BUSINESS ORGANIZATION AND STATEMENT OF ACCOUNTING POLICIES (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Depreciation and amortization expense	$ 43,048,000	$ 39,077,000	$ 89,607,000	$ 84,048,000
Operation and maintenance	88,586,000	90,393,000	174,793,000	175,525,000
Unrealized gain and losses from mark-to-market adjustments [Abstract]
Regulatory Assets - Derivative liabilities	2,167,000	30,375,000	(5,781,000)	70,536,000
Regulatory Liabilities - Derivative assets	40,000	(584,000)	12,000	5,748,000
Numerator: [Abstract]
Net Income attributable to UIL Holdings	11,999,000	14,156,000	59,049,000	66,200,000
Less: Net income allocated to unvested units	18,000	29,000	105,000	143,000
Net income attributable to common shareholders	11,981,000	14,127,000	58,944,000	66,057,000
Denominator: [Abstract]
Basic average number of shares outstanding	50,793	50,628	50,740	50,574
Effect of dilutive securities	248	244	262	250
Diluted average number of shares outstanding	51,041	50,872	51,002	50,824
Earnings per share: [Abstract]
Basic	$ 0.24	$ 0.28	$ 1.16	$ 1.31
Diluted	$ 0.23	$ 0.28	$ 1.16	$ 1.30
Number of shares of common stock outstanding with purchase options not included in diluted earnings per share (in shares)		89,336		97,669
Regulatory assets [Abstract]
Total Regulatory Assets	1,037,978,000		1,037,978,000		1,086,122,000
Less current portion of regulatory assets	123,007,000		123,007,000		102,900,000
Regulatory Assets, Net	914,971,000		914,971,000		983,222,000
Regulatory liabilities [Abstract]
Total regulatory liabilities	459,368,000		459,368,000		446,420,000
Less current portion of regulatory liabilities	28,597,000		28,597,000		26,245,000
Regulatory Liabilities, Net	430,771,000		430,771,000		420,175,000
Derivatives [Abstract]
Gross derivative asset	81,600,000		81,600,000
Regulatory asset	178,300,000		178,300,000
Gross derivative liability	259,900,000		259,900,000
Gross derivative liability, UIL's portion of entity	157,900,000		157,900,000
Equity Investments [Abstract]
Income (loss) from equity investment	3,915,000	2,647,000	8,402,000	4,709,000
Regulatory accounting [Abstract]
Delivery term of related contracts for differences, minimum (in years)			7Y
Delivery term of related contracts for differences, maximum (in years)			15Y
Gross derivative asset	81,600,000		81,600,000
Stock-Based Compensation [Abstract]
Total stock-based compensation expense	900,000	1,400,000	3,700,000	3,300,000
Restricted Stock [Member]
Stock-Based Compensation [Abstract]
Vesting rights			equal annual installments over a five year period.
President and Chief Executive Officer [Member] | Restricted Stock [Member]
Stock-Based Compensation [Abstract]
Number of shares granted (in shares)			2,286
Weighted average grant date fair value (in dollars per share)			$ 34.45
Non-employee directors [Member] | Restricted Stock [Member]
Stock-Based Compensation [Abstract]
Number of shares granted (in shares)			23,461
Weighted average grant date fair value (in dollars per share)			$ 33.42
Vesting rights			shares vest in May 2013
Management [Member] | Performance Shares [Member]
Stock-Based Compensation [Abstract]
Number of shares granted (in shares)			119,640
Weighted average grant date fair value (in dollars per share)			$ 34.45
The Southern Connecticut Gas Company (SCG) [Member]
Regulatory accounting [Abstract]
Maximum receivable amount allowed under weather insurance contract	3,000,000		3,000,000
Maximum payable amount allowed under weather insurance contract	2,000,000		2,000,000
Connecticut Natural Gas Corporation (CNG) [Member]
Regulatory accounting [Abstract]
Maximum receivable amount allowed under weather insurance contract	3,000,000		3,000,000
Maximum payable amount allowed under weather insurance contract	2,000,000		2,000,000
Berkshire Weather Insurance [Member]
Derivatives [Abstract]
Gross derivative asset	1,000,000		1,000,000
Regulatory accounting [Abstract]
Maximum receivable amount allowed under weather insurance contract	1,000,000		1,000,000
Gross derivative asset	1,000,000		1,000,000
GenConn [Member]
Ownership percentage in GenConn [Abstract]
Ownership percentage in joint venture (in hundredths)	50.00%		50.00%
Derivatives [Abstract]
Number of new capacity resources selected	4		4
Number of Contracts for Differences (CfDs) approved by Public Utility Regulatory Authority (PURA)	4		4
Number of Contracts for Differences (CfDs) executed by UI and CL&P	2		2
Percentage of cost borne by UI customers for CFDs (in hundredths)			20.00%
Percentage of cost borne by CL&P customers (in hundredths)			80.00%
Equity Investments [Abstract]
Carrying value of joint venture	126,600,000		126,600,000		131,100,000
Income (loss) from equity investment	3,900,000	2,600,000	8,400,000	4,700,000
Accumulated Deferred Investment Tax Credits [Member]
Regulatory liabilities [Abstract]
Total regulatory liabilities	4,685,000		4,685,000		4,758,000
Remaining period (in years)			32Y
Income Taxes Due Principally To Book Tax Differences Liabilities [Member]
Regulatory liabilities [Abstract]
Total regulatory liabilities	19,628,000		19,628,000		14,445,000
Deferred Gain on Sale of Property [Member]
Regulatory liabilities [Abstract]
Total regulatory liabilities	37,798,000		37,798,000		37,798,000
