QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2010		Sep. 30, 2010		Jun. 30, 2010		Mar. 31, 2010		Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
QUARTERLY FINANCIAL DATA (UNAUDITED) [Abstract]
Operating Revenues	$ 373,918		$ 321,427		$ 314,049		$ 561,053		$ 333,993		$ 236,277		$ 207,116		$ 220,280		$ 1,570,447	$ 997,666	$ 896,550
Operating Income	45,090		34,253		40,224		99,815		32,143		34,763		25,513		32,880		219,382	125,299	122,168
Net income attributable to UIL Holdings	$ 21,276	[1]	$ 12,180	[1]	$ 14,156	[1]	$ 52,044	[1]	$ 10,474	[1]	$ 16,976	[1]	$ 11,489	[1]	$ 15,915	[1]	$ 99,656	$ 54,854	$ 54,317
Earnings Per Share of Common Stock - Basic: (in dollars per share)	$ 0.42	[2]	$ 0.24	[2]	$ 0.28	[2]	$ 1.03	[2]	$ 0.21	[2]	$ 0.52	[2]	$ 0.38	[2]	$ 0.53	[2]	$ 1.96	$ 1.53	$ 1.94
Earnings Per Share of Common Stock - Diluted: (in dollars per share)	$ 0.42	[3]	$ 0.24	[3]	$ 0.28	[3]	$ 1.02	[3]	$ 0.20	[3]	$ 0.52	[3]	$ 0.38	[3]	$ 0.53	[3]	$ 1.95	$ 1.52	$ 1.93

[1] Includes acquisition-related costs in the 2nd, 3rd and 4th quarters of 2010

[2] Based on weighted average number of shares outstanding each quarter.

[3] Based on weighted average number of shares outstanding each quarter. Reflecting the effect of dilutive stock options, performance shares and restricted stock