CONSOLIDATED STATEMENT OF INCOME (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
CONSOLIDATED STATEMENT OF INCOME (Unaudited) [Abstract]
Operating Revenues	$ 584,053	$ 571,162
Operation
Purchased power	97,102	53,130
Natural gas purchased	174,520	214,925
Operation and maintenance	101,347	92,877
Transmission wholesale	19,709	20,911
Depreciation and amortization (Note F)	43,284	40,318
Taxes - other than income taxes (Note F)	41,315	39,536
Merger and acquisition-related expenses (Note A)	6,702	5,051
Total Operating Expenses	483,979	466,748
Operating Income	100,074	104,414
Other Income and (Deductions), net
Acquisition-related bridge facility fees (Note A)	0	(6,413)
Other income and (deductions) (Note F)	4,368	3,862
Total Other Income and (Deductions), net	4,368	(2,551)
Interest Charges, net
Interest on long-term debt	22,225	22,452
Other interest, net	1,232	175
Interest charges, gross	23,457	22,627
Amortization of debt expense and redemption premiums	607	607
Total Interest Charges, net	24,064	23,234
Income from Equity Investments	2,936	3,386
Income Before Income Taxes	83,314	82,015
Income Taxes (Note E)	25,705	26,550
Net Income	57,609	55,465
Less: Preferred Stock Dividends of Subsidiary, Noncontrolling Interests	7	13
Net Income attributable to UIL Holdings	$ 57,602	$ 55,452
Average Number of Common Shares Outstanding - Basic (in shares)	56,881	56,779
Average Number of Common Shares Outstanding - Diluted (in shares)	57,184	57,043
Earnings Per Share of Common Stock - Basic (Note A) (in dollars per share)	$ 1.01	$ 0.98
Earnings Per Share of Common Stock - Diluted (Note A) (in dollars per share)	$ 1.01	$ 0.97
Cash Dividends Declared per share of Common Stock (in dollars per share)	$ 0.432	$ 0.432