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	$ 314,049,000	$ 207,116,000	$ 875,102,000	$ 427,396,000
Purchased power and gas	100,848,000	53,567,000	379,454,000	128,915,000
Operation and maintenance	95,002,000	62,367,000	189,000,000	113,988,000
Transmission wholesale	17,607,000	15,339,000	34,629,000	30,815,000
Depreciation and amortization	34,468,000	27,038,000	70,573,000	54,289,000
Taxes - other than income taxes	25,900,000	16,592,000	61,407,000	34,296,000
Acquisition-related costs	0	6,700,000	0	6,700,000
Operating Income	40,224,000	25,513,000	140,039,000	58,393,000
Other Income and (Deductions), net	5,604,000	4,904,000	10,269,000	8,960,000
Interest Charges, net	23,597,000	10,467,000	47,071,000	21,018,000
Income (Loss) Before Income Taxes and Equity Earnings	22,231,000	19,950,000	103,237,000	46,335,000
Income Taxes	10,708,000	7,564,000	41,718,000	18,042,000
Income (Loss) Before Equity Earnings	11,523,000	12,386,000	61,519,000	28,293,000
Income from Equity Investments	2,647,000	(897,000)	4,709,000	(889,000)
Net Income (Loss)	14,170,000	11,489,000	66,228,000	27,404,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	14,000	0	28,000	0
Net Income (Loss) attributable to UIL Holdings	14,156,000	11,489,000	66,200,000	27,404,000
Total Assets	4,487,033,000		4,487,033,000		4,455,433,000
Electric Distribution Reportable Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	135,437,000	162,142,000	287,066,000	337,232,000
Purchased power and gas	37,311,000	53,567,000	86,574,000	128,915,000
Operation and maintenance	46,259,000	55,382,000	95,599,000	99,697,000
Transmission wholesale	0	0	0	0
Depreciation and amortization	19,526,000	23,788,000	40,742,000	47,857,000
Taxes - other than income taxes	10,860,000	10,408,000	22,298,000	21,844,000
Acquisition-related costs	0	0	0	0
Operating Income	21,481,000	18,997,000	41,853,000	38,919,000
Other Income and (Deductions), net	3,357,000	4,124,000	7,532,000	7,579,000
Interest Charges, net	7,283,000	6,610,000	14,775,000	13,268,000
Income (Loss) Before Income Taxes and Equity Earnings	17,555,000	16,511,000	34,610,000	33,230,000
Income Taxes	9,101,000	6,199,000	17,491,000	12,839,000
Income (Loss) Before Equity Earnings	8,454,000	10,312,000	17,119,000	20,391,000
Income from Equity Investments	2,647,000	(897,000)	4,709,000	(889,000)
Net Income (Loss)	11,101,000	9,415,000	21,828,000	19,502,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0		0
Net Income (Loss) attributable to UIL Holdings	11,101,000		21,828,000
Total Assets	0		0		0
Net plant in service	889,200,000		889,200,000		820,900,000
Electric Transmission Reportable Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	47,999,000	44,971,000	94,596,000	90,157,000
Purchased power and gas	0	0	0	0
Operation and maintenance	7,980,000	6,870,000	14,978,000	13,957,000
Transmission wholesale	17,607,000	15,339,000	34,629,000	30,815,000
Depreciation and amortization	3,094,000	3,238,000	6,164,000	6,410,000
Taxes - other than income taxes	6,699,000	6,184,000	13,524,000	12,452,000
Acquisition-related costs	0	0	0	0
Operating Income	12,619,000	13,340,000	25,301,000	26,523,000
Other Income and (Deductions), net	1,692,000	648,000	3,138,000	1,039,000
Interest Charges, net	3,019,000	2,915,000	5,981,000	5,852,000
Income (Loss) Before Income Taxes and Equity Earnings	11,292,000	11,073,000	22,458,000	21,710,000
Income Taxes	3,408,000	4,396,000	6,876,000	8,619,000
Income (Loss) Before Equity Earnings	7,884,000	6,677,000	15,582,000	13,091,000
Income from Equity Investments	0	0	0	0
Net Income (Loss)	7,884,000	6,677,000	15,582,000	13,091,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0		0