Middletown/Norwalk Local Transmission Network Service Collections [Member]
Regulatory liabilities [Abstract]
Total regulatory liabilities	22,261,000		22,261,000		22,548,000
Remaining period (in years)			38Y
Pension And Other Post-Retirement Benefit Plans [Member]
Regulatory liabilities [Abstract]
Total regulatory liabilities	14,836,000		14,836,000		17,956,000
Remaining period, minimum (in years)			1Y
Remaining period, maximum (in years)			8Y
Deferred Income Taxes [Member]
Regulatory liabilities [Abstract]
Total regulatory liabilities	36,247,000		36,247,000		48,740,000
Asset Retirement Obligation [Member]
Regulatory liabilities [Abstract]
Total regulatory liabilities	8,455,000		8,455,000		8,941,000
Deferred Purchased Gas [Member]
Regulatory liabilities [Abstract]
Total regulatory liabilities	10,787,000		10,787,000		0
Low Income Program [Member]
Regulatory liabilities [Abstract]
Total regulatory liabilities	14,782,000		14,782,000		0
Unfunded Future Income Taxes [Member]
Regulatory liabilities [Abstract]
Total regulatory liabilities	20,534,000		20,534,000		9,735,000
Asset Removal Costs [Member]
Regulatory liabilities [Abstract]
Total regulatory liabilities	233,527,000		233,527,000		224,125,000
Deferred Transmission Expense [Member]
Regulatory liabilities [Abstract]
Total regulatory liabilities	0		0		11,628,000
Other Regulatory Liability [Member]
Regulatory liabilities [Abstract]
Total regulatory liabilities	35,828,000		35,828,000		45,746,000
Nuclear Plant Investments - Above Market [Member]
Regulatory assets [Abstract]
Total Regulatory Assets	262,720,000		262,720,000		272,943,000
Connecticut Yankee [Member]
Regulatory assets [Abstract]
Total Regulatory Assets	12,718,000		12,718,000		14,247,000
Remaining period (in years)			4Y
Unamortized Redemption Costs [Member]
Regulatory assets [Abstract]
Total Regulatory Assets	12,505,000		12,505,000		12,906,000
Remaining period, minimum (in years)			9Y
Remaining period, maximum (in years)			21Y
Pension and Other Post-Retirement Benefit Plans [Member]
Regulatory assets [Abstract]
Total Regulatory Assets	336,656,000		336,656,000		344,746,000
Environmental Remediation Costs [Member]
Regulatory assets [Abstract]
Total Regulatory Assets	19,354,000		19,354,000		19,101,000
Remaining period, minimum (in years)			4Y
Remaining period, maximum (in years)			5Y
Customer Rate Surcharge [Member]
Regulatory assets [Abstract]
Total Regulatory Assets	10,032,000		10,032,000		15,757,000
Low Income Program [Member]
Regulatory assets [Abstract]
Total Regulatory Assets	29,003,000		29,003,000		37,420,000
Deferred transmission income/expense [Member]
Regulatory assets [Abstract]
Total Regulatory Assets	10,650,000		10,650,000		0
Debt Premium [Member]
Regulatory assets [Abstract]
Total Regulatory Assets	44,925,000		44,925,000		48,275,000
Remaining period, minimum (in years)			1Y
Remaining period, maximum (in years)			26Y
Deferred Purchased Gas [Member]
Regulatory assets [Abstract]
Total Regulatory Assets	0		0		15,558,000
Deferred Income Taxes [Member]
Regulatory assets [Abstract]
Total Regulatory Assets	29,279,000		29,279,000		20,994,000
Unfunded Future Income Taxes [Member]
Regulatory assets [Abstract]
Total Regulatory Assets	9,958,000		9,958,000		11,657,000
Contracts For Differences Regulatory Asset [Member]
Regulatory assets [Abstract]
Total Regulatory Assets	178,322,000		178,322,000		184,105,000
Excess Generation Service Charge [Member]
Regulatory assets [Abstract]
Total Regulatory Assets	16,035,000		16,035,000		13,758,000
Storm Costs [Member]
Regulatory assets [Abstract]
Total Regulatory Assets	26,088,000		26,088,000		29,618,000
Other Regulatory Asset [Member]
Regulatory assets [Abstract]
Total Regulatory Assets	39,733,000		39,733,000		45,037,000
Current Portion Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Total derivative assets/(liabilities)	12,410,000		12,410,000		14,189,000
Unrealized gain and losses from mark-to-market adjustments [Abstract]
Contracts for differences	11,410,000		11,410,000		10,678,000
Weather insurance contracts	1,000,000		1,000,000		3,511,000
Noncurrent Portion Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Total derivative assets/(liabilities)	70,161,000		70,161,000		73,264,000
Unrealized gain and losses from mark-to-market adjustments [Abstract]
Contracts for differences	70,161,000		70,161,000		73,264,000
Weather insurance contracts	0		0		0
Current Portion Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Total derivative assets/(liabilities)	(30,332,000)		(30,332,000)		(28,888,000)
Unrealized gain and losses from mark-to-market adjustments [Abstract]
Contracts for differences	(30,332,000)		(30,332,000)		(28,888,000)
Weather insurance contracts	0		0		0
Noncurrent Portion Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Total derivative assets/(liabilities)	(229,563,000)		(229,563,000)		(239,147,000)
Unrealized gain and losses from mark-to-market adjustments [Abstract]
Contracts for differences	(229,563,000)		(229,563,000)		(239,147,000)
Weather insurance contracts	0		0		0
Restatement Adjustment [Member]
Depreciation and amortization expense		4,600,000		13,500,000
Operation and maintenance		$ (4,600,000)		$ (13,500,000)