PENSION AND OTHER BENEFITS (Details) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Estimated Amortizations from Regulatory Assets into Net Periodic Benefit Cost for the period January 1, 2011 - December 31, 2011 [Abstract]
Net pension expense	$ 39,028,000		$ 28,811,000		$ 22,313,000
Assets, Target Allocation [Abstract]
Assumed potential change in discount rate used in pension expense calculation (in hundredths)	0.25%
Financial impact of assumed potential change in discount rate used to calculate pension expense	2,400,000
Assumed potential change in expected return on pension plan assets (in hundredths)	1.00%
Financial impact of assumed potential change in return on plan assets on pension expense	6,000,000
Assumed potential change in discount rate used in OPEB expense calculation (in hundredths)	0.25%
Financial impact of assumed potential change in discount rate used to calculate OPEB expense	300,000
Assumed potential change in expected return on OPEB plan assets (in hundredths)	1.00%
Financial impact of assumed potential change in return on plan assets on OPEB expense	400,000
Percentage of projected benefit obligation or market value of assets for recognition of gains and losses (in hundredths)	5.00%
Purchase Accounting and Other Accounting Matters [Abstract]
Regulatory asset	1,086,122,000		1,041,737,000
Change in Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	543,089,000
Fair value of plan assets at end of year	585,693,000		543,089,000
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates [Abstract]
Effect of one percent increase on aggregate service and interest cost components	936,000
Effect of one percent decrease on aggregate service and interest cost components	(763,000)
Effect of one percent increase on accumulated post-retirement benefit obligation	10,777,000
Effect of one percent decrease on accumulated post-retirement benefit obligation	(8,983,000)
Unrecognized Prior Service Cost and Actuarial Gains (Losses) [Member]
Purchase Accounting and Other Accounting Matters [Abstract]
Amount recognized form previously unrecognized losses and prior service costs			213,000,000
Regulatory asset	146,600,000		136,200,000
Pension Expense [Member]
Purchase Accounting and Other Accounting Matters [Abstract]
Regulatory asset					8,300,000
OPEB Expense [Member]
Purchase Accounting and Other Accounting Matters [Abstract]
Regulatory asset					1,900,000
United IIuminating Company (UI) [Member]
Assets, Target Allocation [Abstract]
Age of non-union employees eligible for partially subsidized benefits	55
Sum of age and years of service of non-union employees at retirement are eligible for benefits partially subsidized by Company.	equal to or greater than 65
Contributions to fund Serial VEBA accounts	4,500,000
Sum of age and years of service at the time of retirement of union employee are eligible for benefits partially subsidized by Company	equal to or greater than 85
Percentage of UI employees represented for collective bargaining purposes (in hundredths)	38.00%
UI Defined Contribution Retirement Plan [Abstract]
Amount of additional cash contribution made to KSOP (in hundredths)	4.00%
Additional cash contribution per year into separate Retiree Medical fund within KSOP	1,000
Maximum vesting percentage for additional cash contribution to KSOP (in hundredths)	100.00%
Period of vesting for additional cash contribution to KSOP (in years)	5Y
Estimated future benefit payments [Abstract]
Matching contribution to the KSOP for union members (in hundredths)	100.00%
Percentage of deferred employee compensation for union members matched at 100% (in hundredths)	3.00%
Percentage of the next deferred employee's compensation for union members matched at 50% (in hundredths)	2.00%
Matching contribution to KSOP based on the next deferred compensation for union members (in hundredths)	50.00%
Maximum matching contribution of annual salary deferred for union members, maximum (in hundredths)	4.00%
Matching contribution to the KSOP for non-union members (in hundredths)	100.00%
Percentage of deferred employee compensation for non-union members matched at 100% (in hundredths)	2.00%
Matching contributions	2,600,000		2,400,000		2,500,000
Pension Plans, Defined Benefit [Member]
Components of net periodic benefit cost:
Service cost	12,574,000		7,675,000		6,133,000
Interest cost	40,484,000		22,702,000		20,928,000
Expected return on plan assets	(42,588,000)		(20,739,000)		(17,113,000)
Amortization of [Abstract]
Prior service costs	643,000		646,000		697,000
Transition obligation (asset)	0		0		0
Actuarial (gain) loss	14,032,000		(23,978,000)		14,425,000
Prior service costs	0		0		(697,000)
Transition obligation (asset)	(626,000)		(646,000)
Actuarial (gain) loss	(14,032,000)		23,978,000		(14,425,000)
Total recognized as regulatory asset	19,866,000		44,757,000		(20,712,000)
Total recognized in net periodic benefit costs and regulatory assets	45,011,000		31,063,000		4,358,000
