CONSOLIDATED STATEMENT OF INCOME (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
CONSOLIDATED STATEMENT OF INCOME [Abstract]
Operating Revenues	$ 1,486,501	$ 1,570,094	$ 997,666
Operating Expenses
Purchased power	154,546	180,149	242,268
Natural gas purchased	370,234	429,079	81,428
Operation and maintenance	356,277	381,814	258,282
Transmission wholesale	79,469	77,997	72,169
Depreciation and amortization (Note F)	181,348	167,462	113,946
Taxes - other than income taxes (Note F)	114,037	114,211	78,702
Acquisition-related costs	0	0	25,572
Total Operating Expenses	1,255,911	1,350,712	872,367
Operating Income	230,590	219,382	125,299
Other Income and (Deductions), net (Note F), (Note H)	25,246	26,932	17,262
Interest Charges, net
Interest on long-term debt	86,469	87,394	50,357
Other interest, net (Note F)	3,636	5,216	1,553
Interest Charges, gross	90,105	92,610	51,910
Amortization of debt expense and redemption premiums	2,437	2,775	1,788
Total Interest Charges, net	92,542	95,385	53,698
Income Before Income Taxes, Equity Earnings	163,294	150,929	88,863
Income Taxes (Note E)	74,866	62,501	35,284
Income Before Equity Earnings	88,428	88,428	53,579
Income from Equity Investments	15,273	11,282	1,278
Net Income	103,701	99,710	54,857
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	64	54	3
Net Income attributable to UIL Holdings	$ 103,637	$ 99,656	$ 54,854
Average Number of Common Shares Outstanding - Basic (in shares)	50,831	50,609	35,722
Average Number of Common Shares Outstanding - Diluted (in shares)	51,108	50,926	36,083
Earnings Per Share of Common Stock - Basic (Note A) (in dollars per share)	$ 2.04	$ 1.96	$ 1.53
Earnings Per Share of Common Stock - Diluted (Note A) (in dollars per share)	$ 2.02	$ 1.95	$ 1.52
Cash Dividends Declared per share of Common Stock (in dollars per share)	$ 1.728	$ 1.728	$ 1.728