	(4,400,000)	(8,500,000)	(8,100,000)
Interest Income	100,000	100,000	100,000	200,000
Other Income/(Loss), Net	(900,000)	(800,000)	(1,900,000)	(400,000)
Income Tax Expense	(2,500,000)	(1,700,000)	(6,100,000)	(6,400,000)
Net Income	4,000,000	2,300,000	9,700,000	5,200,000
Total Assets	863,600,000	874,200,000	863,600,000	874,200,000
Investments in Property, Plant and Equipment			19,500,000	12,800,000
Electric Distribution Member
Segment Reporting Information Line Items
Operating Revenues	794,400,000	884,500,000	1,686,000,000	1,884,500,000
Depreciation And Amortization	(76,800,000)	(111,600,000)	(168,700,000)	(214,700,000)
Operating Expenses	(632,900,000)	(696,300,000)	(1,325,100,000)	(1,497,800,000)
Operating Income	84,700,000	76,600,000	192,200,000	172,000,000
Total Interest Expense	(31,000,000)	(35,700,000)	(60,600,000)	(72,200,000)
Interest Income	600,000	(1,500,000)	1,900,000	(800,000)
Other Income/(Loss), Net	2,800,000	(500,000)	6,600,000	4,200,000
Income Tax Expense	(17,200,000)	(10,500,000)	(43,600,000)	(45,600,000)
Net Income	39,900,000	28,400,000	96,500,000	57,600,000
Net Income Attributable to Noncontrolling Interests	(800,000)	(800,000)	(1,700,000)	(1,600,000)
Net Income Attributable to Controlling Interests	39,100,000	27,600,000	94,800,000	56,000,000
Total Assets	8,836,000,000	8,860,100,000	8,836,000,000	8,860,100,000
Investments in Property, Plant and Equipment			251,100,000	267,900,000
Natural Gas Distribution Member
Segment Reporting Information Line Items
Operating Revenues	78,400,000	73,500,000	258,600,000	245,200,000
Depreciation And Amortization	(6,300,000)	(7,000,000)	(13,100,000)	(10,600,000)
Operating Expenses	(65,700,000)	(62,000,000)	(199,200,000)	(191,700,000)
Operating Income	6,400,000	4,500,000	46,300,000	42,900,000
Total Interest Expense	(5,200,000)	(5,500,000)	(10,400,000)	(10,400,000)
Interest Income	0	0	0	0
Other Income/(Loss), Net	400,000	100,000	800,000	100,000
Income Tax Expense	(400,000)	400,000	(13,000,000)	(13,600,000)
Net Income	1,200,000	(500,000)	23,700,000	19,000,000
Net Income Attributable to Noncontrolling Interests	0	0	0	0
Net Income Attributable to Controlling Interests	1,200,000	(500,000)	23,700,000	19,000,000
Total Assets	1,445,100,000	1,370,800,000	1,445,100,000	1,370,800,000
Investments in Property, Plant and Equipment			45,400,000	28,000,000
The Connecticut Light and Power Company [Member] | Transmission Member
Segment Reporting Information Line Items
Operating Revenues	118,100,000	123,900,000	241,900,000	247,700,000
Depreciation And Amortization	(16,000,000)	(16,800,000)	(33,300,000)	(33,500,000)
Operating Expenses	(33,000,000)	(34,200,000)	(70,100,000)	(70,000,000)
Operating Income	69,100,000	72,900,000	138,500,000	144,200,000
Total Interest Expense	(15,800,000)	(15,700,000)	(29,100,000)	(31,800,000)
Interest Income	100,000	900,000	200,000	1,100,000
Other Income/(Loss), Net	1,500,000	500,000	4,000,000	2,500,000
Income Tax Expense	(22,100,000)	(23,700,000)	(45,800,000)	(47,800,000)
Net Income	32,800,000	34,900,000	67,800,000	68,200,000
Total Assets	2,620,700,000	2,552,700,000	2,620,700,000	2,552,700,000
Investments in Property, Plant and Equipment			60,100,000	59,300,000
Public Service Company of New Hampshire [Member] | Transmission Member
Segment Reporting Information Line Items
Operating Revenues	21,100,000	20,100,000	42,700,000	39,800,000
Depreciation And Amortization	(2,800,000)	(2,600,000)	(5,600,000)	(5,300,000)
Operating Expenses	(7,900,000)	(8,300,000)	(15,900,000)	(15,800,000)
Operating Income	10,400,000	9,200,000	21,200,000	18,700,000
Total Interest Expense	(1,900,000)	(2,100,000)	(3,800,000)	(4,200,000)
