Schedule I - Condensed Financial Statements of Registrant, Statements of Income (Details) (UIL Holdings Corporation [Member], USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
UIL Holdings Corporation [Member]
Condensed Financial Statements, Captions [Line Items]
Operating Revenues	$ 4,193	$ 353	$ 0
Operating Expenses
Operation and maintenance	639	548	451
Depreciation and amortization	2,530	306	0
Taxes - other than income taxes	156	10	7
Acquisition and closing related expenses	0	0	25,572
Total Operating Expenses	3,325	864	26,030
Operating Income	868	(511)	(26,030)
Other Income and (Deductions), net	351	(1,609)	(17)
Interest Charges, net	23,292	22,784	9,111
Equity in Earnings of Subsidiaries	117,083	113,081	77,208
Income Before Income Taxes	95,010	88,177	42,050
Income Taxes	(8,691)	(11,533)	(12,807)
Net Income	103,701	99,710	54,857
Net Income Attributable to Noncontrolling Interest	64	54	3
Net Income attributable to UIL Holdings	$ 103,637	$ 99,656	$ 54,854
Average Number of Common Shares Outstanding - Diluted (in shares)	50,831	50,609	35,722
Average Number of Common Shares Outstanding - Basic (in shares)	51,108	50,926	36,083
Earnings Per Share of Common Stock - Basic: (in dollars per share)	$ 2.04	$ 1.96	$ 1.53
Earnings Per Share of Common Stock - Diluted: (in dollars per share)	$ 2.02	$ 1.95	$ 1.52