PENSION AND OTHER BENEFITS (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
PENSION AND OTHER BENEFITS [Abstract]
Potential funding requirement for 2013, minimum	$ 50,000
Potential funding requirement for 2013, maximum	60,000
Change in the discount rate assumed for pension expense (in hundredths)	0.25%
Effect of the change in the discount rate on pension expense	2,500
Change in the expected return on assets (in hundredths)	1.00%
Effect of the change in the expected return on assets on pension expenses	6,400
Change in the discount rate assumed, other postretirement plans (in hundredths)	0.25%
Effect of the change in the discount rate on other postretirement expense	400
Effect of the change in the discount rate on Projected Benefit Obligation	100,000
Change in the expected return on assets, other postretirement plans (in hundredths)	1.00%
Effect of the change in the expected return on assets on other postretirement expense	400
Target Allocations [Abstract]
Minimum funding requirement for 2013	20,000
Terms of other postretirement benefits plans - non union employess	Non-union employees who are 55 years of age and whose sum of age and years of service at time of retirement is equal to or greater than 65 are eligible for benefits partially subsidized by UI. The amount of benefits subsidized by UI is determined by age and years of service at retirement. For funding purposes, UI established a 401(h) account in connection with the UI Pension Plan and Serial Voluntary Employee Benefit Association Trust (VEBA) accounts for the years 2007 through 2020 to fund other postretirement benefits for UI's non-union employees who retire on or after January 1, 1994. These VEBA accounts were approved by the IRS and UI contributed $4.5 million to fund the Serial VEBA accounts in 2007. UI does not expect to make a contribution in 2013 to fund OPEB for non-union employees.
Terms of other postretirement benefit plans - union employees	Union employees whose sum of age and years of service at the time of retirement is equal to or greater than 85 (or who are 62 with at least 20 years of service) are eligible for benefits partially subsidized by UI. The amount of benefits subsidized by UI is determined by age and years of service at retirement. For funding purposes, UI established a VEBA to fund other postretirement benefits for UI's union employees. The funding strategy for the VEBA is to select funds that most clearly mirror the pension allocation strategy.
Description of alternative method to amortize prior service costs and unrecognized actuarial gains and losses.	UIL Holdings utilizes an alternative method to amortize unrecognized actuarial gains and losses related to the pension and other postretirement benefits plans over the lesser of the average remaining service period or 10 years. For ASC 715 purposes, UIL Holdings does not recognize gains or losses until there is a variance in an amount equal to at least 5% of the greater of the projected benefit obligation or the market-related value of assets.
Change in Benefit Obligation [Roll Forward]
Actuarial (gain) loss	12,500	(9,072)
Change in Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	585,693
Fair value of plan assets at end of year	663,601	585,693
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates [Abstract]
Aggregate service and interest cost components, increase	836
Aggregate service and interest cost components, decrease	(676)
Accumulated post-retirement benefit obligation, increase	12,162
Accumulated post-retirement benefit obligation, decrease	(10,000)
Estimated Future Benefit Payments [Abstract]
2013	49,350
2014	48,447
2015	49,385
2016	49,625
2017	50,484
2018-2022	265,991
Defined Contribution Retirement Plans/401(k) Disclosure [Abstract]
Matching contributions	3,300	2,600	2,400
Gas Companies Employees [Member]
Defined Contribution Retirement Plans/401(k) Disclosure [Abstract]
Matching contributions	1,600	2,000
Union Employees [Member]
Defined Contribution Retirement Plans/401(k) Disclosure [Abstract]
Employer matching contribution of employees percent (in hundredths)	100.00%
Employer matching contribution at 100 percent (in hundredths)	3.00%
