SEGMENT INFORMATION (Details) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Dec. 31, 2012
Segment Reporting Information [Line Items]
Operating Revenues	$ 316,478,000		$ 322,888,000		$ 1,183,591,000		$ 1,063,547,000
Purchased power and gas	74,657,000		95,135,000		421,516,000		360,909,000
Operation and maintenance	89,810,000		87,281,000		273,539,000		260,887,000
Transmission wholesale	28,659,000		26,565,000		65,994,000		59,847,000
Depreciation and amortization	50,761,000		46,279,000		146,537,000		135,886,000
Taxes - other than income taxes	32,389,000		28,502,000		96,335,000		82,924,000
Regulatory disallowances	17,543,000		0		17,543,000		0
Operating Income	22,659,000		39,126,000		162,127,000		163,094,000
Other Income and (Deductions), net	5,153,000		7,416,000		15,718,000		21,205,000
Interest Charges, net	22,399,000		23,353,000		69,533,000		70,335,000
Income from Equity Investments	3,930,000		3,421,000		11,590,000		11,823,000
Income (Loss) Before Income Taxes	9,343,000		26,610,000		119,902,000		125,787,000
Income Taxes	4,186,000		10,835,000		45,004,000		50,924,000
Net Income (Loss)	5,157,000		15,775,000		74,898,000		74,863,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	13,000		26,000		39,000		65,000
Net Income (Loss) attributable to UIL Holdings	5,144,000		15,749,000		74,859,000		74,798,000
Total Capital Expenditure	80,188,000	[1]	69,041,000	[1]	224,172,000	[1]	214,240,000	[1]
Total Assets	4,865,778,000	[2]			4,865,778,000	[2]			4,959,367,000	[2]
Electric Distribution Reportable Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	151,369,000		154,165,000		421,959,000		424,060,000
Purchased power and gas	37,314,000		43,565,000		105,996,000		118,581,000
Operation and maintenance	43,387,000		42,599,000		132,919,000		130,071,000
Transmission wholesale	0		0		0		0
Depreciation and amortization	28,397,000		25,286,000		72,924,000		66,763,000
Taxes - other than income taxes	13,390,000		12,202,000		36,860,000		32,828,000
Regulatory disallowances	17,543,000		0		17,543,000		0
Operating Income	11,338,000		30,513,000		55,717,000		75,817,000
Other Income and (Deductions), net	3,534,000		6,071,000		10,687,000		13,556,000
Interest Charges, net	6,088,000		7,242,000		19,834,000		21,732,000
Income from Equity Investments	3,930,000		3,421,000		11,590,000		11,823,000
Income (Loss) Before Income Taxes	12,714,000		32,763,000		58,160,000		79,464,000
Income Taxes	5,607,000		14,597,000		24,464,000		36,163,000
Net Income (Loss)	7,107,000		18,166,000		33,696,000		43,301,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0		0		0		0
Net Income (Loss) attributable to UIL Holdings	7,107,000		18,166,000		33,696,000		43,301,000
Total Capital Expenditure	0	[1]	0	[1]	0	[1]	0	[1]
Total Assets	0	[2]			0	[2]			0	[2]
Electric Transmission Reportable Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	67,830,000		62,932,000		180,412,000		162,729,000
Purchased power and gas	0		0		0		0
Operation and maintenance	11,532,000		9,840,000		33,212,000		30,791,000
Transmission wholesale	28,659,000		26,565,000		65,994,000		59,847,000
Depreciation and amortization	4,152,000		3,787,000		12,123,000		10,942,000
Taxes - other than income taxes	10,466,000		8,256,000		25,260,000		19,858,000
Regulatory disallowances	0		0		0		0
Operating Income	13,021,000		14,484,000		43,823,000		41,291,000
Other Income and (Deductions), net	746,000		863,000		3,465,000		2,889,000
Interest Charges, net	2,763,000		3,148,000		9,104,000		10,042,000
Income from Equity Investments	0		0		0		0
Income (Loss) Before Income Taxes	11,004,000		12,199,000		38,184,000		34,138,000
Income Taxes	3,890,000		3,955,000		13,474,000		10,709,000
Net Income (Loss)	7,114,000		8,244,000		24,710,000		23,429,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0		0		0		0
Net Income (Loss) attributable to UIL Holdings	7,114,000		8,244,000		24,710,000		23,429,000
Total Capital Expenditure	0	[1]	0	[1]	0	[1]	0	[1]
Total Assets	0	[2]			0	[2]			0	[2]
Total Electric Reportable Segments [Member]
Segment Reporting Information [Line Items]
Operating Revenues	219,199,000		217,097,000		602,371,000		586,789,000
Purchased power and gas	37,314,000		43,565,000		105,996,000		118,581,000
Operation and maintenance	54,919,000		52,439,000		166,131,000		160,862,000
