BUSINESS ORGANIZATION AND STATEMENT OF ACCOUNTING POLICIES (Details) (USD $)	3 Months Ended		6 Months Ended			6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended										1 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Jun. 30, 2011 Nuclear Plant Investments - Above Market [Member]	Dec. 31, 2010 Nuclear Plant Investments - Above Market [Member]	Jun. 30, 2011 Income Taxes Due Principally To Book-Tax Differences [Member]	Dec. 31, 2010 Income Taxes Due Principally To Book-Tax Differences [Member]	Jun. 30, 2011 Connecticut Yankee [Member]	Dec. 31, 2010 Connecticut Yankee [Member]	Jun. 30, 2011 Unamortized Redemption Costs [Member]	Dec. 31, 2010 Unamortized Redemption Costs [Member]	Jun. 30, 2011 Pension and Other Post-Retirement Benefit Plans [Member]	Dec. 31, 2010 Pension and Other Post-Retirement Benefit Plans [Member]	Jun. 30, 2011 Environmental Remediation Costs [Member]	Dec. 31, 2010 Environmental Remediation Costs [Member]	Jun. 30, 2011 Customer Rate Surcharge [Member]	Dec. 31, 2010 Customer Rate Surcharge [Member]	Jun. 30, 2011 Low Income Program [Member]	Dec. 31, 2010 Low Income Program [Member]	Jun. 30, 2011 Debt Premium [Member]	Dec. 31, 2010 Debt Premium [Member]	Jun. 30, 2011 Deferred Purchased Gas [Member]	Dec. 31, 2010 Deferred Purchased Gas [Member]	Jun. 30, 2011 Deferred Income Taxes [Member]	Dec. 31, 2010 Deferred Income Taxes [Member]	Jun. 30, 2011 Deferred Income Taxes [Member]	Dec. 31, 2010 Deferred Income Taxes [Member]	Jun. 30, 2011 Unfunded Future Income Taxes [Member]	Dec. 31, 2010 Unfunded Future Income Taxes [Member]	Jun. 30, 2011 Contracts For Differences Regulatory Asset [Member]	Dec. 31, 2010 Contracts For Differences Regulatory Asset [Member]	Jun. 30, 2011 Excess Generation Service Charge [Member]	Dec. 31, 2010 Excess Generation Service Charge [Member]	Jun. 30, 2011 Other Regulatory Asset [Member]	Dec. 31, 2010 Other Regulatory Asset [Member]	Jun. 30, 2011 Accumulated Deferred Investment Tax Credits [Member]	Dec. 31, 2010 Accumulated Deferred Investment Tax Credits [Member]	Jun. 30, 2011 Deferred Gain on Sale of Property [Member]	Dec. 31, 2010 Deferred Gain on Sale of Property [Member]	Jun. 30, 2011 Middletown/Norwalk Local Transmission Network Service Collections [Member]	Dec. 31, 2010 Middletown/Norwalk Local Transmission Network Service Collections [Member]	Jun. 30, 2011 Pension And Other Post-Retirement Benefit Plans [Member]	Dec. 31, 2010 Pension And Other Post-Retirement Benefit Plans [Member]	Jun. 30, 2011 Asset Retirement Obligation [Member]	Dec. 31, 2010 Asset Retirement Obligation [Member]	Jun. 30, 2011 Deferred Purchased Gas [Member]	Dec. 31, 2010 Deferred Purchased Gas [Member]	Jun. 30, 2011 Asset Removal Costs [Member]	Dec. 31, 2010 Asset Removal Costs [Member]	Jun. 30, 2011 Deferred Transmission Expense [Member]	Dec. 31, 2010 Deferred Transmission Expense [Member]	Jun. 30, 2011 Other Regulatory Liability [Member]	Dec. 31, 2010 Other Regulatory Liability [Member]	Jun. 30, 2011 Current Assets [Member]	Dec. 31, 2010 Current Assets [Member]	Jun. 30, 2011 Deferred Charges and Other Assets [Member]	Dec. 31, 2010 Deferred Charges and Other Assets [Member]	Jun. 30, 2011 Current Liabilities [Member]	Dec. 31, 2010 Current Liabilities [Member]	Jun. 30, 2011 Noncurrent Liabilities [Member]	Dec. 31, 2010 Noncurrent Liabilities [Member]	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]
Business Acquisition [Line Items]
Purchase price of acquisition	$ 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]
Remaining period (in years)						(a)		(b)		5				(c)				(g)		(h)				(i)		(j)				(j)		(d)		(e)		(b)
