CAPITALIZATION (Details) (USD $)	12 Months Ended						12 Months Ended							12 Months Ended				12 Months Ended														12 Months Ended											12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Equity Bridge Loan [Member]	Dec. 31, 2011 Restricted Stock [Member]	Dec. 31, 2010 Restricted Stock [Member]	Dec. 31, 2011 Performance Shares [Member]	Dec. 31, 2011 Stock Options [Member]	Dec. 31, 2010 Stock Options [Member]	Dec. 31, 2009 Stock Options [Member]	Dec. 31, 2008 Stock Options [Member]	Dec. 31, 2011 Common Stock [Member]	Dec. 31, 2010 Common Stock [Member]	Dec. 31, 2010 Additional Common Stock [Member]	Dec. 31, 2010 Public Offering [Member]	Dec. 31, 2011 CNG, 8.00% Noncallable [Member]	Dec. 31, 2011 CNG, 6.00% [Member]	Dec. 31, 2011 Berkshire [Member]	Dec. 31, 2011 United Illuminating Company (UI) [Member] Series A Senior Notes [Member]	Dec. 31, 2010 United Illuminating Company (UI) [Member] Series B Senior Notes [Member]	Dec. 31, 2011 United Illuminating Company (UI) [Member] 1996 Series due 2026 [Member] Pollution Controlled Revenue Bonds [Member]	Dec. 31, 2010 United Illuminating Company (UI) [Member] 1996 Series due 2026 [Member] Pollution Controlled Revenue Bonds [Member]	Dec. 31, 2011 United Illuminating Company (UI) [Member] 2010 Series due 2027 [Member] Pollution Controlled Revenue Bonds [Member]	Dec. 31, 2010 United Illuminating Company (UI) [Member] 2010 Series due 2027 [Member] Pollution Controlled Revenue Bonds [Member]	Dec. 31, 2011 United Illuminating Company (UI) [Member] 1997 Series due 2027 [Member] Pollution Controlled Revenue Bonds [Member]	Dec. 31, 2010 United Illuminating Company (UI) [Member] 1997 Series due 2027 [Member] Pollution Controlled Revenue Bonds [Member]	Dec. 31, 2011 United Illuminating Company (UI) [Member] 2009 Series due 2029 [Member] Pollution Controlled Revenue Bonds [Member]	Dec. 31, 2010 United Illuminating Company (UI) [Member] 2009 Series due 2029 [Member] Pollution Controlled Revenue Bonds [Member]	Dec. 31, 2011 United Illuminating Company (UI) [Member] 2003 Series due 2033 [Member] Pollution Controlled Revenue Bonds [Member]	Dec. 31, 2010 United Illuminating Company (UI) [Member] 2003 Series due 2033 [Member] Pollution Controlled Revenue Bonds [Member]	Feb. 06, 2012 United Illuminating Company (UI) [Member] 2003 Series due 2033 [Member] Pollution Controlled Revenue Bonds [Member]	Dec. 31, 2011 United Illuminating Company (UI) [Member] Series A and B Senior Notes [Member]	Dec. 31, 2010 United Illuminating Company (UI) [Member] Series A and B Senior Notes [Member]	Dec. 31, 2011 United Illuminating Company (UI) [Member] Series C and D Senior Notes [Member]	Dec. 31, 2010 United Illuminating Company (UI) [Member] Series C and D Senior Notes [Member]	Dec. 31, 2011 United Illuminating Company (UI) [Member] Series E and F Senior Notes [Member]	Dec. 31, 2010 United Illuminating Company (UI) [Member] Series E and F Senior Notes [Member]	Dec. 31, 2011 United Illuminating Company (UI) [Member] Series C Senior Notes [Member]	Dec. 31, 2010 United Illuminating Company (UI) [Member] Series C Senior Notes [Member]	Dec. 31, 2011 United Illuminating Company (UI) [Member] Senior Notes due 2040 [Member]	Dec. 31, 2010 United Illuminating Company (UI) [Member] Senior Notes due 2040 [Member]	Dec. 31, 2010 United Illuminating Company (UI) [Member] Equity Bridge Loan [Member]	Dec. 31, 2011 United Illuminating Company (UI) [Member] Senior Notes due 2025 [Member]	Dec. 31, 2010 United Illuminating Company (UI) [Member] Senior Notes due 2025 [Member]	Dec. 31, 2011 The Southern Connecticut Gas Company (SCG) [Member] Secured Medium Term