Net Income (Loss) attributable to UIL Holdings	7,884,000		15,582,000
Total Assets	0		0		0
Net plant in service	537,600,000		537,600,000		512,200,000
Total Electric Reportable Segments [Member]
Segment Reporting Information [Line Items]
Operating Revenues	183,436,000	207,113,000	381,662,000	427,389,000
Purchased power and gas	37,311,000	53,567,000	86,574,000	128,915,000
Operation and maintenance	54,239,000	62,252,000	110,577,000	113,654,000
Transmission wholesale	17,607,000	15,339,000	34,629,000	30,815,000
Depreciation and amortization	22,620,000	27,026,000	46,906,000	54,267,000
Taxes - other than income taxes	17,559,000	16,592,000	35,822,000	34,296,000
Acquisition-related costs	0	0	0	0
Operating Income	34,100,000	32,337,000	67,154,000	65,442,000
Other Income and (Deductions), net	5,049,000	4,772,000	10,670,000	8,618,000
Interest Charges, net	10,302,000	9,525,000	20,756,000	19,120,000
Income (Loss) Before Income Taxes and Equity Earnings	28,847,000	27,584,000	57,068,000	54,940,000
Income Taxes	12,509,000	10,595,000	24,367,000	21,458,000
Income (Loss) Before Equity Earnings	16,338,000	16,989,000	32,701,000	33,482,000
Income from Equity Investments	2,647,000	(897,000)	4,709,000	(889,000)
Net Income (Loss)	18,985,000	16,092,000	37,410,000	32,593,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0		0
Net Income (Loss) attributable to UIL Holdings	18,985,000		37,410,000
Total Assets	2,571,172,000		2,571,172,000		2,371,621,000
Gas Distribution Reportable Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	130,609,000		493,433,000
Purchased power and gas	63,537,000		292,880,000
Operation and maintenance	40,472,000		77,960,000
Transmission wholesale	0		0
Depreciation and amortization	11,766,000		23,573,000
Taxes - other than income taxes	8,340,000		25,584,000
Acquisition-related costs	0		0
Operating Income	6,494,000		73,436,000
Other Income and (Deductions), net	471,000		(450,000)
Interest Charges, net	7,769,000		14,801,000
Income (Loss) Before Income Taxes and Equity Earnings	(804,000)		58,185,000
Income Taxes	656,000		22,224,000
Income (Loss) Before Equity Earnings	(1,460,000)		35,961,000
Income from Equity Investments	0		0
Net Income (Loss)	(1,460,000)		35,961,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	14,000		28,000
Net Income (Loss) attributable to UIL Holdings	(1,474,000)		35,933,000
Total Assets	1,946,319,000		1,946,319,000		2,033,576,000
Net plant in service	302,200,000		302,200,000		298,900,000
Other [Member]
Segment Reporting Information [Line Items]
Operating Revenues	4,000	3,000	7,000	7,000
Purchased power and gas	0	0	0	0
Operation and maintenance	291,000	115,000	463,000	334,000
Transmission wholesale	0	0	0	0
Depreciation and amortization	82,000	12,000	94,000	22,000
Taxes - other than income taxes	1,000	0	1,000	0
Acquisition-related costs	0	6,700,000	0	6,700,000
Operating Income	(370,000)	(6,824,000)	(551,000)	(7,049,000)
Other Income and (Deductions), net	84,000	132,000	49,000	342,000
Interest Charges, net	5,526,000	942,000	11,514,000	1,898,000
Income (Loss) Before Income Taxes and Equity Earnings	(5,812,000)	(7,634,000)	(12,016,000)	(8,605,000)
Income Taxes	(2,457,000)	(3,031,000)	(4,873,000)	(3,416,000)
Income (Loss) Before Equity Earnings	(3,355,000)	(4,603,000)	(7,143,000)	(5,189,000)
Income from Equity Investments	0	0	0	0
Net Income (Loss)	(3,355,000)	(4,603,000)	(7,143,000)	(5,189,000)
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0		0
Net Income (Loss) attributable to UIL Holdings	(3,355,000)		(7,143,000)
Total Assets	$ (30,458,000)		$ (30,458,000)		$ 50,236,000