The allowed distribution return on equity resulting from the final decision issued by the State of Connecticut's Department of Public Utility Control.

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- Definition

The amount of revenue requirements approved by the State of Connecticut's Department of Public Utility Control for the current year, for the peaking generation plant in Devon.

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- Definition

The amount of revenue requirements approved by the State of Connecticut's Department of Public Utility Control for the current year, for the peaking generation plant in Middletown.

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- Definition

The entity's approved return on equity, as per the Massachusetts Department of Public Utilities.

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Balance Type:	na
Period Type:	instant

- Definition

The entity's authorized return on equity resulted from remediated billing issues.

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Balance Type:	na
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- Definition

The ratio of debt to total capitalization (debt plus equity) resulting from the final decision issued by the State of Connecticut's Department of Public Utility Control.

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- Definition

The ratio of equity to total capitalization (debt plus equity) resulting from the final decision issued by the State of Connecticut's Department of Public Utility Control.

+ Details

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Namespace Prefix:	uil
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The cumulative amount of deposits made by the entity, through the balance sheet date, in projects to improve regional energy reliability.

+ Details

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Balance Type:	debit
Period Type:	instant

- Definition

The amount of decoupling charge approved by the State of Connecticut's Department of Public Utility Control as of the date specified, which is to be recovered from ratepayers over a twelve month period commencing in October 2010.

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- Definition

The approximate amount of draft decoupling adjustment approved by PURA to be credited to ratepayers over a twelve months period.

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- Definition

The duration (in years) of the rate plan approved by the Massachusetts Department of Public Utilities.

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Balance Type:	na
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- Definition

The duration of the pilot program (in years) provided by the State of Connecticut's Department of Public Utility Control, for full decoupling of distribution revenues from sales.

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- Details

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- Definition

The amount of the entity's requested rate increase (decrease), filed with the State of Connecticut's Department of Public Utility Control.

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The expected amount of investment for ownership of specific transmission assets as they are placed in service, based on previously projected costs.

+ Details

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Namespace Prefix:	uil
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The face amount of secured medium-term notes approved for issuance by PURA.

+ Details

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Namespace Prefix:	uil
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Balance Type:	credit
Period Type:	instant

- Details

Name:	uil_GasDistributionRatesAbstract
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Balance Type:	na
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- Definition

The amount of income from the entity's equity investment in joint venture.

+ Details

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Namespace Prefix:	uil
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of long-term debt which is in the initial process for issue for Southern Connecticut Gas

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The interim rate increase (decrease) for the entity, ordered by the State of Connecticut's Department of Public Utility Control.

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Definition

The maximum maturity (in years) of secured medium-term notes approved for issuance by PURA.

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Balance Type:	na
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- Definition

The minimum maturity (in years) of secured medium-term notes approved for issuance by PURA.

+ Details

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Namespace Prefix:	uil
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	instant

- Definition

The maximum fixed coupon rate of secured medium-term notes approved for issuance by PURA.

+ Details

Name:	uil_MaximumFixedCouponRate
Namespace Prefix:	uil
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The maximum period over which the entity expects to make remaining investments.

+ Details

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Namespace Prefix:	uil
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

- Definition

The minimum amount of investment for ownership of specific transmission assets as they are placed in service.

+ Details

Name:	uil_MinimumAmountOfEntitysInvestmentForOwnershipOfSpecificTransmissionAssets
Namespace Prefix:	uil
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The minimum period over which the entity expects to make remaining investments.

+ Details

Name:	uil_MinimumPeriodOverWhichRemainingInvestmentsAreExpectedToBeMade
Namespace Prefix:	uil
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of inter-related transmission projects to improve regional energy reliability.

+ Details

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Namespace Prefix:	uil
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of inter-related transmission projects to improve regional energy reliability, with portions sited in Connecticut.

+ Details

Name:	uil_NumberOfInterRelatedTransmissionProjectsWithPortionsSited
Namespace Prefix:	uil
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of new peaking generation plants to help address the need for power generation during the heaviest load periods.

+ Details

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Namespace Prefix:	uil
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of ownership in a joint venture project selected to build and operate new peaking generation plants.