Notes due 2041 [Member]	Dec. 31, 2011 The Southern Connecticut Gas Company (SCG) [Member] Senior Secured Medium Term Notes [Member]	Dec. 31, 2011 The Southern Connecticut Gas Company (SCG) [Member] Secured Medium Term Notes due 2021 [Member]	Dec. 31, 2011 UIL Holding [Member] Pollution Controlled Revenue Bonds [Member]	Dec. 31, 2011 UIL Holding [Member] Series A Senior Notes [Member]	Dec. 31, 2010 UIL Holding [Member] Series A Senior Notes [Member]	Dec. 31, 2011 UIL Holding [Member] Series B Senior Notes [Member]	Dec. 31, 2010 UIL Holding [Member] Series B Senior Notes [Member]	Dec. 31, 2011 UIL Holding [Member] Unsecured Notes [Member]	Dec. 31, 2010 UIL Holding [Member] Unsecured Notes [Member]	Dec. 31, 2011 UIL Holding [Member] Series D Unsecured Medium Term Notes due 2013 [Member]	Dec. 31, 2011 UIL Holding [Member] Series B Senior Unsecured Notes due 2022 [Member]	Dec. 31, 2011 Gas Companies [Member] Secured Medium Term Notes due 2011 [Member]	Dec. 31, 2010 Gas Companies [Member] Secured Medium Term Notes due 2011 [Member]	Dec. 31, 2011 Gas Companies [Member] Secured Medium Term Notes due 2018-2041 [Member]	Dec. 31, 2010 Gas Companies [Member] Secured Medium Term Notes due 2018-2041 [Member]	Dec. 31, 2011 Gas Companies [Member] Secured Medium Term Notes due 2025-2037 [Member]	Dec. 31, 2010 Gas Companies [Member] Secured Medium Term Notes due 2025-2037 [Member]	Dec. 31, 2011 Gas Companies [Member] Secured Medium Term Notes due 2026-2028 [Member]	Dec. 31, 2010 Gas Companies [Member] Secured Medium Term Notes due 2026-2028 [Member]	Dec. 31, 2011 Gas Companies [Member] First Motgage Bond Series due 2019 [Member]	Dec. 31, 2010 Gas Companies [Member] First Motgage Bond Series due 2019 [Member]	Dec. 31, 2011 Gas Companies [Member] Senior Unsecured Notes due 2011-2021 [Member]	Dec. 31, 2010 Gas Companies [Member] Senior Unsecured Notes due 2011-2021 [Member]	Dec. 31, 2011 Gas Companies [Member] Series A Unsecured Medium Term Notes due 2012-2017 [Member]	Dec. 31, 2010 Gas Companies [Member] Series A Unsecured Medium Term Notes due 2012-2017 [Member]	Dec. 31, 2011 Gas Companies [Member] Series D Unsecured Medium Term Notes due 2013 [Member]	Dec. 31, 2010 Gas Companies [Member] Series D Unsecured Medium Term Notes due 2013 [Member]	Dec. 31, 2011 Gas Companies [Member] Series B Unsecured Medium Term Notes due 2014-2024 [Member]	Dec. 31, 2010 Gas Companies [Member] Series B Unsecured Medium Term Notes due 2014-2024 [Member]	Dec. 31, 2011 Gas Companies [Member] Series C Unsecured Medium Term Notes due 2035-2037 [Member]	Dec. 31, 2010 Gas Companies [Member] Series C Unsecured Medium Term Notes due 2035-2037 [Member]
Temporary Equity [Line Items]
Common stock outstanding (in shares)												50,545,487	50,443,083	2,655,000	17,700,000
Common stock par value (in dollars per share)		$ 25.75										$ 0.00			$ 25.75
Net proceeds of offering	$ 200,000	$ 502,220,000	$ 92,225,000											$ 501,500,000
Consolidated redeemable preferred stock, noncontrolling [Abstract]
Per value per share (in dollars per share)																$ 3.125	$ 100
Redemption price per share (in dollars per share)																$ 0	$ 110
Shares issued and outstanding (in shares)	112,810															108,706	4,104
Amount	750,000															340,000	410,000
Stock authorized but unissued (in shares)																775,609	9,994,964
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock (in shares)					100,003	62,368
Number of options [Roll Forward]
Beginning balance (in shares)								134,994	168,501	403,472
Granted (in shares)								0	0	0
Forfeited (in shares)								(7,910)	(3,202)	(234,971)
