QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
QUARTERLY FINANCIAL DATA (UNAUDITED) [Abstract]
Operating Revenues	$ 420,846		$ 323,809		$ 283,508		$ 458,338		$ 373,565		$ 321,427		$ 314,049		$ 561,053		$ 1,486,501	$ 1,570,094	$ 997,666
Operating Income	67,496		39,126		36,507		87,461		45,090		34,253		40,224		99,815		230,590	219,382	125,299
Net income attributable to UIL Holdings	$ 28,839		$ 15,749		$ 11,999		$ 47,050		$ 21,276		$ 12,180		$ 14,156		$ 52,044		$ 103,637	$ 99,656	$ 54,854
Earnings Per Share of Common Stock - Basic: (in dollars per share)	$ 0.57	[1]	$ 0.31	[1]	$ 0.24	[1]	$ 0.93	[1]	$ 0.42	[1]	$ 0.24	[1]	$ 0.28	[1]	$ 1.03	[1]	$ 2.04	$ 1.96	$ 1.53
Earnings Per Share of Common Stock - Diluted: (in dollars per share)	$ 0.56	[2]	$ 0.31	[2]	$ 0.23	[2]	$ 0.92	[2]	$ 0.42	[2]	$ 0.24	[2]	$ 0.28	[2]	$ 1.02	[2]	$ 2.02	$ 1.95	$ 1.52

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

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