CAPITALIZATION (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Weighted average grant date fair value [Abstract]
Weighted average grant date fair value (in dollars per share)	$ 34.33		$ 33.33		$ 28.43
Average exercise price [Abstract]
Weighted-average remaining contractual life (in years)	3 months 18 days		6 months		1 year 3 months 18 days
Performance share costs	$ 2,400,000
Weighted-average period over which compensation costs will be recognized, average (in months)	0 months
Weighted-average period over which performance-share costs will be recognized, average (in months)	10
Weighted-average period over which restricted stock costs will be recognized, average (in months)	8
Long-term debt [Abstract]
Long-term debt	1,610,550,000		1,517,005,000
Less: Current portion of long-term debt	48,296,000	[1]	13,712,000	[1]
Less: Unamortized discount	2,916,000		3,221,000
Plus: Unamortized premium	41,016,000		48,275,000
Net Long-Term Debt	1,600,354,000		1,548,347,000
Interest rate on tax-exempt bond (in hundredths)	0.41%
Restricted net assets	578,900,000
Tax-exempt bond interest ending period	2013-01-18
Auction rate interest rate reset period (in days)	35
Discounted net proceed from note issued	203,500,000		51,050,000		556,109,000
Fair value long-term debt	1,900,000,000		1,700,000,000
Weighted average maturity term of long-term debt (in years)	12
Long term debt average interest (in hundredths)	5.60%
Maturities [Abstract]
2013	41,455,000
2014	6,455,000
2015	1,455,000
2016	11,455,000
2017 & thereafter	1,549,730,000
Berkshire [Member] | Unsecured Notes [Member]
Maturities [Abstract]
Balance of debt repaid			3,000,000
Interest rate on note repaid (in hundredths)			4.76%
United Illuminating Company (UI) [Member]
Long-term debt [Abstract]
Restricted net assets, UI equity investment	124,700,000
Maturities [Abstract]
Amount of note purchase agreement	203,500,000
Remaining amount of note purchase agreement	100,000,000
United Illuminating Company (UI) [Member] | Pollution Control Revenue Bonds [Member]
Long-term debt [Abstract]
Bonds refinanced, aggregate principle amount outstanding	103,500,000
United Illuminating Company (UI) [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Discounted net proceed from note issued	103,500,000
United Illuminating Company (UI) [Member] | Series A Senior Notes Due 2018 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Interest rate (in hundredths)	6.46%		6.46%
United Illuminating Company (UI) [Member] | Series A Senior Notes Due 2019 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Long-term debt payable	31,000,000		0
Interest rate (in hundredths)	2.98%		0.00%
Due date	2019-01-31
United Illuminating Company (UI) [Member] | Series A and B Senior Notes Due 2018 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Long-term debt payable	100,000,000		100,000,000
Due date	2018
United Illuminating Company (UI) [Member] | Series B Senior Notes Due 2018 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Interest rate (in hundredths)	6.51%		6.51%
United Illuminating Company (UI) [Member] | 1996 Series Due 2026 [Member] | Pollution Control Revenue Bonds [Member]
Long-term debt [Abstract]
Long-term debt payable	0	[2]	7,500,000	[2]
Interest rate (in hundredths)	0.00%		5.75%	[2]
Due date	2026	[2]
United Illuminating Company (UI) [Member] | 2010 Series Due 2027 [Member] | Pollution Control Revenue Bonds [Member]
Long-term debt [Abstract]
Long-term debt payable	27,500,000		27,500,000
Interest rate (in hundredths)	4.50%		4.50%
Due date	2027
United Illuminating Company (UI) [Member] | 1997 Series Due 2027 [Member] | Pollution Control Revenue Bonds [Member]
Long-term debt [Abstract]
Long-term debt payable	0	[2]	71,000,000	[2]
Interest rate (in hundredths)			7.13%	[2]
Due date	2027	[2]