Exercised (in shares)								(28,864)	(30,305)	0
Ending balance (in shares)								98,220	134,994	168,501	403,472
Option price per share [Abstract]
Beginning balance, minimum (in dollars per share)								$ 21.68	$ 21.68	$ 21.68	$ 21.68
Beginning balance, maximum (in dollars per share)								$ 33.96	$ 34.51	$ 34.51	$ 34.52
Exercised, minimum (in dollars per share)								$ 21.68	$ 21.68	$ 0
Exercised, maximum (in dollars per share)								$ 31.25	$ 23.64
Ending balance, minimum (in dollars per share)								$ 21.68	$ 21.68	$ 21.68	$ 21.68
Ending balance, maximum (in dollars per share)								$ 33.96	$ 34.51	$ 34.51	$ 34.52
Average exercise price [Abstract]
Beginning balance (in dollars per share)								$ 31.70	$ 30.32	$ 32.07
Granted (in dollars per share)								$ 0	$ 0	$ 0
Forfeited (in dollars per share)								$ 0	$ 0	$ 0
Exercised (in dollars per share)								$ 0	$ 0	$ 0
Ending balance (in dollars per share)								$ 33.39	$ 31.70	$ 30.32	$ 32.07
Number of options (in shares)								98,220	134,994	168,501
Option price per share, minimum (in dollars per share)								$ 21.68	$ 21.68	$ 21.68	$ 21.68
Option price per share, maximum (in dollars per share)								$ 33.96	$ 34.51	$ 34.51	$ 34.52
Exercisable at end of period (in shares)								$ 33.39	$ 31.70	$ 30.32
Intrinsic value	200,000							200,000
Weighted-average contractual life (in years)								0.5	1.3	2.0
Total stock option compensation costs							0
Performance share costs	2,300,000
Stock options outstanding and exercisable, intrinsic value	200,000							200,000
Weighted-average remaining contractual lives for stock options outstanding (in years)	0.5	1.3	2.0
Compensation cost related to non-vested awards not yet recognized						1,100,000
Weighted-average period over which compensation costs will be recognized, minimum (in months)					0		0	0
Weighted-average period over which compensation costs will be recognized, average (in months)					9M		9M	9M
Weighted-average period over which compensation costs will be recognized, maximum (in months)					12M		12M	12M
Cash received from options exercised								800,000	700,000	0
Tax benefit realized for the tax deductions from the option exercises								100,000	100,000	0
Schedule of Capitalization, Long-term Debt [Line Items]
Redemption of outstanding cumulative preferred stock at par																		776
Percentage interest rate on cumulative preferred stock at par (in hundredths)																		4.80%
Long-term debt [Abstract]
Long-term debt payable				0															100,000,000	100,000,000	7,500,000	7,500,000	27,500,000	27,500,000	71,000,000	71,000,000	25,000,000	25,000,000	64,460,000	64,460,000		70,000,000	70,000,000	77,000,000	77,000,000	28,000,000	28,000,000	50,000,000	50,000,000	100,000,000	100,000,000	61,783,000	50,000,000	50,000,000					0	4,286,000	0	45,000,000	450,000,000	450,000,000			0	30,000,000	100,000,000	50,000,000	85,000,000	85,000,000	29,000,000	29,000,000	10,000,000	10,000,000	22,545,000	27,000,000	55,000,000	55,000,000	20,000,000	20,000,000	10,000,000	10,000,000	65,000,000	65,000,000
Interest rate (in hundredths)																			6.46%	6.51%	5.75%	5.75%	4.50%	4.50%	7.13%	7.13%	6.88%	6.88%				6.06%	6.06%	6.26%	6.26%	6.51%	6.51%	6.61%	6.61%	6.09%	6.09%		5.61%	5.61%	5.39%		3.88%		7.23%	7.23%	7.38%	7.38%	4.625%	4.625%	4.89%	3.61%	6.59%	6.59%	3.88%	3.88%	5.772%	5.772%	6.88%	6.88%	10.06%	10.06%	4.76%	4.76%	6.85%	6.85%	6.50%	6.50%	8.12%	8.12%	5.63%	5.63%
Interest rate, maximum (in hundredths)																																																											7.50%	7.50%	6.38%	6.38%	7.95%	7.95%			9.60%	9.60%	9.10%	9.10%			8.49%	8.49%	6.66%	6.66%
