SEGMENT INFORMATION (Details) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010		Dec. 31, 2010
SEGMENT INFORMATION [Abstract]
Number of reporting segments					3
Segment information [Abstract]
Operating Revenues	$ 321,427,000		$ 236,277,000		$ 1,196,529,000		$ 663,673,000
Purchased power and gas	101,688,000		65,616,000		481,142,000		194,531,000
Operation and maintenance	95,539,000		57,372,000		284,539,000		171,360,000
Transmission wholesale	25,180,000		23,434,000		59,809,000		54,249,000
Depreciation and amortization	38,368,000		28,383,000		108,941,000		82,672,000
Taxes - other than income taxes	26,399,000		20,238,000		87,806,000		54,534,000
Acquisition-related expenses	0		6,471,000		0		13,171,000
Operating Income	34,253,000		34,763,000		174,292,000		93,156,000
Other Income and (Deductions), net	8,392,000		5,591,000		18,661,000		14,551,000
Interest Charges, net	24,343,000		11,609,000		71,414,000		32,627,000
Income Before Income Taxes and Equity Earnings	18,302,000		28,745,000		121,539,000		75,080,000
Income Taxes	9,629,000		12,114,000		51,347,000		30,156,000
Income Before Equity Earnings	8,673,000		16,631,000		70,192,000		44,924,000
Income from Equity Investments	3,521,000		345,000		8,230,000		(544,000)
Net Income	12,194,000		16,976,000		78,422,000		44,380,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	14,000		0		42,000		0
Net Income attributable to UIL Holdings	12,180,000		16,976,000		78,380,000		44,380,000
Total Assets	4,582,306,000				4,582,306,000				4,455,433,000
Goodwill	302,184,000				302,184,000				298,890,000
Electric Distribution Reportable Segment [Member]
Segment information [Abstract]
Operating Revenues	165,528,000		181,816,000		452,594,000		519,048,000
Purchased power and gas	52,974,000		65,616,000		139,548,000		194,531,000
Operation and maintenance	51,699,000		50,644,000		147,298,000		150,341,000
Transmission wholesale	0		0		0		0
Depreciation and amortization	22,899,000		25,367,000		63,641,000		73,224,000
Taxes - other than income taxes	13,040,000		12,005,000		35,338,000		33,849,000
Acquisition-related expenses	0		0		0		0
Operating Income	24,916,000		28,184,000		66,769,000		67,103,000
Other Income and (Deductions), net	3,159,000		4,805,000		10,691,000		12,384,000
Interest Charges, net	7,834,000		7,823,000		22,609,000		21,091,000
Income Before Income Taxes and Equity Earnings	20,241,000		25,166,000		54,851,000		58,396,000
Income Taxes	10,660,000		10,624,000		28,151,000		23,406,000
Income Before Equity Earnings	9,581,000		14,542,000		26,700,000		34,990,000
Income from Equity Investments	3,521,000		345,000		8,230,000		(544,000)
Net Income	13,102,000		14,887,000		34,930,000		34,446,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0				0
Net Income attributable to UIL Holdings	13,102,000				34,930,000
Total Assets	0				0
Net plant in service	982,300,000				982,300,000				820,900,000
Electric Transmission Reportable Segment [Member]
Segment information [Abstract]
Operating Revenues	55,292,000		54,458,000		149,888,000		144,615,000
Purchased power and gas	0		0		0		0
Operation and maintenance	7,535,000		6,649,000		22,513,000		20,606,000
Transmission wholesale	25,180,000		23,434,000		59,809,000		54,249,000
Depreciation and amortization	3,182,000		3,005,000		9,346,000		9,415,000
Taxes - other than income taxes	6,086,000		8,226,000		19,610,000		20,678,000
Acquisition-related expenses	0		0		0		0
Operating Income	13,309,000		13,144,000		38,610,000		39,667,000
Other Income and (Deductions), net	1,598,000		757,000		4,736,000		1,796,000
Interest Charges, net	3,752,000		2,859,000		9,733,000		8,711,000
Income Before Income Taxes and Equity Earnings	11,155,000		11,042,000		33,613,000		32,752,000
Income Taxes	3,560,000		4,598,000		10,436,000		13,251,000
Income Before Equity Earnings	7,595,000		6,444,000		23,177,000		19,501,000
Income from Equity Investments	0		0				0
Net Income	7,595,000		6,444,000		23,177,000		19,501,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0				0
Net Income attributable to UIL Holdings	7,595,000				23,177,000
Total Assets	0				0
Net plant in service	484,600,000				484,600,000				512,200,000
Total Electric Reportable Segments [Member]
Segment information [Abstract]
Operating Revenues	220,820,000		236,274,000		602,482,000		663,663,000