Employer matching contribution of employees next 2 percent (in hundredths)	50.00%
Employer matching contribution at 50 percent (in hundredths)	2.00%
Employer maximum annual matching contribution percent (in hundredths)	4.00%
Non Union Employees [Member]
Defined Contribution Retirement Plans/401(k) Disclosure [Abstract]
Employer matching contribution of employees percent (in hundredths)	100.00%
Employer matching contribution at 100 percent (in hundredths)	2.00%
Employer matching contribution of employees next 2 percent (in hundredths)	50.00%
Employer matching contribution at 50 percent (in hundredths)	2.00%
Pension Benefits [Member]
Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	792,101	776,131
Net transfer in due to acquisition of the Gas Companies	0	0
Service cost	12,032	12,574	7,675
Interest cost	41,470	40,484	22,702
Participant contributions	0	0
Actuarial (gain) loss	149,088	5,492	(23,978)
Benefits paid (including expenses)	(44,083)	(42,580)
Benefit obligation at end of year	950,608	792,101	776,131
Change in Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	548,122	502,327
Net transfer in due to acquisitions of Gas Companies	0	0
Actual Return on Plan Assets	71,595	13,848
Employer contributions	49,427	74,542
Participant contributions	0	0
Benefits paid (including expenses)	(44,083)	(42,596)
Fair value of plan assets at end of year	625,061	548,122	502,327
Funded Status of Plan [Abstract]
Projected benefits (less than) greater than plan assets	325,547	243,980
Amounts Recognized in the Statement of Financial Position consist of [Abstract]
Non-current assets	0	0
Current liabilities	3,488	914
Non-current liabilities	322,059	243,065
Amounts Recognized as a Regulatory Asset [Abstract]
Transition obligation (asset)	0	0
Prior Service Cost	859	1,506
Net (gain) loss	244,032	134,838
Total Regulatory Assets	244,891	136,344
Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets [Abstract]
Projected benefit obligation	950,608	776,133
Accumulated benefit obligation	858,803	709,235
Fair value of plan assets	625,060	532,595
Components of net periodic benefit cost:
Service cost	12,032	12,574	7,675
Interest cost	41,470	40,484	22,702
Expected return on plan assets	(44,874)	(42,588)	(20,739)
Amortization of:
Prior service costs	647	643	646
Transition obligation	0	0	0
Actuarial (gain) loss	13,173	14,032	(23,978)
Net periodic benefit cost	22,448	25,145	(13,694)
Pension Benefits [Member] | Minimum [Member]
Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Average wage increase (in hundredths)	3.50%	3.50%
Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate (in hundredths)	5.05%	5.10%	5.00%
Average wage increase (in hundredths)	3.50%	3.50%	3.80%
Return on plan assets (in hundredths)	7.75%	8.25%	8.25%
Pension Benefits [Member] | Maximum [Member]
Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Average wage increase (in hundredths)	3.80%	3.80%
Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate (in hundredths)	5.30%	5.35%	5.35%
Average wage increase (in hundredths)	3.80%	3.80%	4.00%
Return on plan assets (in hundredths)	8.00%	8.50%	8.50%
Pension Benefits [Member] | Non Qualified Plans [Member]
Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate (in hundredths)	4.00%	5.05%
Pension Benefits [Member] | Qualified Plans [Member]
Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate (in hundredths)	4.25%	5.30%
Pension Benefits [Member] | Other Changes in Plans Assets and Benefit Obligations Recognized as a Regulatory Assets [Member]
Components of net periodic benefit cost:
Net (gain) loss	122,368	34,524	21,425
Amortization of:
Prior service costs	(647)	(626)	(646)
Transition obligation	0	0	0
Actuarial (gain) loss	(13,173)	(14,032)	23,978
Total Recognized as Regulatory Asset	108,548	19,866	44,757
Total recognized in net periodic benefit costs and regulatory asset	130,996	45,011	31,063
Pension Benefits [Member] | Estimated amortization from Regulatory Assets [Member]
Amortization of:
Prior service costs	605	647	643
Transition obligation	0	0	0
Amortization of net (gain) loss	20,808	13,173	14,032
Total estimated amortizations	21,413	13,820	14,675
Other Post-Retirement Benefits [Member]
Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	122,382	130,633
Net transfer in due to acquisition of the Gas Companies	0	0
Service cost	1,604	2,164	1,450
Interest cost	6,246	6,634	4,285
Participant contributions	1,305	3,020
Benefits paid (including expenses)	(7,350)	(10,997)
Benefit obligation at end of year	136,687	122,382	130,633
Change in Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	37,573	40,762
Net transfer in due to acquisitions of Gas Companies	0	0
Actual Return on Plan Assets	4,092	501
Employer contributions	0	0
Participant contributions	1,305	3,020
Benefits paid (including expenses)	(4,430)	(6,711)
Fair value of plan assets at end of year	38,540	37,573	40,762
Funded Status of Plan [Abstract]
Projected benefits (less than) greater than plan assets	98,147	84,810
Amounts Recognized in the Statement of Financial Position consist of [Abstract]
Non-current assets	0	0
Current liabilities	214	194
Non-current liabilities	97,933	84,615
Amounts Recognized as a Regulatory Asset [Abstract]
Transition obligation (asset)	0	392
Prior Service Cost	45	(23)
Net (gain) loss	19,643	9,847
Total Regulatory Assets	19,688	10,216
Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Health care trend rate (current year) (in hundredths)	7.50%	8.00%
Components of net periodic benefit cost:
Service cost	1,604	2,164	1,450
Interest cost	6,246	6,634	4,285
Expected return on plan assets	(2,503)	(2,965)	(1,910)
Amortization of:
Prior service costs	(69)	(101)	(103)
Transition obligation	392	1,020	1,058
Actuarial (gain) loss	965	2,008	690
Net periodic benefit cost	6,635	8,760	5,470
Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Composite health care trend rate (current year), minimum (in hundredths)	8.00%
Composite health care trend rate (2019 forward), minimum (in hundredths)	5.00%
Other Post-Retirement Benefits [Member] | Minimum [Member]
Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate (in hundredths)	4.00%	5.05%
Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate (in hundredths)	5.05%	5.15%	5.00%
Return on plan assets (in hundredths)	5.56%	5.86%	5.89%
Composite health care trend rate (current year), minimum (in hundredths)		7.80%	8.10%
Composite health care trend rate (2019 forward), minimum (in hundredths)		4.50%	4.50%
Other Post-Retirement Benefits [Member] | Maximum [Member]
Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate (in hundredths)	4.25%	5.30%
Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate (in hundredths)	5.30%	5.30%	5.30%
Return on plan assets (in hundredths)	8.00%	8.25%	8.25%
Composite health care trend rate (current year), minimum (in hundredths)		8.50%	8.50%
Composite health care trend rate (2019 forward), minimum (in hundredths)		5.00%	5.00%
Other Post-Retirement Benefits [Member] | Other Changes in Plans Assets and Benefit Obligations Recognized as a Regulatory Assets [Member]
Components of net periodic benefit cost:
Net (gain) loss	10,760	(6,608)	2,173
Amortization of:
Prior service costs	69	101	103
Transition obligation	(392)	(1,020)	(1,058)
Actuarial (gain) loss	(965)	(2,008)	(690)
Total Recognized as Regulatory Asset	9,472	(9,535)	528
Total recognized in net periodic benefit costs and regulatory asset	16,107	(775)	5,998
Other Post-Retirement Benefits [Member] | Estimated amortization from Regulatory Assets [Member]
Amortization of:
Prior service costs	(50)	(69)	(101)
Transition obligation	0	392	1,020
Amortization of net (gain) loss	1,939	965	2,008
Total estimated amortizations	1,889	1,288	2,927
UI [Member]
Defined Benefit Plan Disclosure [Line Items]
Account contributions to fund Serial VEBA Accounts in 2007	4,500
Percentage of employees represented by Local 470-1 Utility Workers Union of America (in hundredths)	51.00%
Regulatory assets of pension and other postretirement plans	$ 264,600	$ 146,600
Equity Securities [Member]
Target Allocations [Abstract]
Target allocation of equity securities for 2013 (in hundredths)	50.00%
Debt Securities [Member]
Target Allocations [Abstract]
Target allocation of equity securities for 2013 (in hundredths)	40.00%
Other Securities [Member]
Target Allocations [Abstract]
Target allocation of equity securities for 2013 (in hundredths)	10.00%