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	$ 283,508,000		$ 314,049,000	$ 741,846,000		$ 875,102,000
Purchased power and gas	74,311,000		100,848,000	265,774,000		379,454,000
Operation and maintenance	88,586,000		90,393,000	174,793,000		175,525,000
Transmission wholesale	17,234,000		17,607,000	33,282,000		34,629,000
Depreciation and amortization	43,048,000		39,077,000	89,607,000		84,048,000
Taxes - other than income taxes	23,822,000		25,900,000	54,422,000		61,407,000
Operating Income	36,507,000		40,224,000	123,968,000		140,039,000
Other Income and (Deductions), net	4,872,000		5,604,000	13,789,000		10,269,000
Interest Charges, net	23,736,000		23,597,000	46,982,000		47,071,000
Income Before Income Taxes, Equity Earnings	17,643,000		22,231,000	90,775,000		103,237,000
Income Taxes	9,546,000		10,708,000	40,089,000		41,718,000
Income Before Equity Earnings	8,097,000		11,523,000	50,686,000		61,519,000
Income from Equity Investments	3,915,000		2,647,000	8,402,000		4,709,000
Net Income	12,012,000		14,170,000	59,088,000		66,228,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	13,000		14,000	39,000		28,000
Net Income attributable to UIL Holdings	11,999,000		14,156,000	59,049,000		66,200,000
Total Assets	4,644,217,000			4,644,217,000			4,744,609,000
Number of reporting segments				3
Electric Distribution Reportable Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	128,782,000		135,437,000	269,895,000		287,066,000
Purchased power and gas	34,358,000		37,311,000	75,016,000		86,574,000
Operation and maintenance	42,403,000		46,259,000	87,472,000		95,599,000
Transmission wholesale	0		0	0		0
Depreciation and amortization	20,998,000		19,526,000	41,477,000		40,742,000
Taxes - other than income taxes	9,917,000		10,860,000	20,626,000		22,298,000
Operating Income	21,106,000		21,481,000	45,304,000		41,853,000
Other Income and (Deductions), net	2,429,000		3,357,000	7,485,000		7,532,000
Interest Charges, net	7,212,000		7,283,000	14,490,000		14,775,000
Income Before Income Taxes, Equity Earnings	16,323,000		17,555,000	38,299,000		34,610,000
Income Taxes	9,333,000		9,101,000	21,566,000		17,491,000
Income Before Equity Earnings	6,990,000		8,454,000	16,733,000		17,119,000
Income from Equity Investments	3,915,000		2,647,000	8,402,000		4,709,000
Net Income	10,905,000		11,101,000	25,135,000		21,828,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0		0	0		0
Net Income attributable to UIL Holdings	10,905,000		11,101,000	25,135,000		21,828,000
Total Assets	0	[1]		0	[1]		0	[1]
Electric Transmission Reportable Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	50,404,000		47,999,000	99,797,000		94,596,000
Purchased power and gas	0		0	0		0
Operation and maintenance	9,854,000		7,980,000	20,951,000		14,978,000
Transmission wholesale	17,234,000		17,607,000	33,282,000		34,629,000
Depreciation and amortization	3,662,000		3,094,000	7,155,000		6,164,000
Taxes - other than income taxes	5,893,000		6,699,000	11,602,000		13,524,000
Operating Income	13,761,000		12,619,000	26,807,000		25,301,000
Other Income and (Deductions), net	1,005,000		1,692,000	2,026,000		3,138,000
Interest Charges, net	3,333,000		3,019,000	6,894,000		5,981,000
Income Before Income Taxes, Equity Earnings	11,433,000		11,292,000	21,939,000		22,458,000
Income Taxes	3,663,000		3,408,000	6,754,000		6,876,000
Income Before Equity Earnings	7,770,000		7,884,000	15,185,000		15,582,000
Income from Equity Investments	0		0	0		0
Net Income	7,770,000		7,884,000	15,185,000		15,582,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0		0	0		0
Net Income attributable to UIL Holdings	7,770,000		7,884,000	15,185,000		15,582,000
Total Assets	0	[1]		0	[1]		0	[1]
Total Electric Reportable Segments [Member]
Segment Reporting Information [Line Items]
Operating Revenues	179,186,000		183,436,000	369,692,000		381,662,000
Purchased power and gas	34,358,000		37,311,000	75,016,000		86,574,000
Operation and maintenance	52,257,000		54,239,000	108,423,000		110,577,000
Transmission wholesale	17,234,000		17,607,000	33,282,000		34,629,000
