STATEMENT OF ACCOUNTING POLICIES (Details) (USD $)	1 Months Ended	3 Months Ended																12 Months Ended			1 Months Ended						12 Months Ended																									12 Months Ended									12 Months Ended		12 Months Ended																																						12 Months Ended		12 Months Ended						12 Months Ended										12 Months Ended
	Jan. 31, 2012	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	May 31, 2011 Non-Employee Directors [Member] Restricted Stock [Member]	Apr. 30, 2011 Non-Employee Directors [Member] Restricted Stock [Member]	Feb. 28, 2011 President and Chief Executive Officer [Member] Restricted Stock [Member]	May 31, 2011 Management [Member] Performance Shares [Member]	Apr. 30, 2011 Management [Member] Performance Shares [Member]	Feb. 28, 2011 Management [Member] Performance Shares [Member]	Dec. 31, 2011 Management [Member] Performance Shares [Member]	Dec. 31, 2011 GenConn [Member]	Dec. 31, 2011 Electric Distribution Plant [Member]	Dec. 31, 2010 Electric Distribution Plant [Member]	Dec. 31, 2011 Electric Transmission Plant [Member]	Dec. 31, 2010 Electric Transmission Plant [Member]	Dec. 31, 2011 Gas Distribution Plant [Member]	Dec. 31, 2010 Gas Distribution Plant [Member]	Dec. 31, 2011 Software [Member]	Dec. 31, 2010 Software [Member]	Dec. 31, 2011 Land [Member]	Dec. 31, 2010 Land [Member]	Dec. 31, 2011 Other Plant [Member]	Dec. 31, 2010 Other Plant [Member]	Dec. 31, 2011 Weather Insurance Contracts [Member]	Dec. 31, 2011 Weather Insurance Contracts [Member] Southern Connecticut Gas Company (SCG) [Member]	Dec. 31, 2011 Weather Insurance Contracts [Member] Connecticut Natural Gas Corporation (CNG) [Member]	Dec. 31, 2011 Current Assets [Member]	Dec. 31, 2010 Current Assets [Member]	Dec. 31, 2011 Deferred Charges and Other Assets [Member]	Dec. 31, 2010 Deferred Charges and Other Assets [Member]	Dec. 31, 2011 Current Liabilities [Member]	Dec. 31, 2010 Current Liabilities [Member]	Dec. 31, 2011 Noncurrent Liabilities [Member]	Dec. 31, 2010 Noncurrent Liabilities [Member]	Dec. 31, 2011 Accumulated Deferred Investment Tax Credits [Member]	Dec. 31, 2010 Accumulated Deferred Investment Tax Credits [Member]	Dec. 31, 2011 Deferred Gain on Sale of Property [Member]		Dec. 31, 2010 Deferred Gain on Sale of Property [Member]		Dec. 31, 2011 Income Taxes Due Principally To Book Tax Differences Liabilities [Member]		Dec. 31, 2010 Income Taxes Due Principally To Book Tax Differences Liabilities [Member]	Dec. 31, 2011 Middletown/Norwalk Local Transmission Network Service Collections [Member]	Dec. 31, 2010 Middletown/Norwalk Local Transmission Network Service Collections [Member]	Dec. 31, 2011 Pension And Other Post-Retirement Benefit Plans [Member]	Dec. 31, 2010 Pension And Other Post-Retirement Benefit Plans [Member]	Dec. 31, 2011 Deferred Income Taxes [Member]		Dec. 31, 2010 Deferred Income Taxes [Member]		Dec. 31, 2011 Asset Retirement Obligation [Member]		Dec. 31, 2010 Asset Retirement Obligation [Member]		Dec. 31, 2011 Deferred Purchased Gas [Member]		Dec. 31, 2010 Deferred Purchased Gas [Member]		Dec. 31, 2011 Unfunded Future Income Taxes [Member]		Dec. 31, 2010 Unfunded Future Income Taxes [Member]		Dec. 31, 2011 Asset Removal Costs [Member]		Dec. 31, 2010 Asset Removal Costs [Member]		Dec. 31, 2011 Deferred Transmission Expense [Member]		Dec. 31, 2010 Deferred Transmission Expense [Member]		Dec. 31, 2011 Other Regulatory Liability [Member]		Dec. 31, 2010 Other Regulatory Liability [Member]		Dec. 31, 2011 Nuclear Plant Investments - Above Market [Member]		Dec. 31, 2010 Nuclear Plant Investments - Above Market [Member]		Dec. 31, 2011 Income Taxes Due Principally To Book Tax Differences Assets [Member]		Dec. 31, 2010 Income Taxes Due Principally To Book Tax Differences Assets [Member]		Dec. 31, 2011 Connecticut Yankee [Member]	Dec. 31, 2010 Connecticut Yankee [Member]	Dec. 31, 2011 Unamortized Redemption Costs [Member]	Dec. 31, 2010 Unamortized Redemption Costs [Member]	Dec. 31, 2011 Pension and Other Post-Retirement Benefit Plans [Member]		Dec. 31, 2010 Pension and Other Post-Retirement Benefit Plans [Member]		Dec. 31, 2011 Environmental Remediation Costs [Member]	Dec. 31, 2010 Environmental Remediation Costs [Member]	Dec. 31, 2011 Customer Rate Surcharge [Member]		Dec. 31, 2010 Customer Rate Surcharge [Member]		Dec. 31, 2011 Low Income Program [Member]		Dec. 31, 2010 Low Income Program [Member]		Dec. 31, 2011 Debt Premium [Member]	Dec. 31, 2010 Debt Premium [Member]	Dec. 31, 2011 Deferred Purchased Gas [Member]	Dec. 31, 2010 Deferred Purchased Gas [Member]		Dec. 31, 2011 Deferred Income Taxes [Member]		Dec. 31, 2010 Deferred Income Taxes [Member]		Dec. 31, 2011 Unfunded Future Income Taxes [Member]		Dec. 31, 2010 Unfunded Future Income Taxes [Member]		Dec. 31, 2011 Contracts For Differences Regulatory Asset [Member]		Dec. 31, 2010 Contracts For Differences Regulatory Asset [Member]		Dec. 31, 2011 Excess Generation Service Charge [Member]		Dec. 31, 2010 Excess Generation Service Charge [Member]		Dec. 31, 2011 Storm Costs [Member]		Dec. 31, 2010 Storm Costs [Member]		Dec. 31, 2011 Other Regulatory Asset [Member]		Dec. 31, 2010 Other Regulatory Asset [Member]
STATEMENT OF ACCOUNTING POLICIES [Abstract]
Purchase price of acquiring Gas Companies		$ 1,296,000,000																$ 1,296,000,000
Ownership percentage in GenConn [Abstract]
Ownership percentage in joint venture (in hundredths)		50.00%																50.00%
Regulatory Assets [Line Items]
Total Regulatory Assets		1,086,122,000								1,041,737,000								1,086,122,000	1,041,737,000																																																																										272,943,000	[1]	293,388,000	[1]	0	[2]	11,910,000	[2]	14,247,000	17,175,000	12,906,000	13,708,000	344,746,000	[3]	351,610,000	[3]	19,101,000	17,285,000	15,757,000	[4]	12,816,000	[4]	37,420,000	[5]	40,674,000	[5]	48,275,000	56,865,000	15,558,000	23,330,000	[6]	20,994,000	[7]	5,859,000	[7]	11,657,000	[7]	25,684,000	[7]	184,105,000	[8]	114,662,000	[8]	13,758,000	[9]	8,711,000	[9]	29,618,000	[10]	7,460,000	[10]	45,037,000	[2]	40,600,000	[2]
Less current portion of regulatory assets		102,900,000								115,848,000								102,900,000	115,848,000