Purchased power and gas	52,974,000		65,616,000		139,548,000		194,531,000
Operation and maintenance	59,234,000		57,293,000		169,811,000		170,947,000
Transmission wholesale	25,180,000		23,434,000		59,809,000		54,249,000
Depreciation and amortization	26,081,000		28,372,000		72,987,000		82,639,000
Taxes - other than income taxes	19,126,000		20,231,000		54,948,000		54,527,000
Acquisition-related expenses	0		0		0		0
Operating Income	38,225,000		41,328,000		105,379,000		106,770,000
Other Income and (Deductions), net	4,757,000		5,562,000		15,427,000		14,180,000
Interest Charges, net	11,586,000		10,682,000		32,342,000		29,802,000
Income Before Income Taxes and Equity Earnings	31,396,000		36,208,000		88,464,000		91,148,000
Income Taxes	14,220,000		15,222,000		38,587,000		36,657,000
Income Before Equity Earnings	17,176,000		20,986,000		49,877,000		54,491,000
Income from Equity Investments	3,521,000		345,000		8,230,000		(544,000)
Net Income	20,697,000		21,331,000		58,107,000		53,947,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0				0
Net Income attributable to UIL Holdings	20,697,000				58,107,000
Total Assets	2,628,282,000	[1]			2,628,282,000	[1]			2,371,621,000	[1]
Gas Distribution Reportable Segment [Member]
Segment information [Abstract]
Operating Revenues	100,579,000				594,012,000
Purchased power and gas	48,714,000				341,594,000
Operation and maintenance	36,492,000				114,452,000
Transmission wholesale	0				0
Depreciation and amortization	12,188,000				35,761,000
Taxes - other than income taxes	7,266,000				32,850,000
Acquisition-related expenses	0				0
Operating Income	(4,081,000)				69,355,000
Other Income and (Deductions), net	3,237,000				2,787,000
Interest Charges, net	7,251,000				22,052,000
Income Before Income Taxes and Equity Earnings	(8,095,000)				50,090,000
Income Taxes	(1,865,000)				20,359,000
Income Before Equity Earnings	(6,230,000)				29,731,000
Income from Equity Investments	0
Net Income	(6,230,000)				29,731,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	14,000				42,000
Net Income attributable to UIL Holdings	(6,244,000)				29,689,000
Total Assets	1,915,613,000				1,915,613,000				2,033,576,000
Goodwill	302,200,000	[2]			302,200,000	[2]			298,890,000	[2]
Other [Member]
Segment information [Abstract]
Operating Revenues	28,000	[3]	3,000	[3]	35,000	[3]	10,000	[3]
Purchased power and gas	0	[3]	0	[3]	0	[3]	0	[3]
Operation and maintenance	(187,000)	[3]	79,000	[3]	276,000	[3]	413,000	[3]
Transmission wholesale	0	[3]	0	[3]	0	[3]	0	[3]
Depreciation and amortization	99,000	[3]	11,000	[3]	193,000	[3]	33,000	[3]
Taxes - other than income taxes	7,000	[3]	7,000	[3]	8,000	[3]	7,000	[3]
Acquisition-related expenses	0	[3]	6,471,000	[3]	0	[3]	13,171,000	[3]
Operating Income	109,000	[3]	(6,565,000)	[3]	(442,000)	[3]	(13,614,000)	[3]
Other Income and (Deductions), net	398,000	[3]	29,000	[3]	447,000	[3]	371,000	[3]
Interest Charges, net	5,506,000	[3]	927,000	[3]	17,020,000	[3]	2,825,000	[3]
Income Before Income Taxes and Equity Earnings	(4,999,000)	[3]	(7,463,000)	[3]	(17,015,000)	[3]	(16,068,000)	[3]
Income Taxes	(2,726,000)	[3]	(3,108,000)	[3]	(7,599,000)	[3]	(6,501,000)	[3]
Income Before Equity Earnings	(2,273,000)	[3]	(4,355,000)	[3]	(9,416,000)	[3]	(9,567,000)	[3]
Income from Equity Investments	0	[3]	0	[3]			0	[3]
Net Income	(2,273,000)	[3]	(4,355,000)	[3]	(9,416,000)	[3]	(9,567,000)	[3]
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0	[3]			0	[3]
Net Income attributable to UIL Holdings	(2,273,000)	[3]			(9,416,000)	[3]
Total Assets	$ 38,411,000	[3]			$ 38,411,000	[3]			$ 50,236,000	[3]

[1] Information for segmenting total assets between Distribution and Transmission is not available. Total UI assets are disclosed in the Total UI column. Net plant in service is segregated by segment and, as of September 30, 2011, was $982.3 million and $484.6 million for Distribution and Transmission, respectively. As of December 31, 2010, net plant in service was $820.9 million and $512.2 million for Distribution and Transmission, respectively.

[2] Includes $302.2 million and $298.9 million of goodwill in the Gas Distribution segment as of September 30, 2011 and December 31, 2010, respectively.

[3] Includes UIL Holdings Corporate and UIL Holdings' non-utility activities, and the elimination of intercompany accounts and notes receivable