Net periodic benefit cost	25,145,000	[1]	(13,694,000)	[1]	25,070,000	[1]
Other Changes in Plan Assets and Benefit Obligations Recognized as a Regulatory Asset [Abstract]
Net (gain) loss	34,524,000		21,425,000		(5,590,000)
Estimated Amortizations from Regulatory Assets into Net Periodic Benefit Cost for the period January 1, 2011 - December 31, 2011 [Abstract]
Amortization of transition obligation	0		0		0
Amortization of prior service cost	647,000		643,000		645,000
Amortization of net (gain) loss	13,173,000		14,032,000		12,309,000
Total estimated amortizations	13,820,000		14,675,000		12,954,000
Assets, Target Allocation [Abstract]
Equity securities (in hundredths)	50.00%
Debt securities (in hundredths)	40.00%
Other (in hundredths)	10.00%
Estimated future employer contributions in next fiscal year	40,000,000
Estimated future employer contributions in next fiscal year, minimum	50,000,000
Estimated future employer contributions in next fiscal year, maximum	60,000,000
Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	776,131,000		371,802,000
Net transfer in due to acquistion of the Gas Companies	0		383,233,000
Service cost	12,574,000		7,675,000		6,133,000
Interest cost	40,484,000		22,702,000		20,928,000
Participant contributions	0		0
Actuarial (gain) loss	5,492,000		14,336,000
Benefits paid (including expenses) - benefit obligation	(42,580,000)		(23,617,000)
Benefit obligation at end of year	792,101,000		776,131,000		371,802,000
Change in Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	502,327,000		231,308,000
Net transfer in due to acquistion of the Gas Companies	0		236,682,000
Actual return on plan assets	13,848,000		49,937,000
Employer contributions	74,542,000		8,017,000
Participant contributions	0		0
Benefits paid (including expenses) - plan assets	(42,596,000)		(23,617,000)
Fair value of plan assets at end of year	548,121,000		502,327,000		231,308,000
Funded Status [Abstract]
Projected benefits (less than) greater than plan assets	243,980,000		273,804,000
Amounts Recognized in Statement of Financial Position consist of [Abstract]
Non-current assets	0		456,000
Current liabilities	914,000		917,000
Non-current liabilities	243,065,000		273,343,000
Amounts Recognized as a Regulatory Asset consist of [Abstract]
Transition obligation (asset)	0		0
Prior service cost	1,506,000		2,132,000
Net (gain) loss	134,838,000		114,346,000
Total recognized as a regulatory asset	136,344,000		116,478,000
Information on Pension Plans with an Accumulated Benefit Obligation in excess of Plan Assets [Abstract]
Projected benefit obligation	776,133,000		760,658,000
Accumulated benefit obligation	709,235,000		697,081,000
Fair value of plan assets	532,595,000		486,398,000
The following actuarial weighted average assumptions were used in calculating net periodic benefit cost:
Discount rate (in hundredths)					6.20%
Discount rate, minimum (in hundredths)	5.10%		5.00%
Discount rate, maximum (in hundredths)	5.35%		5.35%
Average wage increase (in hundredths)					3.80%
Average wage increase, minimum (in hundredths)	3.50%		3.80%
Average wage increase, maximum (in hundredths)	3.80%		4.00%
Return on plan assets (in hundredths)					8.50%
Return on plan assets, minimum (in hundredths)	8.25%		8.25%
Return on plan assets, maximum (in hundredths)	8.50%		8.50%
Estimated future benefit payments [Abstract]
2012	42,406,000
2013	47,270,000
2014	44,744,000
2015	47,002,000
2016	48,409,000
2017-2021	262,764,000
Pension Plans, Defined Benefit [Member] | Qualified Plans [Member]
Weighted-average assumptions used in calculating benefit obligation Abstract]
Discount rate, minimum (in hundredths)	5.30%		5.00%
Discount rate, maximum (in hundredths)			5.35%
Average wage increase, minimum (in hundredths)	3.50%		3.80%
Average wage increase, maximum (in hundredths)	3.80%		4.00%
Pension Plans, Defined Benefit [Member] | Non-Qualified Plan [Member]
Weighted-average assumptions used in calculating benefit obligation Abstract]
Discount rate, minimum (in hundredths)	5.05%		5.10%
Discount rate, maximum (in hundredths)			5.15%
Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of net periodic benefit cost:
Service cost	2,164,000		1,450,000		1,334,000
Interest cost	6,634,000		4,285,000		4,138,000
Expected return on plan assets	(2,965,000)		(1,910,000)		(1,640,000)
Amortization of [Abstract]
Prior service costs	(101,000)		(103,000)		(101,000)
Transition obligation (asset)	1,020,000		1,058,000		1,058,000
Actuarial (gain) loss	2,008,000		690,000		2,686,000
Prior service costs	(1,020,000)		(1,058,000)		101,000
Transition obligation (asset)	101,000		103,000		(1,058,000)
Actuarial (gain) loss	(2,008,000)		(690,000)		(2,686,000)
Total recognized as regulatory asset	(9,535,000)		528,000		(8,313,000)
Total recognized in net periodic benefit costs and regulatory assets	(775,000)		5,998,000		(838,000)
Net periodic benefit cost	8,760,000	[1]	5,470,000	[1]	7,475,000	[1]
