SEGMENT INFORMATION (Details) (USD $)	3 Months Ended																12 Months Ended
	Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2010		Sep. 30, 2010		Jun. 30, 2010		Mar. 31, 2010		Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
SEGMENT INFORMATION [Abstract]
Number of reporting segments																	3
Segment information [Abstract]
Operating Revenues	$ 373,918,000		$ 321,427,000		$ 314,049,000		$ 561,053,000		$ 333,993,000		$ 236,277,000		$ 207,116,000		$ 220,280,000		$ 1,570,447,000		$ 997,666,000		$ 896,550,000
Purchased power and gas																	609,228,000		323,696,000		333,339,000
Operation and maintenance																	382,167,000		258,282,000		225,853,000
Transmission wholesale																	77,997,000		72,169,000		57,012,000
Depreciation and amortization (Note F)																	167,462,000		113,946,000		98,116,000
Taxes - other than income taxes																	114,211,000		78,702,000		60,062,000
Acquisition related costs																	0		25,572,000		0
Operating Income (Loss)	45,090,000		34,253,000		40,224,000		99,815,000		32,143,000		34,763,000		25,513,000		32,880,000		219,382,000		125,299,000		122,168,000
Other Income and (Deductions), net																	26,932,000		17,262,000		5,586,000
Interest Charges, net																	95,385,000		53,698,000		40,400,000
Income Before Income Taxes and Equity Earnings																	150,929,000		88,863,000		87,354,000
Income Taxes																	62,501,000		35,284,000		33,096,000
Income (Losses) Before Equity Earnings																	88,428,000		53,579,000		54,258,000
Income (Losses) from Equity Investments																	11,282,000		1,278,000		59,000
Net Income																	99,710,000		54,857,000		54,317,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests																	54,000		3,000		0
Net income attributable to UIL Holdings	21,276,000	[1]	12,180,000	[1]	14,156,000	[1]	52,044,000	[1]	10,474,000	[1]	16,976,000	[1]	11,489,000	[1]	15,915,000	[1]	99,656,000		54,854,000		54,317,000
Total Assets	4,744,609,000	[2]							4,481,838,000	[2]							4,744,609,000	[2]	4,481,838,000	[2]	2,221,760,000
Goodwill	266,797,000								298,890,000								266,797,000		298,890,000
Electric Distribution Reportable Segment [Member]
Segment information [Abstract]
Operating Revenues																	599,153,000		667,737,000		726,562,000
Purchased power and gas																	180,149,000		242,268,000		333,339,000
Operation and maintenance																	216,361,000		210,646,000		195,894,000
Transmission wholesale																	0		0		0
Depreciation and amortization (Note F)																	83,725,000		96,007,000		85,617,000
Taxes - other than income taxes																	45,967,000		44,206,000		40,978,000
Acquisition related costs																	0		0		0
Operating Income (Loss)																	72,951,000		74,610,000		70,734,000
Other Income and (Deductions), net																	14,697,000		13,101,000		5,586,000
Interest Charges, net																	30,489,000		28,539,000		24,592,000
Income Before Income Taxes and Equity Earnings																	57,159,000		59,172,000		51,728,000
Income Taxes																	30,865,000		25,026,000		20,106,000
Income (Losses) Before Equity Earnings																	26,294,000		34,146,000		31,622,000
Income (Losses) from Equity Investments																	11,282,000		1,278,000		59,000
Net Income																	37,576,000		35,424,000		31,681,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests																	0		0
Net income attributable to UIL Holdings																	37,576,000		35,424,000		31,681,000
Total Assets	0	[3]							0	[3]							0	[3]	0	[3]	0	[3]
Net plant in service	1,029,800,000								820,900,000								1,029,800,000		820,900,000		691,100,000
Electric Transmission Reportable Segment [Member]
Segment information [Abstract]
Operating Revenues																	198,503,000		191,810,000		169,119,000
Purchased power and gas																	0		0		0
Operation and maintenance																	32,027,000		27,699,000		29,027,000
Transmission wholesale																	77,997,000		72,169,000		57,012,000
Depreciation and amortization (Note F)																	12,690,000		12,402,000		12,349,000
Taxes - other than income taxes																	24,736,000		27,435,000		19,080,000
Acquisition related costs																	0		0		0
Operating Income (Loss)																	51,053,000		52,105,000		51,651,000
Other Income and (Deductions), net																	6,651,000		2,939,000		(33,000)
Interest Charges, net																	13,235,000		12,034,000		11,699,000
Income Before Income Taxes and Equity Earnings																	44,469,000		43,010,000		39,919,000
Income Taxes																	13,186,000		14,682,000		14,627,000
Income (Losses) Before Equity Earnings																	31,283,000		28,328,000		25,292,000
Income (Losses) from Equity Investments																	0		0		0
Net Income																	31,283,000		28,328,000		25,292,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests																	0		0