Transmission wholesale	28,659,000		26,565,000		65,994,000		59,847,000
Depreciation and amortization	32,549,000		29,073,000		85,047,000		77,705,000
Taxes - other than income taxes	23,856,000		20,458,000		62,120,000		52,686,000
Regulatory disallowances	17,543,000		0		17,543,000		0
Operating Income	24,359,000		44,997,000		99,540,000		117,108,000
Other Income and (Deductions), net	4,280,000		6,934,000		14,152,000		16,445,000
Interest Charges, net	8,851,000		10,390,000		28,938,000		31,774,000
Income from Equity Investments	3,930,000		3,421,000		11,590,000		11,823,000
Income (Loss) Before Income Taxes	23,718,000		44,962,000		96,344,000		113,602,000
Income Taxes	9,497,000		18,552,000		37,938,000		46,872,000
Net Income (Loss)	14,221,000		26,410,000		58,406,000		66,730,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0		0		0		0
Net Income (Loss) attributable to UIL Holdings	14,221,000		26,410,000		58,406,000		66,730,000
Total Capital Expenditure	41,310,000	[1]	39,584,000	[1]	124,437,000	[1]	134,769,000	[1]
Total Assets	2,847,227,000	[2]			2,847,227,000	[2]			2,875,765,000	[2]
Gas Distribution Reportable Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	97,244,000		105,755,000		581,115,000		476,644,000
Purchased power and gas	37,343,000		51,570,000		315,520,000		242,328,000
Operation and maintenance	36,271,000		35,788,000		111,496,000		101,944,000
Transmission wholesale	0		0		0		0
Depreciation and amortization	17,202,000		16,410,000		58,515,000		56,499,000
Taxes - other than income taxes	8,410,000		8,011,000		33,769,000		30,111,000
Regulatory disallowances	0		0		0		0
Operating Income	(1,982,000)		(6,024,000)		61,815,000		45,762,000
Other Income and (Deductions), net	234,000		372,000		(298,000)		4,286,000
Interest Charges, net	7,507,000		7,070,000		22,415,000		21,281,000
Income from Equity Investments	0		0		0		0
Income (Loss) Before Income Taxes	(9,255,000)		(12,722,000)		39,102,000		28,767,000
Income Taxes	(2,970,000)		(5,423,000)		14,770,000		10,807,000
Net Income (Loss)	(6,285,000)		(7,299,000)		24,332,000		17,960,000
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	13,000		26,000		39,000		65,000
Net Income (Loss) attributable to UIL Holdings	(6,298,000)		(7,325,000)		24,293,000		17,895,000
Total Capital Expenditure	25,968,000	[1]	22,968,000	[1]	63,771,000	[1]	55,840,000	[1]
Total Assets	1,934,249,000				1,934,249,000				1,993,560,000
Other [Member]
Segment Reporting Information [Line Items]
Operating Revenues	35,000		36,000		105,000		114,000
Purchased power and gas	0		0		0		0
Operation and maintenance	(1,380,000)		(946,000)		(4,088,000)		(1,919,000)
Transmission wholesale	0		0		0		0
Depreciation and amortization	1,010,000		796,000		2,975,000		1,682,000
Taxes - other than income taxes	123,000		33,000		446,000		127,000
Regulatory disallowances	0		0		0		0
Operating Income	282,000		153,000		772,000		224,000
Other Income and (Deductions), net	639,000		110,000		1,864,000		474,000
Interest Charges, net	6,041,000		5,893,000		18,180,000		17,280,000
Income from Equity Investments	0		0		0		0
Income (Loss) Before Income Taxes	(5,120,000)		(5,630,000)		(15,544,000)		(16,582,000)
Income Taxes	(2,341,000)		(2,294,000)		(7,704,000)		(6,755,000)
Net Income (Loss)	(2,779,000)		(3,336,000)		(7,840,000)		(9,827,000)
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	0		0		0		0
Net Income (Loss) attributable to UIL Holdings	(2,779,000)		(3,336,000)		(7,840,000)		(9,827,000)
Total Capital Expenditure	12,910,000	[1]	6,489,000	[1]	35,964,000	[1]	23,631,000	[1]
Total Assets	84,302,000				84,302,000				90,042,000
Distribution Segment [Member]
Net plant in service segregated by segment [Abstract]
Net plant in service	1,099,100,000				1,099,100,000				1,058,600,000
Transmission Segment [Member]
Net plant in service segregated by segment [Abstract]
Net plant in service	$ 644,700,000				$ 644,700,000				$ 607,800,000

[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 Distribution and Transmission column.  Net plant in service is segregated by segment and, as of September 30, 2013, was $1,099.1 million and $644.7 million for Distribution and Transmission, respectively.  As of December 31, 2012, net plant in service was $1,058.6 million and $607.8 million for Distribution and  Transmission, respectively.