United Illuminating Company (UI) [Member] | 2009 Series Due 2029 [Member] | Pollution Control Revenue Bonds [Member]
Long-term debt [Abstract]
Long-term debt payable	0	[2]	25,000,000	[2]
Interest rate (in hundredths)			6.88%	[2]
Due date	2029	[2]
United Illuminating Company (UI) [Member] | 2003 Series Due 2033 [Member] | Pollution Control Revenue Bonds [Member]
Long-term debt [Abstract]
Long-term debt payable	64,460,000	[3]	64,460,000	[3]
Interest rate, other	Auction Rate	[3]	Auction Rate	[3]
Due date	2033	[3]
United Illuminating Company (UI) [Member] | Series A and B Senior Notes Due 2017 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Long-term debt payable	70,000,000		70,000,000
Interest rate (in hundredths)	6.06%		6.06%
Due date	2017
United Illuminating Company (UI) [Member] | Series C and D Senior Notes Due 2022 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Interest rate (in hundredths)	6.26%		6.26%
United Illuminating Company (UI) [Member] | Series E and F Senior Notes Due 2037 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Long-term debt payable	28,000,000		28,000,000
Interest rate (in hundredths)	6.51%		6.51%
Due date	2037
United Illuminating Company (UI) [Member] | Series C Senior Notes Due 2020 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Long-term debt payable	50,000,000		50,000,000
Interest rate (in hundredths)	6.61%		6.61%
Due date	2020
United Illuminating Company (UI) [Member] | Senior Notes Due 2040 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Long-term debt payable	100,000,000		100,000,000
Interest rate (in hundredths)	6.09%		6.09%
Due date	2040
United Illuminating Company (UI) [Member] | Series D and E Senior Notes Due 2042 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Long-term debt payable	87,000,000		0
Interest rate (in hundredths)	4.89%
Due date	2042
United Illuminating Company (UI) [Member] | Series B and C Senior Notes [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Interest rate (in hundredths)	3.61%		3.61%
United Illuminating Company (UI) [Member] | Series B, C and D Senior Notes Due 2022 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Long-term debt payable	162,500,000		77,000,000
Due date	2022
United Illuminating Company (UI) [Member] | Series C Senior Notes Due 2022 [Member]
Long-term debt [Abstract]
Principal amount of note outstanding	34,000,000
United Illuminating Company (UI) [Member] | Series C Senior Notes Due 2022 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Interest rate (in hundredths)	3.61%
Due date	2022-01-31
United Illuminating Company (UI) [Member] | Series D Senior Note Due 2042 [Member]
Long-term debt [Abstract]
Interest rate (in hundredths)	4.89%
Principal amount of note outstanding	52,000,000
Due date	2042-01-30
United Illuminating Company (UI) [Member] | Series E Senior Notes Due 2042 [Member]
Long-term debt [Abstract]
Interest rate (in hundredths)	4.89%
Principal amount of note outstanding	35,000,000
Due date	2042-01-30
United Illuminating Company (UI) [Member] | Senior Notes Due 2025 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Long-term debt payable	50,000,000		50,000,000
Interest rate (in hundredths)	5.61%		5.61%
Due date	2025
United Illuminating Company (UI) [Member] | Series B Senior Unsecured Notes due 2022 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Interest rate (in hundredths)	3.61%
Principal amount of note outstanding	51,500,000
Due date	2022-01-31
The Southern Connecticut Gas Company (SCG) [Member] | Secured Medium Term Notes Due 2011 [Member]
Maturities [Abstract]
Balance of debt repaid			30,000,000
Interest rate on note repaid (in hundredths)			6.59%
The Southern Connecticut Gas Company (SCG) [Member] | Secured Medium Term Note IV, Due 2018-2041 [Member]
Long-term debt [Abstract]
Long-term debt payable	25,000,000
Interest rate (in hundredths)	5.39%