Interest rate, other																													Auction rate	Auction rate
Due date																			2018	2018	2026	2026	2027	2027	2027	2027	2029	2029	2033	2033		2017	2017	2022	2022	2037	2037	2020	2020	2040	2040		2025	2025					2011	2011	2011	2011	2020	2020			2011	2011							2019	2019					2013	2013
Due date, minimum																																																											2018	2018	2025	2025	2026	2026			2011	2011	2012	2012			2014	2014	2035	2035
Due date, maximum																																																											2041	2041	2037	2037	2028	2028			2021	2021	2017	2017			2024	2024	2037	2037
Long-Term Debt	1,517,005,000	1,612,529,000
Less: Current portion of long-term debt	13,712,000	154,114,000
Less: Unamortized discount	3,221,000	3,512,000
Plus: Unamortized premium	48,275,000	56,865,000
Net Long-Term Debt	1,548,347,000	1,511,768,000
Due date to remarket interest rate																					February 1, 2012				February 1, 2012		February 1, 2012
Bond refinanced date																					January 2012				January 2012		January 2012
Fixed interest rate, end date																							February 1, 2010
Date bond was refunded																							February 1, 2010
Proceed from issuance of tax exempt bond																							27,500,000
Interest rate on tax-exempt bond (in hundredths)																							4.50%
Interest period on tax-exempt bond																							five-year, 5-month
Auction rate interest rate reset period (in days)																													35D
Auction bond rate at year end (in hundredths)																															0.52%
Balance of debt repaid				63,000,000														3,000,000																												30,000,000			4,300,000		45,000,000
Percentage of discounted price of note issued (in hundredths)																																																						99.204%
Discounted net proceed from note issued	51,050,000	556,109,000	182,773,000																																													103,500,000						443,500,000
Note issuance date																																									July 27, 2010				September 22, 2011		September 22, 2011
Fair value long-term debt	1,700,000,000	1,600,000,000
Maturities [Abstract]
2012	6,455,000
2013	41,455,000
2014	6,455,000
2015	1,455,000
2016 & thereafter	1,461,185,000
Bonds refinanced, aggregate principal amount outstanding																																																103,500,000
Amount of note purchase agreement	203,500,000
Issuances of long term debt	51,050,000	556,109,000	182,773,000																																													103,500,000						443,500,000
Principal amount of note outstanding																																													25,000,000		25,000,000								52,000,000	51,500,000
Interest rate (in hundredths)																			6.46%	6.51%	5.75%	5.75%	4.50%	4.50%	7.13%	7.13%	6.88%	6.88%				6.06%	6.06%	6.26%	6.26%	6.51%	6.51%	6.61%	6.61%	6.09%	6.09%		5.61%	5.61%	5.39%		3.88%		7.23%	7.23%	7.38%	7.38%	4.625%	4.625%	4.89%	3.61%	6.59%	6.59%	3.88%	3.88%	5.772%	5.772%	6.88%	6.88%	10.06%	10.06%	4.76%	4.76%	6.85%	6.85%	6.50%	6.50%	8.12%	8.12%	5.63%	5.63%
Due date																																													September 22, 2041		September 22, 2021								January 30, 2042	January 31, 2022
Balance of debt repaid				$ 63,000,000														$ 3,000,000																												$ 30,000,000			$ 4,300,000		$ 45,000,000
Interest rate on note repaid (in hundredths)																		4.76%																												6.59%
Note issuance date																																									July 27, 2010				September 22, 2011		September 22, 2011
Weighted average maturity term of long-term debt (in years)	11.3
Long term debt average interest (in hundredths)	5.90%