+ Details

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Namespace Prefix:	uil
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Balance Type:	na
Period Type:	instant

- Definition

The amount of pension and earnings sharing over-recovery credit approved by the State of Connecticut's Department of Public Utility Control as of the date specified, which is to be refunded to ratepayers over a twelve month period commencing in October 2010.

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

The approximate percentage of increase (decrease) of the entity's requested amount of rate increase (decrease), filed with the State of Connecticut's Department of Public Utility Control.

+ Details

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Balance Type:	na
Period Type:	instant

- Definition

The percentage of earnings over the allowed twelve month level, that is retained by the entity, as per the earnings sharing mechanism structure.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of earnings over the allowed twelve month level, that is returned to customers, as per the earnings sharing mechanism structure.

+ Details

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Namespace Prefix:	uil
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of the interim rate increase (decrease) for the entity, ordered by the State of Connecticut's Department of Public Utility Control.

+ Details

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Namespace Prefix:	uil
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of standard service customers with wholesale power supply agreements in place, for the first twelve month period following the end of the current fiscal year.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of standard service customers with wholesale power supply agreements in place, for the second twelve month period following the end of the current fiscal year.

+ Details

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Namespace Prefix:	uil
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of transmission construction work in progress costs on which recovery of return is authorized, as per the Order issued by FERC.

+ Details

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Namespace Prefix:	uil
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

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Name:	uil_PowerSupplyArrangementsAbstract
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Balance Type:	na
Period Type:	duration

- Definition

The allowed distribution return on equity prior to the final decision issued by the State of Connecticut's Department of Public Utility Control.

+ Details

Name:	uil_PreviouslyApprovedAllowedDistributionReturnOnEquity
Namespace Prefix:	uil
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The ratio of debt to total capitalization (debt plus equity) prior to the final decision issued by the State of Connecticut's Department of Public Utility Control.

+ Details

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Data Type:	num:percentItemType
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Period Type:	instant

- Definition

The ratio of equity to total capitalization (debt plus equity) prior to the final decision issued by the State of Connecticut's Department of Public Utility Control.

+ Details

Name:	uil_PreviouslyApprovedCapitalStructureEquityRatio
Namespace Prefix:	uil
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount which may be refinanced with the proceeds of the sales of the MTN's - Southern Connecticut Gas

+ Details

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Namespace Prefix:	uil
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The projected cost of the Connecticut portion of projects.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	uil_ProjectsToImproveRegionalEnergyReliabilityAbstract
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- Definition

The amount of recoverable carrying charges to be recorded.

+ Details

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Balance Type:	credit
Period Type:	duration

- Definition

The threshold percentage of another entity's costs for the Connecticut portions of the projects, by which the entity has the right to invest up to this amount, given that it is greater than the defined minimum amount.