Regulatory Assets, Net		983,222,000								925,889,000								983,222,000	925,889,000
Remaining period (in years)																																																																																																					4Y
Remaining period, minimum (in years)																																																																																																							10Y						4Y										1Y
Remaining period, maximum (in years)																																																																																																							22Y						5Y										26Y
Regulatory Liabilities [Line Items]
Total regulatory liabilities		446,420,000								435,967,000								446,420,000	435,967,000																																	4,758,000	4,905,000	37,798,000	[1]	37,798,000	[1]	14,445,000	[2]	0	22,548,000	23,121,000	17,956,000	33,685,000	48,740,000	[7]	29,793,000	[7]	8,941,000	[11]	5,690,000	[11]	0	[6]	8,217,000	[6]	9,735,000	[7]	163,000	[7]	224,125,000	[2]	219,121,000	[2]	11,628,000	[12]	27,036,000	[12]	45,746,000	[2]	46,438,000	[2]
Less current portion of regulatory liabilities		26,245,000								53,601,000								26,245,000	53,601,000
Regulatory Liabilities, Net		420,175,000								382,366,000								420,175,000	382,366,000
Delivery term of related contracts for differences, minimum (in years)																		9Y
Delivery term of related contracts for differences, maximum (in years)																		16Y
Recovery period, revenue received for excess refund of overearnings, minimum (in years)																		2Y
Recovery period, revenue received for excess refund of overearnings, maximum (in years)																		3Y
Remaining period (in years)																																																				32Y									39Y
Remaining period, minimum (in years)																																																															4Y
Remaining period, maximum (in years)																																																															8Y
Derivative fair value, net [Abstract]
Derivative assets/(liabilities), gross																																												14,189,000	6,057,000	73,264,000	28,131,000	(28,888,000)	(13,246,000)	(239,147,000)	(129,560,000)
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%
Derivative [Line Items]
Gross derivative asset		83,900,000																83,900,000																							3,500,000
Regulatory asset		184,100,000																184,100,000
Gross derivative liability		268,000,000																268,000,000
Gross derivative liability, portion of CL&P		163,100,000																163,100,000
Capacity of plant that exploded (in megawatts)																		620
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
Unrealized gain and losses from mark-to-market adjustments [Abstract]
Regulatory Assets - Derivative Assets																		69,397,000	(23,022,000)
Regulatory Liabilities - Derivative liabilities																		5,736,000	584,000
Property, Plant and Equipment [Line Items]
Gross property, plant, & equipment		3,132,256,000								2,944,223,000								3,132,256,000	2,944,223,000										845,252,000	777,916,000	549,995,000	489,223,000	1,318,917,000	1,275,133,000	139,241,000	129,202,000	42,625,000	39,008,000	236,226,000	233,741,000
Less accumulated depreciation		929,401,000								859,461,000								929,401,000	859,461,000
Total		2,202,855,000								2,084,762,000								2,202,855,000	2,084,762,000
Construction work in progress		367,500,000								242,688,000								367,500,000	242,688,000
Net Property, Plant and Equipment		2,570,355,000								2,327,450,000								2,570,355,000	2,327,450,000
Asset retirement obligations		18,100,000								17,800,000								18,100,000	17,800,000
Asset removal costs		224,100,000								219,100,000								224,100,000	219,100,000
Weighted-average allowance for funds used during construction (in hundredths)																		6.31%	6.65%	2.44%
Annual depreciation rate (in hundredths)		3.30%								3.60%								3.30%	3.60%	3.70%
Restricted cash		6,500,000								2,400,000								6,500,000	2,400,000
Equity Investments [Abstract]
Carrying value of joint venture										62,786,000									62,786,000									131,100,000
Conversion of loans to equity in joint venture																		63,000,000	55,500,000
Income (loss) from equity investment																		11,282,000	1,278,000	59,000
Distributions received from joint venture																												8,000,000
Subsequent distributions received from joint venture	3,300,000
Undistributed earnings in equity method investmeent		4,600,000																4,600,000
Numerator [Abstract]
Net income attributable to UIL Holdings		21,276,000	[13]	12,180,000	[13]	14,156,000	[13]	52,044,000	[13]	10,474,000	[13]	16,976,000	[13]	11,489,000	[13]	15,915,000	[13]	99,656,000	54,854,000	54,317,000
Less: Net income allocated to unvested units																		210,000	149,000	84,000
Net Income attributable to common shareholders																		99,446,000	54,705,000	54,233,000
Denominator [Abstract]
Basic average number of shares outstanding (in shares)																		50,609,000	35,722,000	28,027,000
Effect of dilutive securities (in shares)																		317,000	361,000	246,000
Diluted average number of shares outstanding (in shares)																		50,926,000	36,083,000	28,273,000
Earnings per share [Abstract]
Basic (in dollars per share)		$ 0.42	[14]	$ 0.24	[14]	$ 0.28	[14]	$ 1.03	[14]	$ 0.21	[14]	$ 0.52	[14]	$ 0.38	[14]	$ 0.53	[14]	$ 1.96	$ 1.53	$ 1.94
Diluted (in dollars per share)		$ 0.42	[15]	$ 0.24	[15]	$ 0.28	[15]	$ 1.02	[15]	$ 0.20	[15]	$ 0.52	[15]	$ 0.38	[15]	$ 0.53	[15]	$ 1.95	$ 1.52	$ 1.93
Number of shares of common stock outstanding with purchase options not included in diluted earnings per share (in shares)																		89,336	98,079	140,152
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)																											3Y
Number of shares granted (in shares)																					23,394	9,011	2,566	3,627	5,314	111,230
Weighted average grant date fair value (in dollars per share)																					$ 33.51	$ 30.77	$ 30.69	$ 31.85	$ 31.76	$ 30.69
Vesting rights																							20% annually for a five year period.
Total stock-based compensation expense								$ 600,000										$ 5,300,000	$ 4,100,000	$ 3,600,000

