Schedule I - Condensed Financial Statements of Registrant, Statements of Cash Flows (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 Cash provided by Operating Activities	$ 89,875	$ 162,538	$ 30,415
Cash Flows from Investing Activities
Acquisition of Gas Companies, net of cash acquired	0	11,211	(917,929)
Plant expenditures	(29,689)	(20,168)	0
Capital contributions to subsidiaries	(100,000)	(60,000)	0
Other	304	18	0
Net Cash (used in) Investing Activities	(129,385)	(68,939)	(917,929)
Cash Flows from Financing Activities
Issuance of common stock	1,943	200	502,220
Issuances of long-term debt	0	0	446,559
Payments on long-term debt	0	(49,286)	(4,286)
Line of credit borrowings (repayments), net	122,000	35,000	0
Payment of common stock dividend	(87,490)	(87,274)	(51,836)
Other	508	(107)	(3,931)
Net Cash (used in) Financing Activities	36,961	(101,467)	888,726
Unrestricted Cash and Temporary Cash Investments:
Net change for the period	(2,549)	(7,868)	1,212
Balance at beginning of period	2,664	10,532	9,320
Balance at end of period	$ 115	$ 2,664	$ 10,532