Net income attributable to UIL Holdings																	31,283,000		28,328,000		25,292,000
Total Assets	0	[3]							0	[3]							0	[3]	0	[3]	0	[3]
Net plant in service	495,800,000								512,200,000								495,800,000		512,200,000		461,800,000
Total Electric Reportable Segments [Member]
Segment information [Abstract]
Operating Revenues																	797,656,000		859,547,000		895,681,000
Purchased power and gas																	180,149,000		242,268,000		333,339,000
Operation and maintenance																	248,388,000		238,345,000		224,921,000
Transmission wholesale																	77,997,000		72,169,000		57,012,000
Depreciation and amortization (Note F)																	96,415,000		108,409,000		97,966,000
Taxes - other than income taxes																	70,703,000		71,641,000		60,058,000
Acquisition related costs																	0		0		0
Operating Income (Loss)																	124,004,000		126,715,000		122,385,000
Other Income and (Deductions), net																	21,348,000		16,040,000		5,553,000
Interest Charges, net																	43,724,000		40,573,000		36,291,000
Income Before Income Taxes and Equity Earnings																	101,628,000		102,182,000		91,647,000
Income Taxes																	44,051,000		39,708,000		34,733,000
Income (Losses) Before Equity Earnings																	57,577,000		62,474,000		56,914,000
Income (Losses) from Equity Investments																	11,282,000		1,278,000		59,000
Net Income																	68,859,000		63,752,000		56,973,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests																	0		0
Net income attributable to UIL Holdings																	68,859,000		63,752,000		56,973,000
Total Assets	2,716,460,000	[3]							2,383,057,000	[3]							2,716,460,000	[3]	2,383,057,000	[3]	2,203,062,000	[3]
Gas Distribution Reportable Segment [Member]
Segment information [Abstract]
Operating Revenues																	772,315,000		138,105,000
Purchased power and gas																	429,079,000		81,428,000
Operation and maintenance																	133,126,000		19,297,000
Transmission wholesale																	0		0
Depreciation and amortization (Note F)																	70,694,000		5,492,000
Taxes - other than income taxes																	43,494,000		7,054,000
Acquisition related costs																	0		0
Operating Income (Loss)																	95,922,000		24,834,000
Other Income and (Deductions), net																	6,571,000		107,000
Interest Charges, net																	28,939,000		4,014,000
Income Before Income Taxes and Equity Earnings																	73,554,000		20,927,000
Income Taxes																	29,721,000		8,026,000
Income (Losses) Before Equity Earnings																	43,833,000		12,901,000
Income (Losses) from Equity Investments																	0		0
Net Income																	43,833,000		12,901,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests																	54,000		3,000
Net income attributable to UIL Holdings																	43,779,000		12,898,000
Total Assets	1,953,079,000	[2]							2,050,332,000	[2]							1,953,079,000	[2]	2,050,332,000	[2]
Goodwill	266,800,000		298,900,000	[2]													266,800,000
Other [Member]
Segment information [Abstract]
Operating Revenues																	476,000		14,000		869,000
Purchased power and gas																	0		0		0
Operation and maintenance																	653,000		640,000		932,000
Transmission wholesale																	0		0		0
Depreciation and amortization (Note F)																	353,000		45,000		150,000
Taxes - other than income taxes																	14,000		7,000		4,000
Acquisition related costs																	0		25,572,000		0
Operating Income (Loss)																	(544,000)		(26,250,000)		(217,000)
Other Income and (Deductions), net																	(987,000)		1,115,000		33,000
Interest Charges, net																	22,722,000		9,111,000		4,109,000
Income Before Income Taxes and Equity Earnings																	(24,253,000)		(34,246,000)		(4,293,000)
Income Taxes																	(11,271,000)		(12,450,000)		(1,637,000)
Income (Losses) Before Equity Earnings																	(12,982,000)		(21,796,000)		(2,656,000)
Income (Losses) from Equity Investments																	0		0		0
Net Income																	(12,982,000)		(21,796,000)		(2,656,000)
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests																	0		0
Net income attributable to UIL Holdings																	(12,982,000)		(21,796,000)		(2,656,000)
Total Assets	$ 75,070,000	[2]							$ 48,449,000	[2]							$ 75,070,000	[2]	$ 48,449,000	[2]	$ 18,698,000

[1] Includes acquisition-related costs in the 2nd, 3rd and 4th quarters of 2010

[2] Includes $266.8 million and $298.9 million of goodwill in the Gas Distribution segment as of December 31, 2011 and December 31, 2010.

[3] 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 December 31, 2011, was $1,029.8 million and $495.8 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. As of December 31, 2009, net plant in service was $691.1 million and $461.8 million for Distribution and Transmission, respectively.