Due date	2041-09-22
The Southern Connecticut Gas Company (SCG) [Member] | Secured Medium Term Notes Due 2021 [Member]
Long-term debt [Abstract]
Interest rate (in hundredths)	3.88%
Principal amount of note outstanding	25,000,000
Due date	2021-09-22
UIL Holdings [Member] | Unsecured Senior Notes Due 2020 [Member]
Long-term debt [Abstract]
Long-term debt payable	450,000,000		450,000,000
Interest rate (in hundredths)	4.625%		4.625%
Due date	2020
UIL Holdings [Member] | Series A Senior Notes [Member]
Maturities [Abstract]
Balance of debt repaid			4,300,000
Interest rate on note repaid (in hundredths)			7.23%
UIL Holdings [Member] | Series B Senior Notes [Member]
Maturities [Abstract]
Balance of debt repaid			45,000,000
Interest rate on note repaid (in hundredths)			7.38%
UIL Holdings [Member] | Equity Bridge Loan [Member]
Maturities [Abstract]
Balance of debt repaid			63,000,000
Gas Companies [Member] | Senior Unsecured Notes Due 2020 - 2021 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Long-term debt payable	21,090,000		22,545,000
Interest rate, minimum (in hundredths)	4.76%		4.76%
Interest rate, maximum (in hundredths)	9.60%		9.60%
Due date, minimum	2020
Due date, maximum	2021
Gas Companies [Member] | Secured Medium Term Note IV, Due 2018-2041 [Member] | Secured Notes [Member]
Long-term debt [Abstract]
Long-term debt payable	100,000,000		100,000,000
Interest rate, minimum (in hundredths)	3.88%		3.88%
Interest rate, maximum (in hundredths)	7.50%		7.50%
Due date, minimum	2018
Due date, maximum	2041
Gas Companies [Member] | Secured Medium Term Notes III, Due 2025-2037 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Due date, minimum	2035
Gas Companies [Member] | Secured Medium Term Notes III, Due 2025-2037 [Member] | Secured Notes [Member]
Long-term debt [Abstract]
Long-term debt payable	85,000,000		85,000,000
Interest rate, minimum (in hundredths)	5.77%		5.77%
Interest rate, maximum (in hundredths)	6.38%		6.38%
Due date, minimum	2025
Due date, maximum	2037
Gas Companies [Member] | Secured Medium Term Notes I, Due 2026-2028 [Member] | Secured Notes [Member]
Long-term debt [Abstract]
Long-term debt payable	29,000,000		29,000,000
Interest rate, minimum (in hundredths)	6.88%		6.88%
Interest rate, maximum (in hundredths)	7.95%		7.95%
Due date, minimum	2026
Due date, maximum	2028
Gas Companies [Member] | First Mortgage Bond Series P Due 2019 [Member] | Secured Notes [Member]
Long-term debt [Abstract]
Long-term debt payable	10,000,000		10,000,000
Interest rate (in hundredths)	10.06%		10.06%
Due date	2019
Gas Companies [Member] | Series A Unsecured Medium Term Notes Due 2013-2017 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Long-term debt payable	50,000,000		55,000,000
Interest rate, minimum (in hundredths)	6.85%		6.85%
Interest rate, maximum (in hundredths)	9.10%		9.10%
Due date, minimum	2013
Due date, maximum	2017
Gas Companies [Member] | Series D Unsecured Medium Term Notes Due 2013 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Long-term debt payable	20,000,000		20,000,000
Interest rate (in hundredths)	6.50%		6.50%
Due date	2013
Gas Companies [Member] | Series B Unsecured Medium Term Notes Due 2014-2024 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Long-term debt payable	10,000,000		10,000,000
Interest rate, minimum (in hundredths)	8.12%		8.12%
Interest rate, maximum (in hundredths)	8.49%		8.49%
Due date, minimum	2014
Due date, maximum	2024
Gas Companies [Member] | Series C Unsecured Medium Term Notes Due 2035-2037 [Member] | Unsecured Notes [Member]
Long-term debt [Abstract]
Long-term debt payable	65,000,000		65,000,000
Interest rate, minimum (in hundredths)	5.63%		5.63%
Interest rate, maximum (in hundredths)	6.66%		6.66%
Due date, maximum	2037
Restricted Stock [Member]
Number of shares [Abstract]
