Schedule I - Condensed Financial Statements of Registrant, Balance Sheet (Details) (UIL Holdings Corporation [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
UIL Holdings Corporation [Member]
Current Assets
Unrestricted cash and temporary cash investments	$ 115	$ 2,664	$ 10,532	$ 9,320
Intercompany accounts receivable	3,264	6,398
Refundable taxes	24,101	0
Deferred income taxes	2,145	25,114
Other	4,553	16,697
Total Current Assets	34,178	50,873
Investments in Subsidiaries	1,689,917	1,546,033
Net Property, Plant and Equipment	60,588	27,870
Other Assets	6,095	7,434
Total Assets	1,790,778	1,632,210
Current Liabilities
Line of credit borrowings	157,000	35,000
Accounts payable	22,242	10,325
Dividends payable	21,887	21,847
Other	21,079	19,970
Total Current Liabilities	222,208	87,142
Other Liabilities	4,934	3,928
CAPITALIZATION
Long-term debt, net of unamortized discount and premium	447,083	446,779
Net Common Stock Equity	1,116,553	1,094,361
Total Capitalization	1,563,636	1,541,140
Total Liabilities and Capitalization	$ 1,790,778	$ 1,632,210