SEGMENT INFORMATION (Details) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013		Dec. 31, 2013
Segment Reporting Information [Line Items]
Operating Revenues	$ 293,026,000		$ 316,478,000		$ 1,198,982,000		$ 1,183,591,000
Purchased power and gas	68,776,000		74,657,000		446,067,000		421,516,000
Operation and maintenance	95,251,000		89,810,000		290,828,000		273,539,000
Transmission wholesale	25,802,000		28,659,000		65,777,000		65,994,000
Depreciation and amortization	35,578,000		50,761,000		112,408,000		146,537,000
Taxes - other than income taxes	32,897,000		32,389,000		102,974,000		96,335,000
Acquisition-related expenses	570,000		0		6,090,000		0
Rate case disallowances	0		17,543,000		0		17,543,000
Operating Income	34,152,000		22,659,000		174,838,000		162,127,000
Other Income and (Deductions), net	3,487,000		5,153,000		(2,366,000)		15,718,000
Interest Charges, net	23,641,000		22,399,000		70,322,000		69,533,000
Income from Equity Investments	3,492,000		3,930,000		10,398,000		11,590,000
Income (Loss) Before Income Taxes	17,490,000		9,343,000		112,548,000		119,902,000
Income Taxes	4,986,000		4,186,000		35,276,000		45,004,000
Net Income (Loss)	12,504,000		5,157,000		77,272,000		74,898,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	6,000		13,000		(21,000)		39,000
Net Income (Loss) attributable to UIL Holdings	12,498,000		5,144,000		77,293,000		74,859,000
Total Capital Expenditure	70,630,000	[1]	80,188,000	[1]	196,171,000		224,172,000
Total Assets	4,857,741,000				4,857,741,000				5,144,220,000
Net plant in service segregated by segment [Abstract]
Goodwill	266,205,000				266,205,000				266,205,000
Reporting Segments [Member] | Electric Distribution Reportable Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	134,289,000		151,369,000		397,384,000		421,959,000
Purchased power and gas	37,962,000		37,314,000		123,771,000		105,996,000
Operation and maintenance	42,819,000		43,387,000		135,282,000		132,919,000
Transmission wholesale	0		0		0		0
Depreciation and amortization	11,943,000		28,397,000		35,902,000		72,924,000
Taxes - other than income taxes	14,244,000		13,390,000		38,597,000		36,860,000
Acquisition-related expenses	0				0
Rate case disallowances			17,543,000				17,543,000
Operating Income	27,321,000		11,338,000		63,832,000		55,717,000
Other Income and (Deductions), net	2,877,000		3,534,000		9,503,000		10,687,000
Interest Charges, net	7,590,000		6,088,000		22,661,000		19,834,000
Income from Equity Investments	3,492,000		3,930,000		10,398,000		11,590,000
Income (Loss) Before Income Taxes	26,100,000		12,714,000		61,072,000		58,160,000
Income Taxes	8,173,000		5,607,000		18,015,000		24,464,000
Net Income (Loss)	17,927,000		7,107,000		43,057,000		33,696,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0		0		0		0
Net Income (Loss) attributable to UIL Holdings	17,927,000		7,107,000		43,057,000		33,696,000
Total Capital Expenditure	0	[1]	0	[1]	0	[1]	0	[1]
Total Assets	0	[2]			0	[2]			0	[2]
Reporting Segments [Member] | Electric Transmission Reportable Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	62,926,000		67,830,000		183,715,000		180,412,000
Purchased power and gas	0		0		0		0
Operation and maintenance	11,456,000		11,532,000		36,049,000		33,212,000
Transmission wholesale	25,802,000		28,659,000		65,777,000		65,994,000
Depreciation and amortization	4,208,000		4,152,000		12,635,000		12,123,000
Taxes - other than income taxes	10,441,000		10,466,000		26,832,000		25,260,000
Acquisition-related expenses	0				0
Rate case disallowances			0				0
Operating Income	11,019,000		13,021,000		42,422,000		43,823,000
Other Income and (Deductions), net	881,000		746,000		2,793,000		3,465,000
Interest Charges, net	3,375,000		2,763,000		9,832,000		9,104,000
Income from Equity Investments	0		0		0		0
Income (Loss) Before Income Taxes	8,525,000		11,004,000		35,383,000		38,184,000
Income Taxes	3,048,000		3,890,000		12,262,000		13,474,000
Net Income (Loss)	5,477,000		7,114,000		23,121,000		24,710,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0		0		0		0
Net Income (Loss) attributable to UIL Holdings	5,477,000		7,114,000		23,121,000		24,710,000
Total Capital Expenditure	0	[1]	0	[1]	0	[1]	0	[1]
Total Assets	0	[2]			0	[2]			0	[2]
Reporting Segments [Member] | Total Electric Reportable Segments [Member]
Segment Reporting Information [Line Items]
Operating Revenues	197,215,000		219,199,000		581,009,000		602,371,000
Purchased power and gas	37,962,000		37,314,000		123,771,000		105,996,000
Operation and maintenance	54,275,000		54,919,000		171,331,000		166,131,000
Transmission wholesale	25,802,000		28,659,000		65,777,000		65,994,000
Depreciation and amortization	16,151,000		32,549,000		48,537,000		85,047,000
Taxes - other than income taxes	24,685,000		23,856,000		65,429,000		62,120,000
Acquisition-related expenses	0				0
Rate case disallowances			17,543,000				17,543,000