Beginning Balance (in shares)	107,241
Granted (in shares)	25,747
Forfeited (in shares)	0
Vested (in shares)	(58,857)
Ending Balance (in shares)	74,131
Weighted average grant date fair value [Abstract]
Beginning Balance (in dollars per share)	$ 28.12
Granted (in dollars per share)	$ 35.51
Vested (in dollars per share)	$ 27.48
Ending Balance (in dollars per share)	$ 30.49
Restricted stock [Abstract]
Restricted stock (in shares)	209,469		100,003
Average exercise price [Abstract]
Restricted stock costs	700,000
Performance Shares [Member]
Number of shares [Abstract]
Beginning Balance (in shares)	324,601
Granted (in shares)	122,302
Forfeited (in shares)	(10,220)
Vested (in shares)	(110,795)
Ending Balance (in shares)	325,888
Weighted average grant date fair value [Abstract]
Beginning Balance (in dollars per share)	$ 27.43
Granted (in dollars per share)	$ 34.2
Forfeited (in dollars per share)	$ 31.10
Vested (in dollars per share)	$ 22.90
Ending Balance (in dollars per share)	$ 31.39
Average exercise price [Abstract]
Total stock option compensation costs	0
Stock Options [Member]
Number of options [Roll Forward]
Beginning balance (in shares)	98,220		134,994		168,501
Granted (in shares)	0		0		0
Forfeited (in shares)	(38,000)		(7,910)		(3,202)
Exercised (in shares)	(56,887)		(28,864)		(30,305)
Ending balance (in shares)	3,333		98,220		134,994
Option price per share [Abstract]
Beginning balance, minimum (in dollars per share)	$ 21.68		$ 21.68
Beginning balance, maximum (in dollars per share)	$ 33.96		$ 34.51
Exercised, minimum (in dollars per share)	$ 31.25		$ 21.68		$ 21.68
Exercised, maximum (in dollars per share)	$ 33.96		$ 31.25		$ 23.64
Ending balance, minimum (in dollars per share)	$ 21.68		$ 21.68		$ 21.68
Ending balance, maximum (in dollars per share)	$ 21.68		$ 33.96		$ 34.51
Average exercise price [Abstract]
Beginning balance (in dollars per share)	$ 33.39		$ 31.70		$ 30.32
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)	$ 21.68		$ 33.39		$ 31.70
Number of exercisable stock options (in shares)	3,333	[4]	98,220		134,994
Option price per share, minimum (in dollars per share)	$ 21.68		$ 21.68		$ 21.68
Option price per share, maximum (in dollars per share)	$ 21.68		$ 33.96		$ 34.51
Average exercisable at end of period (in shares)	$ 21.68		$ 33.39		$ 31.70
Intrinsic value	50,000
Cash received from options exercised under all share-based payment arrangements	1,900,000		800,000		700,000
Tax benefit realized for the tax deductions from the option exercises	100,000		100,000		100,000
Common Stock [Member]
Temporary Equity [Line Items]
Common stock outstanding (in shares)	50,665,114		50,545,487
Common stock par value (in dollars per share)	$ 0		$ 0
CNG, 8.00% Noncallable [Member]
Consolidated redeemable preferred stock, noncontrolling [Abstract]
Dividend rate of preferred stock (in hundredths)	8.00%
Per value per share (in dollars per share)	$ 3.125
Shares issued and outstanding (in shares)	108,706
Value of shares issued and outstanding	$ 300,000
Stock authorized but unissued (in shares)	775,609
CNG, 6.00% [Member]
Consolidated redeemable preferred stock, noncontrolling [Abstract]
Dividend rate of preferred stock (in hundredths)	6.00%
Per value per share (in dollars per share)	$ 100
Shares issued and outstanding (in shares)	4,104
Stock authorized but unissued (in shares)	9,999,068

[1] Includes the current portion of unamortized premium.

[2] The interest rates on these bonds were due to be remarketed on February 1, 2012; however such bonds were refinanced in January 2012, as discussed below.

[3] The interest rate on these Bonds is reset through an auction held every 35 days. On January 18, 2013, the interest rate on the Bonds was 0.41%.

[4] The intrinsic value of exercisable stock options at December 31, 2012 was $0.05 million.