SEGMENT INFORMATION (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Segment Reporting Information [Line Items]
Operating Revenues	$ 1,861,529,000	$ 1,114,892,000	$ 4,589,835,000	$ 3,397,624,000
Depreciation And Amortization	(231,300,000)	(129,000,000)	(546,800,000)	(361,400,000)
Operating Expenses	(1,217,300,000)	(782,100,000)	(3,256,200,000)	(2,426,900,000)
Operating Income	412,905,000	203,816,000	786,813,000	609,307,000
Total Interest Expense	(90,361,000)	(63,932,000)	(245,856,000)	(184,716,000)
Other Income/(Loss), Net	4,324,000	1,430,000	14,904,000	19,077,000
Income Tax Expense	(117,360,000)	(49,883,000)	(199,379,000)	(157,934,000)
Net Income	209,508,000	91,431,000	356,482,000	285,734,000
Net Income Attributable to Noncontrolling Interests	(1,880,000)	(1,470,000)	(5,253,000)	(4,340,000)
Net Income Attributable to Controlling Interests	207,628,000	89,961,000	351,229,000	281,394,000
Total Assets	27,757,562,000		27,757,562,000		15,647,066,000
Investments in Property, Plant and Equipment			1,081,750,000	749,060,000
The Connecticut Light And Power Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	658,111,000	673,666,000	1,812,218,000	1,955,361,000
Operating Income	139,709,000	137,737,000	291,954,000	378,519,000
Total Interest Expense	(33,591,000)	(35,219,000)	(100,869,000)	(99,270,000)
Other Income/(Loss), Net	2,889,000	(2,356,000)	8,636,000	4,308,000
Income Tax Expense	(34,121,000)	(33,634,000)	(63,917,000)	(100,057,000)
Net Income	74,886,000	66,528,000	135,804,000	183,500,000
Total Assets	8,959,350,000		8,959,350,000		8,791,396,000
Investments in Property, Plant and Equipment			332,323,000	305,595,000
Public Service Company Of New Hampshire [Member]
Segment Reporting Information [Line Items]
Operating Revenues	256,949,000	259,648,000	755,051,000	769,309,000
Operating Income	61,298,000	48,468,000	153,730,000	133,343,000
Total Interest Expense	(12,607,000)	(10,368,000)	(38,204,000)	(31,224,000)
Other Income/(Loss), Net	(353,000)	3,293,000	2,237,000	12,112,000
Income Tax Expense	(21,106,000)	(15,759,000)	(48,037,000)	(39,468,000)
Net Income	27,232,000	25,634,000	69,726,000	74,763,000
Total Assets	3,021,083,000		3,021,083,000		3,116,541,000
Investments in Property, Plant and Equipment			161,021,000	167,383,000
Western Massachusetts Electric Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	112,470,000	104,515,000	333,331,000	309,589,000
Operating Income	28,145,000	19,786,000	81,861,000	59,315,000
Total Interest Expense	(6,769,000)	(5,992,000)	(20,033,000)	(17,026,000)
Other Income/(Loss), Net	685,000	(722,000)	1,965,000	259,000
Income Tax Expense	(7,977,000)	(4,638,000)	(24,385,000)	(15,977,000)
Net Income	14,084,000	8,434,000	39,408,000	26,571,000
Total Assets	1,657,371,000		1,657,371,000		1,502,893,000
Investments in Property, Plant and Equipment			218,184,000	153,470,000
Electric Distribution Member
Segment Reporting Information [Line Items]
Operating Revenues	1,483,700,000	878,600,000	3,499,700,000	2,564,700,000
Depreciation And Amortization	(172,600,000)	(98,700,000)	(398,100,000)	(266,300,000)
Operating Expenses	(1,027,400,000)	(666,800,000)	(2,654,400,000)	(1,993,100,000)
Operating Income	283,700,000	113,100,000	447,200,000	305,300,000
Total Interest Expense	(45,500,000)	(30,300,000)	(123,400,000)	(90,900,000)
Interest Income	(300,000)	900,000	1,800,000	2,700,000
Other Income/(Loss), Net	2,600,000	600,000	6,900,000	7,100,000
Income Tax Expense	(88,800,000)	(25,500,000)	(117,200,000)	(69,000,000)
Net Income	151,700,000	58,800,000	215,300,000	155,200,000
Net Income Attributable to Noncontrolling Interests	(1,200,000)	(800,000)	(3,200,000)	(2,400,000)
Net Income Attributable to Controlling Interests	150,500,000	58,000,000	212,100,000	152,800,000
Total Assets	17,576,900,000		17,576,900,000		9,653,100,000
Investments in Property, Plant and Equipment			461,300,000	385,000,000
Electric Distribution Member | The Connecticut Light And Power Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	531,500,000	552,600,000	1,448,300,000	1,592,500,000
Net Income	34,600,000	35,400,000	29,100,000	84,600,000
Total Assets	6,217,700,000		6,217,700,000		6,161,000,000
Electric Distribution Member | Public Service Company Of New Hampshire [Member]
Segment Reporting Information [Line Items]
Operating Revenues	232,400,000	237,500,000	681,400,000	704,500,000
Net Income	21,200,000	20,600,000	51,100,000	58,100,000
Total Assets	2,433,700,000		2,433,700,000		2,551,300,000
Electric Distribution Member | Western Massachusetts Electric Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	88,100,000	88,700,000	261,700,000	268,000,000
Net Income	5,300,000	2,800,000	12,800,000	12,500,000
Total Assets	934,800,000		934,800,000		942,600,000
Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Operating Revenues	91,300,000	59,600,000	361,500,000	318,100,000
Depreciation And Amortization	(12,600,000)	(7,200,000)	(32,700,000)	(20,200,000)
Operating Expenses	(77,200,000)	(52,300,000)	(292,900,000)	(251,500,000)
Operating Income	1,500,000	100,000	35,900,000	46,400,000
Total Interest Expense	(8,700,000)	(5,300,000)	(22,900,000)	(15,600,000)
Interest Income	0	0	0	0
Other Income/(Loss), Net	200,000	400,000	300,000	1,200,000
Income Tax Expense	2,600,000	1,800,000	(5,000,000)	(11,300,000)
Net Income	(4,400,000)	(3,000,000)	8,300,000	20,700,000
Net Income Attributable to Noncontrolling Interests	0	0	0	0
Net Income Attributable to Controlling Interests	(4,400,000)	(3,000,000)	8,300,000	20,700,000
Total Assets	2,744,000,000		2,744,000,000		1,511,300,000
Investments in Property, Plant and Equipment			105,900,000	73,600,000
Transmission [Member]
Segment Reporting Information [Line Items]
Operating Revenues	235,600,000	159,100,000	627,200,000	469,400,000
Depreciation And Amortization	(29,700,000)	(19,500,000)	(79,500,000)	(64,300,000)
Operating Expenses	(66,300,000)	(46,700,000)	(179,500,000)	(139,200,000)
Operating Income	139,600,000	92,900,000	368,200,000	265,900,000
Total Interest Expense	(26,100,000)	(21,300,000)	(71,900,000)	(56,600,000)
Interest Income	100,000	100,000	400,000	400,000
Other Income/(Loss), Net	1,900,000	(1,100,000)	5,800,000	7,000,000
Income Tax Expense	(43,700,000)	(28,500,000)	(119,300,000)	(86,400,000)
Net Income	71,800,000	42,100,000	183,200,000	130,300,000
Net Income Attributable to Noncontrolling Interests	(700,000)	(600,000)	(2,100,000)	(1,900,000)
Net Income Attributable to Controlling Interests	71,100,000	41,500,000	181,100,000	128,400,000
Total Assets	6,109,300,000		6,109,300,000		3,792,900,000
Investments in Property, Plant and Equipment			476,000,000	254,200,000
Transmission [Member] | The Connecticut Light And Power Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	126,600,000	121,100,000	363,900,000	362,900,000
Net Income	40,300,000	31,100,000	106,700,000	98,900,000
Total Assets	2,741,700,000		2,741,700,000		2,630,400,000
Transmission [Member] | Public Service Company Of New Hampshire [Member]
Segment Reporting Information [Line Items]
Operating Revenues	24,500,000	22,100,000	73,700,000	64,800,000
Net Income	6,000,000	5,000,000	18,600,000	16,700,000
Total Assets	587,400,000		587,400,000		565,200,000
Transmission [Member] | Western Massachusetts Electric Company [Member]
Segment Reporting Information [Line Items]
Operating Revenues	24,400,000	15,800,000	71,600,000	41,600,000
Net Income	8,800,000	5,600,000	26,600,000	14,100,000
Total Assets	722,600,000		722,600,000		560,300,000
Other Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	219,500,000	142,300,000	582,900,000	403,500,000
Depreciation And Amortization	(17,500,000)	(4,400,000)	(39,100,000)	(12,700,000)
Operating Expenses	(216,800,000)	(135,700,000)	(614,500,000)	(397,400,000)
Operating Income	(14,800,000)	2,200,000	(70,700,000)	(6,600,000)
Total Interest Expense	(12,200,000)	(8,100,000)	(32,700,000)	(25,300,000)
Interest Income	2,200,000	1,300,000	4,800,000	4,000,000
Other Income/(Loss), Net	324,900,000	86,300,000	565,300,000	321,200,000
Income Tax Expense	13,800,000	4,100,000	44,900,000	11,900,000
Net Income	313,900,000	85,800,000	511,600,000	305,200,000
Net Income Attributable to Noncontrolling Interests	0	0	0	0
Net Income Attributable to Controlling Interests	313,900,000	85,800,000	511,600,000	305,200,000
Total Assets	18,396,600,000		18,396,600,000		6,618,000,000
Investments in Property, Plant and Equipment			38,600,000	36,300,000
Eliminations Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	(168,600,000)	(124,700,000)	(481,500,000)	(358,100,000)
Depreciation And Amortization	1,100,000	800,000	2,600,000	2,100,000
Operating Expenses	170,400,000	119,400,000	485,100,000	354,300,000
Operating Income	2,900,000	(4,500,000)	6,200,000	(1,700,000)
Total Interest Expense	2,100,000	1,100,000	5,000,000	3,700,000
Interest Income	(2,100,000)	(1,300,000)	(4,800,000)	(4,000,000)
Other Income/(Loss), Net	(325,100,000)	(85,800,000)	(565,500,000)	(320,600,000)
Income Tax Expense	(1,300,000)	(1,800,000)	(2,800,000)	(3,100,000)
Net Income	(323,500,000)	(92,300,000)	(561,900,000)	(325,700,000)
Net Income Attributable to Noncontrolling Interests	0	0	0	0
Net Income Attributable to Controlling Interests	(323,500,000)	(92,300,000)	(561,900,000)	(325,700,000)
Total Assets	(17,069,200,000)		(17,069,200,000)		(5,928,200,000)
Investments in Property, Plant and Equipment			0	0
Total [Member]
Segment Reporting Information [Line Items]
Interest Income	(100,000)	1,000,000	2,200,000	3,100,000
Other Income/(Loss), Net	$ 4,500,000	$ 400,000	$ 12,800,000	$ 15,900,000