Remaining period, minimum (in years)												11				4		2				1										9
Remaining period, maximum (in years)												23				5		3				27										16
Total regulatory assets	1,044,493,000		1,044,493,000		1,041,737,000	283,165,000	293,388,000	7,562,000	11,910,000	15,759,000	17,175,000	13,307,000	13,708,000	335,740,000	351,610,000	2,441,000	17,285,000	17,118,000	12,816,000	35,331,000	40,674,000	52,361,000	56,865,000	0	26,650,000	12,349,000	5,859,000			35,675,000	25,684,000	185,244,000	114,662,000	6,520,000	8,711,000	41,921,000	44,740,000
Less current portion of regulatory assets	81,257,000		81,257,000		115,848,000
Regulatory Assets, Net	963,236,000		963,236,000		925,889,000
Regulatory Liabilities [Line Items]
Remaining period (in years)																												(j)										33		(a)		39				(k)		< 1		(b)		(f)		(b)
Remaining period, minimum (in years)																																												4
Remaining period, maximum (in years)																																												7
Total regulatory liabilities	445,231,000		445,231,000		435,967,000																							16,228,000	29,793,000									4,831,000	4,905,000	37,798,000	37,798,000	22,835,000	23,121,000	24,379,000	33,685,000	7,979,000	8,451,000	24,721,000	15,275,000	223,611,000	219,121,000	38,494,000	27,036,000	44,355,000	36,782,000
Less current portion of regulatory liabilities	78,315,000		78,315,000		53,601,000
Regulatory Liabilities, Net	366,916,000		366,916,000		382,366,000
Derivative fair value, net [Abstract]
Derivative assets/(liabilities), gross																																																								10,615,000	6,057,000	73,679,000	28,131,000	(28,905,000)	(13,246,000)	(240,601,000)	(129,560,000)
Derivative [Line Items]
Number of Contracts for Differences (CfDs) executed by UI and CL&P	2		2
Percentage of cost borne by UI customers (in hundredths)			20.00%
Percentage of cost borne by CL&P customers (in hundredths)			80.00%
Gross derivative asset	84,300,000		84,300,000
Regulatory asset	185,200,000		185,200,000
Gross derivative liability	269,500,000		269,500,000
Gross derivative liability, portion of CL&P	164,200,000		164,200,000
Capacity of plant (in megawatts)			620
Unrealized gains and losses from mark-to-market adjustments [Abstract]
Regulatory Assets - Derivative liabilities	29,722,000	4,905,000	70,536,000	(57,552,000)
Regulatory Liabilities - Derivative assets	68,000	(192,000)	5,748,000	(93,000)
Asset Retirement Obligations [Abstract]
Asset retirement obligations (ARO)	18,300,000		18,300,000		17,800,000
Numerator:
Net income attributable to UIL Holdings	14,156,000	11,489,000	66,200,000	27,404,000
Less: Net income allocated to unvested units	29,000	41,000	143,000	103,000
Net income attributable to common shareholders	14,127,000	11,448,000	66,057,000	27,301,000
Denominator:
Basic average number of shares outstanding (in shares)	50,628,000	30,093,000	50,574,000	30,037,000
Effect of dilutive securities (in shares)	244,000	220,000	250,000	280,000
Diluted average number of shares outstanding (in shares)	50,872,000	30,313,000	50,824,000	30,317,000
Earnings per share:
Basic (in dollars per share)	$ 0.28	$ 0.38	$ 1.31	$ 0.91
Diluted (in dollars per share)	$ 0.28	$ 0.38	$ 1.30	$ 0.90
Equity Investments [Abstract]
Carrying value	69,500,000		69,500,000		62,800,000
Loan UI made to GenConn	63,000,000		63,000,000
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	89,336	136,945	97,669	136,945
Income Taxes [Abstract]
Impact of change in accounting method on income tax expense		1,400,000		1,200,000
Stock-Based Compensation [Abstract]
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	$ 1,400,000	$ 900,000	$ 3,300,000	$ 2,100,000