Schedule I - Condensed Financial Statements of Registrant, Statements of Comprehensive Income (Details) (UIL Holdings Corporation [Member], USD $) In Thousands, 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]
Net Income	$ 103,701	$ 99,710	$ 54,857
Other Comprehensive Income (Loss)	(74)	(541)	166
Comprehensive Income (Loss)	103,627	99,169	55,023
Less:
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	64	54	3
Comprehensive Income Attributable to UIL Holdings	$ 103,563	$ 99,115	$ 55,020