Depreciation and amortization	24,660,000		22,620,000	48,632,000		46,906,000
Taxes - other than income taxes	15,810,000		17,559,000	32,228,000		35,822,000
Operating Income	34,867,000		34,100,000	72,111,000		67,154,000
Other Income and (Deductions), net	3,434,000		5,049,000	9,511,000		10,670,000
Interest Charges, net	10,545,000		10,302,000	21,384,000		20,756,000
Income Before Income Taxes, Equity Earnings	27,756,000		28,847,000	60,238,000		57,068,000
Income Taxes	12,996,000		12,509,000	28,320,000		24,367,000
Income Before Equity Earnings	14,760,000		16,338,000	31,918,000		32,701,000
Income from Equity Investments	3,915,000		2,647,000	8,402,000		4,709,000
Net Income	18,675,000		18,985,000	40,320,000		37,410,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0		0	0		0
Net Income attributable to UIL Holdings	18,675,000		18,985,000	40,320,000		37,410,000
Total Assets	2,720,923,000	[1]		2,720,923,000	[1]		2,716,460,000	[1]
Gas Distribution Reportable Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	103,618,000		130,609,000	370,889,000		493,433,000
Purchased power and gas	39,953,000		63,537,000	190,758,000		292,880,000
Operation and maintenance	36,240,000		35,863,000	66,156,000		64,485,000
Transmission wholesale	0		0	0		0
Depreciation and amortization	17,948,000		16,375,000	40,089,000		37,048,000
Taxes - other than income taxes	7,921,000		8,340,000	22,100,000		25,584,000
Operating Income	1,556,000		6,494,000	51,786,000		73,436,000
Other Income and (Deductions), net	1,268,000		471,000	3,914,000		(450,000)
Interest Charges, net	7,391,000		7,769,000	14,211,000		14,801,000
Income Before Income Taxes, Equity Earnings	(4,567,000)		(804,000)	41,489,000		58,185,000
Income Taxes	(1,366,000)		656,000	16,230,000		22,224,000
Income Before Equity Earnings	(3,201,000)		(1,460,000)	25,259,000		35,961,000
Income from Equity Investments	0		0	0		0
Net Income	(3,201,000)		(1,460,000)	25,259,000		35,961,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	13,000		14,000	39,000		28,000
Net Income attributable to UIL Holdings	(3,214,000)		(1,474,000)	25,220,000		35,933,000
Total Assets	1,863,947,000			1,863,947,000			1,953,079,000
Other [Member]
Segment Reporting Information [Line Items]
Operating Revenues	704,000		4,000	1,265,000		7,000
Purchased power and gas	0		0	0		0
Operation and maintenance	89,000		291,000	214,000		463,000
Transmission wholesale	0		0	0		0
Depreciation and amortization	440,000		82,000	886,000		94,000
Taxes - other than income taxes	91,000		1,000	94,000		1,000
Operating Income	84,000		(370,000)	71,000		(551,000)
Other Income and (Deductions), net	170,000		84,000	364,000		49,000
Interest Charges, net	5,800,000		5,526,000	11,387,000		11,514,000
Income Before Income Taxes, Equity Earnings	(5,546,000)		(5,812,000)	(10,952,000)		(12,016,000)
Income Taxes	(2,084,000)		(2,457,000)	(4,461,000)		(4,873,000)
Income Before Equity Earnings	(3,462,000)		(3,355,000)	(6,491,000)		(7,143,000)
Income from Equity Investments	0		0	0		0
Net Income	(3,462,000)		(3,355,000)	(6,491,000)		(7,143,000)
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0		0	0		0
Net Income attributable to UIL Holdings	(3,462,000)		(3,355,000)	(6,491,000)		(7,143,000)
Total Assets	59,347,000			59,347,000			75,070,000
Distribution Segment [Member]
Net plant in service segregated by segment [Abstract]
Net plant in service	1,012,700,000			1,012,700,000			1,029,800,000
Transmission Segment [Member]
Net plant in service segregated by segment [Abstract]
Net plant in service	$ 566,900,000			$ 566,900,000			$ 495,800,000

[1] Information for segmenting total assets between Distribution and Transmission is not available. Total UI assets are disclosed in theTotal UI column. Net plant in service is segregated by segment and, as of June 30, 2012, was $1,012.7 million and $566.9 million for Distribution and Transmission, respectively. As of December 31, 2011, net plant in service was $1,029.8 million and $495.8 million for Distribution and Transmission, respectively.