Interest Income	0	200,000	100,000	100,000
Other Income/(Loss), Net	500,000	200,000	1,000,000	500,000
Income Tax Expense	(3,300,000)	(2,800,000)	(6,900,000)	(5,800,000)
Net Income	5,700,000	4,700,000	11,600,000	9,300,000
Total Assets	507,300,000	475,600,000	507,300,000	475,600,000
Investments in Property, Plant and Equipment			21,800,000	18,900,000
Western Massachusetts Electric Company [Member] | Transmission Member
Segment Reporting Information Line Items
Operating Revenues	13,000,000	10,200,000	25,800,000	20,400,000
Depreciation And Amortization	(2,600,000)	(2,100,000)	(5,900,000)	(3,000,000)
Operating Expenses	(3,300,000)	(2,900,000)	(6,500,000)	(6,600,000)
Operating Income	7,100,000	5,200,000	13,400,000	10,800,000
Total Interest Expense	(1,400,000)	(1,300,000)	(2,500,000)	(2,500,000)
Interest Income	0	100,000	0	200,000
Other Income/(Loss), Net	1,100,000	700,000	2,700,000	800,000
Income Tax Expense	(2,600,000)	(1,800,000)	(5,200,000)	(3,600,000)
Net Income	4,200,000	2,900,000	8,400,000	5,700,000
Total Assets	402,300,000	290,200,000	402,300,000	290,200,000
Investments in Property, Plant and Equipment			57,400,000	33,600,000
Transmission Member
Segment Reporting Information Line Items
Operating Revenues	152,100,000	154,200,000	310,300,000	307,900,000
Depreciation And Amortization	(21,400,000)	(21,600,000)	(44,800,000)	(41,800,000)
Operating Expenses	(44,200,000)	(45,400,000)	(92,500,000)	(92,400,000)
Operating Income	86,500,000	87,200,000	173,000,000	173,700,000
Total Interest Expense	(19,100,000)	(19,100,000)	(35,400,000)	(38,500,000)
Interest Income	100,000	1,200,000	300,000	1,400,000
Other Income/(Loss), Net	3,300,000	1,500,000	8,100,000	3,900,000
Income Tax Expense	(28,000,000)	(28,300,000)	(57,900,000)	(57,200,000)
Net Income	42,800,000	42,500,000	88,100,000	83,300,000
Net Income Attributable to Noncontrolling Interests	(600,000)	(600,000)	(1,200,000)	(1,200,000)
Net Income Attributable to Controlling Interests	42,200,000	41,900,000	86,900,000	82,100,000
Total Assets	3,551,400,000	3,327,700,000	3,551,400,000	3,327,700,000
Investments in Property, Plant and Equipment			146,000,000	113,100,000
Other Segment Member
Segment Reporting Information Line Items
Operating Revenues	130,800,000	133,800,000	261,200,000	257,900,000
Depreciation And Amortization	(3,900,000)	(3,500,000)	(8,300,000)	(7,300,000)
Operating Expenses	(126,800,000)	(117,100,000)	(261,700,000)	(232,800,000)
Operating Income	100,000	13,200,000	(8,800,000)	17,800,000
Total Interest Expense	(8,500,000)	(8,100,000)	(17,100,000)	(15,700,000)
Interest Income	1,500,000	1,400,000	2,700,000	2,700,000
Other Income/(Loss), Net	85,400,000	72,100,000	234,900,000	183,800,000
Income Tax Expense	2,000,000	(1,000,000)	7,800,000	(2,600,000)
Net Income	80,500,000	77,600,000	219,500,000	186,000,000
Net Income Attributable to Noncontrolling Interests	0	0	0	0
Net Income Attributable to Controlling Interests	80,500,000	77,600,000	219,500,000	186,000,000
Total Assets	6,208,600,000	6,043,900,000	6,208,600,000	6,043,900,000
Investments in Property, Plant and Equipment			26,000,000	33,400,000
Eliminations Segment Member
Segment Reporting Information Line Items
Operating Revenues	(108,200,000)	(134,600,000)	(233,400,000)	(244,700,000)
Depreciation And Amortization	400,000	700,000	1,300,000	1,600,000
Operating Expenses	108,200,000	130,700,000	234,900,000	241,700,000
Operating Income	400,000	(3,200,000)	2,800,000	(1,400,000)
Total Interest Expense	1,600,000	1,200,000	2,700,000	2,400,000
Interest Income	(1,500,000)	(2,300,000)	(2,800,000)	(3,700,000)
Other Income/(Loss), Net	(85,300,000)	(70,400,000)	(234,800,000)	(182,000,000)