[1] Asset/Liability relates to the Competitive Transition Assessment (CTA). Total CTA costs recovery is currently projected to be completed in 2015, with stranded cost amortization expected to end in 2013.

[2] Amortization period and/or balance vary depending on the nature, cost of removal and/or remaining life of the underlying assets/liabilities.

[3] Asset life is dependent upon timing of final pension plan distribution; balance is recalculated each year in accordance with ASC 715 "Compensation-Retirement Benefits" (Note G).

[4] Deferral of revenue received for excess refund of overearnings to be recovered over 2 - 3 years.

[5] Various hardship and payment plan programs approved for recovery.

[6] Deferred purchase gas costs balances at the end of the rate year are normally recorded/returned in the next year.

[7] The balance will be extinguished when the asset or liability has been realized or settled, respectively.

[8] Asset life is equal to delivery term of related contracts (which vary from approximately 9 - 16 years); balance fluctuates based upon quarterly market analysis performed on the related derivatives (Note K).

[9] Working capital allowance for generation service charge; this amount fluctuates based upon cash inflows and outflows in a given period.

[10] Storm costs include accumulated costs for major storms occurring from January 2009 forward. UIL will seek recovery of these costs during its next general rate proceeding.

[11] The liability will be extinguished simultaneous with the retirement of the assets and settlement of the corresponding asset retirement obligation.

[12] Regulatory asset or liability which defers transmission income or expense and fluctuates based upon actual revenues and revenue requirements.

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

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

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