Operating Income	38,340,000		24,359,000		106,254,000		99,540,000
Other Income and (Deductions), net	3,758,000		4,280,000		12,296,000		14,152,000
Interest Charges, net	10,965,000		8,851,000		32,493,000		28,938,000
Income from Equity Investments	3,492,000		3,930,000		10,398,000		11,590,000
Income (Loss) Before Income Taxes	34,625,000		23,718,000		96,455,000		96,344,000
Income Taxes	11,221,000		9,497,000		30,277,000		37,938,000
Net Income (Loss)	23,404,000		14,221,000		66,178,000		58,406,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0		0		0		0
Net Income (Loss) attributable to UIL Holdings	23,404,000		14,221,000		66,178,000		58,406,000
Total Capital Expenditure	31,523,000	[1]	41,310,000	[1]	91,600,000	[1]	124,437,000	[1]
Total Assets	2,709,623,000	[2]			2,709,623,000	[2]			2,950,707,000	[2]
Reporting Segments [Member] | Gas Distribution Reportable Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	94,708,000		97,244,000		616,780,000		581,115,000
Purchased power and gas	30,504,000		37,343,000		321,986,000		315,520,000
Operation and maintenance	44,950,000		36,271,000		130,761,000		111,496,000
Transmission wholesale	0		0		0		0
Depreciation and amortization	16,477,000		17,202,000		55,994,000		58,515,000
Taxes - other than income taxes	9,082,000		8,410,000		37,080,000		33,769,000
Acquisition-related expenses	0				0
Rate case disallowances			0				0
Operating Income	(6,305,000)		(1,982,000)		70,959,000		61,815,000
Other Income and (Deductions), net	702,000		234,000		(587,000)		(298,000)
Interest Charges, net	7,116,000		7,507,000		21,320,000		22,415,000
Income from Equity Investments	0		0		0		0
Income (Loss) Before Income Taxes	(12,719,000)		(9,255,000)		49,052,000		39,102,000
Income Taxes	(6,229,000)		(2,970,000)		18,749,000		14,770,000
Net Income (Loss)	(6,490,000)		(6,285,000)		30,303,000		24,332,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	6,000		13,000		(21,000)		39,000
Net Income (Loss) attributable to UIL Holdings	(6,496,000)		(6,298,000)		30,324,000		24,293,000
Total Capital Expenditure	32,926,000	[1]	25,968,000	[1]	78,278,000	[1]	63,771,000	[1]
Total Assets	1,996,825,000	[3]			1,996,825,000	[3]			2,010,246,000	[3]
Net plant in service segregated by segment [Abstract]
Goodwill	266,200,000				266,200,000				266,200,000
Reporting Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Operating Revenues	1,103,000		35,000		1,103,000		105,000
Purchased power and gas	310,000		0		310,000		0
Operation and maintenance	(3,974,000)		(1,380,000)		(11,264,000)		(4,088,000)
Transmission wholesale	0		0		0		0
Depreciation and amortization	2,950,000		1,010,000		7,877,000		2,975,000
Taxes - other than income taxes	(870,000)		123,000		465,000		446,000
Acquisition-related expenses	570,000				6,090,000
Rate case disallowances			0				0
Operating Income	2,117,000		282,000		(2,375,000)		772,000
Other Income and (Deductions), net	(973,000)		639,000		(14,075,000)		1,864,000
Interest Charges, net	5,560,000		6,041,000		16,509,000		18,180,000
Income from Equity Investments	0		0		0		0
Income (Loss) Before Income Taxes	(4,416,000)		(5,120,000)		(32,959,000)		(15,544,000)
Income Taxes	(6,000)		(2,341,000)		(13,750,000)		(7,704,000)
Net Income (Loss)	(4,410,000)		(2,779,000)		(19,209,000)		(7,840,000)
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0		0		0		0
Net Income (Loss) attributable to UIL Holdings	(4,410,000)		(2,779,000)		(19,209,000)		(7,840,000)
Total Capital Expenditure	6,181,000	[1]	12,910,000	[1]	26,293,000	[1]	35,964,000	[1]
Total Assets	151,293,000				151,293,000				183,267,000
Reporting Segments [Member] | Distribution Segment [Member]
Net plant in service segregated by segment [Abstract]
Net plant in service	1,236,800,000				1,236,800,000				1,181,100,000
Reporting Segments [Member] | Transmission Segment [Member]
Net plant in service segregated by segment [Abstract]
Net plant in service	$ 658,200,000				$ 658,200,000				$ 656,000,000

[1] Information for segmenting total capital expenditures between Distribution and Transmission is not available. Total Electric Distribution and Transmission capital expenditures are disclosed in the Total Electric Distribution and Transmission column.

[2] Information for segmenting total assets between Distribution and Transmission is not available. Total Electric Distribution and Transmission assets are disclosed in the Total Electric and Distribution and Transmission column. Net plant in service is segregated by segment and, as of September 30, 2014, was $1,236.8 million and $658.2 million for Distribution and Transmission, respectively. As of December 31, 2013, net plant in service was $1,181.1 million and $656.0 million for Distribution and Transmission, respectively.

[3] Includes $266.2 million of goodwill in the Gas Distribution segment as of September 30, 2014 and December 31, 2013.