Income Tax Expense	(900,000)	0	(1,400,000)	(200,000)
Net Income	(85,700,000)	(74,700,000)	(233,500,000)	(184,900,000)
Net Income Attributable to Noncontrolling Interests	0	0	0	0
Net Income Attributable to Controlling Interests	(85,700,000)	(74,700,000)	(233,500,000)	(184,900,000)
Total Assets	(5,598,800,000)	(5,372,800,000)	(5,598,800,000)	(5,372,800,000)
Investments in Property, Plant and Equipment			0	0
The Connecticut Light and Power Company [Member] | Total Member
Segment Reporting Information Line Items
Interest Income	700,000	1,400,000	1,400,000	2,000,000
Other Income/(Loss), Net	1,300,000	(700,000)	5,300,000	3,600,000
Public Service Company of New Hampshire [Member] | Total Member
Segment Reporting Information Line Items
Interest Income	0	(2,000,000)	500,000	(1,800,000)
Other Income/(Loss), Net	4,400,000	1,800,000	8,300,000	4,000,000
Western Massachusetts Electric Company [Member] | Total Member
Segment Reporting Information Line Items
Interest Income	100,000	200,000	100,000	400,000
Other Income/(Loss), Net	200,000	(100,000)	800,000	400,000
Total Member
Segment Reporting Information Line Items
Interest Income	700,000	(1,200,000)	2,100,000	(400,000)
Other Income/(Loss), Net	6,600,000	2,800,000	15,600,000	10,000,000
The Connecticut Light and Power Company [Member]
Segment Reporting Information Line Items
Operating Revenues	608,013,000	707,917,000	1,281,695,000	1,502,897,000
Depreciation And Amortization	(52,100,000)	(107,500,000)	(111,000,000)	(200,000,000)
Operating Expenses	(441,100,000)	(494,200,000)	(929,900,000)	(1,071,200,000)
Operating Income	114,799,000	106,157,000	240,782,000	231,656,000
Total Interest Expense	(34,298,000)	(37,207,000)	(64,050,000)	(75,734,000)
Other Income/(Loss), Net	2,058,000	745,000	6,663,000	5,678,000
Income Tax Expense	(29,924,000)	(25,610,000)	(66,423,000)	(69,102,000)
Net Income	52,635,000	44,085,000	116,972,000	92,498,000
Total Assets	8,222,022,000	8,227,700,000	8,222,022,000	8,227,700,000	8,255,192,000
Investments in Property, Plant and Equipment			201,966,000	191,667,000
Public Service Company of New Hampshire [Member]
Segment Reporting Information Line Items
Operating Revenues	240,191,000	238,322,000	509,661,000	496,890,000
Depreciation And Amortization	(33,600,000)	(16,600,000)	(80,200,000)	(39,300,000)
Operating Expenses	(168,700,000)	(178,300,000)	(344,600,000)	(374,300,000)
Operating Income	37,939,000	43,384,000	84,874,000	83,311,000
Total Interest Expense	(10,401,000)	(11,969,000)	(20,857,000)	(24,381,000)
Other Income/(Loss), Net	4,361,000	(197,000)	8,820,000	2,215,000
Income Tax Expense	(10,234,000)	(9,602,000)	(23,708,000)	(23,719,000)
Net Income	21,665,000	21,616,000	49,129,000	37,426,000
Total Assets	2,884,492,000	2,791,800,000	2,884,492,000	2,791,800,000	2,879,121,000
Investments in Property, Plant and Equipment			111,459,000	141,709,000
Western Massachusetts Electric Company [Member]
Segment Reporting Information Line Items
Operating Revenues	98,390,000	92,473,000	205,074,000	192,680,000
Depreciation And Amortization	(12,500,000)	(9,000,000)	(22,400,000)	(17,300,000)
Operating Expenses	(67,400,000)	(69,200,000)	(143,200,000)	(144,700,000)
Operating Income	18,477,000	14,265,000	39,529,000	30,710,000
Total Interest Expense	(5,460,000)	(5,657,000)	(11,034,000)	(10,601,000)
Other Income/(Loss), Net	242,000	161,000	981,000	765,000
Income Tax Expense	(5,088,000)	(3,520,000)	(11,339,000)	(9,966,000)
Net Income	8,171,000	5,249,000	18,137,000	10,908,000
Total Assets	1,265,863,000	1,164,400,000	1,265,863,000	1,164,400,000	1,199,559,000
Investments in Property, Plant and Equipment			$ 76,898,000	$ 46,354,000