+ Details

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Period Type:	instant

- Definition

The aggregate fair value of additional assets that would be required to be posted as collateral for derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5624258-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 133
-Paragraph 44D
-Subparagraph c
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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REGULATORY PROCEEDINGS (Details) (USD $) In Millions, unless otherwise specified		3 Months Ended		6 Months Ended		7 Months Ended	12 Months Ended			6 Months Ended				6 Months Ended				6 Months Ended
	Jun. 30, 2011	Jun. 30, 2011 United Illuminating Company (UI) [Member]	Jun. 30, 2010 United Illuminating Company (UI) [Member]	Jun. 30, 2011 United Illuminating Company (UI) [Member]	Jun. 30, 2010 United Illuminating Company (UI) [Member]	Dec. 31, 2011 United Illuminating Company (UI) [Member]	Dec. 31, 2011 United Illuminating Company (UI) [Member]	Dec. 31, 2009 United Illuminating Company (UI) [Member]	Sep. 01, 2010 United Illuminating Company (UI) [Member]	Jun. 30, 2011 The Southern Connecticut Gas Company (SCG) [Member]	Aug. 31, 2009 The Southern Connecticut Gas Company (SCG) [Member]	Jan. 31, 2009 The Southern Connecticut Gas Company (SCG) [Member]	Oct. 24, 2008 The Southern Connecticut Gas Company (SCG) [Member]	Jun. 30, 2011 Connecticut Natural Gas Corporation (CNG) [Member]	Jul. 31, 2009 Connecticut Natural Gas Corporation (CNG) [Member]	Jan. 31, 2009 Connecticut Natural Gas Corporation (CNG) [Member]	Aug. 06, 2008 Connecticut Natural Gas Corporation (CNG) [Member]	Jun. 30, 2011 The Berkshire Gas Company [Member]
Electric Distribution and Transmission - Rates [Abstract]
Allowed distribution return on equity (in hundredths)								8.75%
Previously approved allowed distribution return on equity (in hundredths)								9.75%
Capital structure, equity ratio (in hundredths)								50.00%
Capital structure, debt ratio (in hundredths)								50.00%
Previously approved capital structure, equity ratio (in hundredths)								48.00%
Previously approved capital structure, debt ratio (in hundredths)								52.00%
Percentage of earnings returned to customers (in hundredths)								50.00%
Percentage of earnings retained by the entity (in hundredths)								50.00%
Duration of pilot program (in years)								2
Decoupling charge approved by the DPUC									$ 1.6
Pension and earnings sharing over-recovery credit approved by the DPUC									3.6
Draft decoupling adjustment approved by PURA		1.3		1.3
Power Supply Arrangements [Abstract]
Percentage of standard service customers with wholesale power supply agreements in place for 2012 (in hundredths)		80.00%		80.00%
Percentage of standard service customers with wholesale power supply agreements in place for 2013 (in hundredths)		20.00%		20.00%
Collateral required to be posted if the entity's credit rating declined two ratings to fall below investment grade		12.3		12.3
New England East-West Solution [Abstract]
Number of inter-related transmission projects		4		4
Percentage of transmission construction work in progress costs on which recovery of return is authorized (in hundredths)		100.00%		100.00%
Number of inter-related transmission projects with portions sited in Connecticut		3		3
Projected cost of the Connecticut portion of projects		779.0		779.0
Minimum amount of entity's investment for ownership of specific transmission assets		60.0		60.0
Threshold percentage of CL&P's costs for the Connecticut portions of projects (in hundredths)		8.40%		8.40%
Expected amount of investment		65.0		65.0
Cumulative amount of deposits in projects to improve regional energy reliability		8.3		8.3
Minimum period over which remaining investments are expected to be made (in years)				3
Maximum period over which remaining investments are expected to be made (in years)				5
Equity Investment in Peaking Generation [Abstract]
Ownership percentage in joint venture (in hundredths)	50.00%
Number of new peaking generation plants		2		2
Approved 2011 revenue requirements for the GenConn Devon facility							36.8
Approved 2011 revenue requirements for the GenConn Middletown facility						22.6
Income from equity investment		2.6	(0.9)	4.7	(0.9)
Gas Distribution - Rates [Abstract]
Interim rate increase (decrease) ordered by the DPUC											12.5		15.1		16.2		15.5
Percentage of interim rate increase (decrease) (in hundredths)											3.20%		3.40%		4.20%		3.28%
Entitys Requested Rate Increase Decrease												50.1				16.4
Percentage increase (decrease) in entity's requested rate increase (decrease) (in hundredths)												15.20%				4.40%
Authorized return on equity (in hundredths)											9.26%				9.31%
Authorized return on equity resulting from remediated billing issues (in hundredths)											9.26%				9.31%
Duration of approved rate plan (in years)																		10
Approved return on equity (in hundredths)										9.36%				9.41%				10.50%
Recoverable carrying charges										2.0				2.0
Approval for Issuance of Debt [Abstract]
Face amount of secured medium-term notes approved for issuance										50.0
Maximum fixed coupon rate (in hundredths)										7.00%
Principal amount of maturing existing debt - Southern Connecticut Gas	30.0
Initiated long term debt Southern Connecticut Gas	$ 50.0
Maturity of debt approved for issuance, minimum (in years)										1
Maturity of debt approved for issuance, maximum (in years)										40