Other Changes in Plan Assets and Benefit Obligations Recognized as a Regulatory Asset [Abstract]
Net (gain) loss	(6,608,000)		2,173,000		(4,670,000)
Estimated Amortizations from Regulatory Assets into Net Periodic Benefit Cost for the period January 1, 2011 - December 31, 2011 [Abstract]
Amortization of transition obligation	392,000		1,020,000		1,059,000
Amortization of prior service cost	(69,000)		(101,000)		(103,000)
Amortization of net (gain) loss	965,000		2,008,000		1,950,000
Total estimated amortizations	1,288,000		2,927,000		2,906,000
Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	130,633,000		69,415,000
Net transfer in due to acquistion of the Gas Companies	0		57,180,000
Service cost	2,164,000		1,450,000		1,334,000
Interest cost	6,634,000		4,285,000		4,138,000
Participant contributions	3,020,000		1,520,000
Actuarial (gain) loss	(9,072,000)		1,665,000
Benefits paid (including expenses) - benefit obligation	(10,997,000)		(4,882,000)
Benefit obligation at end of year	122,382,000		130,633,000		69,415,000
Change in Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	40,762,000		22,194,000
Net transfer in due to acquistion of the Gas Companies	0		18,422,000
Actual return on plan assets	501,000		2,662,000
Employer contributions	0		846,000
Participant contributions	3,020,000		1,520,000
Benefits paid (including expenses) - plan assets	(6,711,000)		(4,882,000)
Fair value of plan assets at end of year	37,572,000		40,762,000		22,194,000
Funded Status [Abstract]
Projected benefits (less than) greater than plan assets	84,810,000		89,871,000
Amounts Recognized in Statement of Financial Position consist of [Abstract]
Non-current assets	0		0
Current liabilities	194,000		216,000
Non-current liabilities	84,615,000		89,654,000
Amounts Recognized as a Regulatory Asset consist of [Abstract]
Transition obligation (asset)	392,000		1,411,000
Prior service cost	(23,000)		(125,000)
Net (gain) loss	9,847,000		18,464,000
Total recognized as a regulatory asset	10,216,000		19,750,000
Information on Pension Plans with an Accumulated Benefit Obligation in excess of Plan Assets [Abstract]
Projected benefit obligation	0		0
Accumulated benefit obligation	0		0
Fair value of plan assets	0		0
Weighted-average assumptions used in calculating benefit obligation Abstract]
Discount rate (Other Post-Retirement Benefits), minimum	5.05%
Discount rate (Other Post-Retirement Benefits), maximum (in hundredths)	5.30%
Assumed Health Care Cost Trend Rates [Abstract]
Health care trend rate (current year) (in hundredths)	8.00%
Health care trend rate (current year), minimum	8.00%
Health care trend rate (2019-2028 forward), minimum	5.00%
Estimated future benefit payments [Abstract]
2012	7,616,000
2013	7,816,000
2014	7,884,000
2015	7,984,000
2016	8,029,000
2017-2021	$ 40,860,000
Other Postretirement Benefit Plans, Defined Benefit [Member] | United IIuminating Company (UI) [Member]
Assets, Target Allocation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	5.30%		5.30%
Weighted-average assumptions used in calculating benefit obligation Abstract]
Discount rate (Other Post-Retirement Benefits), minimum			5.15%
Discount rate (Other Post-Retirement Benefits), maximum (in hundredths)			5.30%
Assumed Health Care Cost Trend Rates [Abstract]
Health care trend rate (current year) (in hundredths)			7.80%
Health care trend rate (current year), minimum			7.80%
Health care trend rate (current year), maximum			9.00%
Health care trend rate (2019-2028 forward), minimum			4.50%
Health care trend rate (2019-2028 forward), maximum			5.00%
The following actuarial weighted average assumptions were used in calculating net periodic benefit cost:
Discount rate (in hundredths)					6.10%
Discount rate, minimum (in hundredths)	5.15%		5.00%
Discount rate, maximum (in hundredths)	5.30%		5.30%
Return on plan assets (in hundredths)					8.50%
Return on plan assets, minimum (in hundredths)	5.86%		5.89%
Return on plan assets, maximum (in hundredths)	8.25%		8.25%
Health care trend rate (current year) (in hundredths)					10.00%
Health care trend rate (current year), minimum (in hundredths)	7.80%		8.10%
Health care trend rate (current year), maximum (in hundredths)	8.50%		8.50%
Health care trend rate (2019 forward) (in hundredths)					5.00%
Health care trend rate (2019 forward), maximum (in hundredths)	4.50%		4.50%
Health care trend rate (2019 forward), maximum (in hundredths)	5.00%		5.00%
Other Postretirement Benefit Plans, Defined Benefit [Member] | Gas Companies [Member]
Assets, Target Allocation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	5.05%		5.15%

[1] (1) For the year ended December 31, 2009, UI recorded $8.3 million of pension expense and $1.9 million of OPEB expense as a regulatory asset. These amounts were approved by the DPUC to address the actual increase in pension and postretirement expense for 2009 (see Note (C), Regulatory Proceedings).