Name:	uil_AllowedDistributionReturnOnEquity
Namespace Prefix:	uil_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

The allowed distribution return on equity resulting from the 2013 final decision issued by the state of Connecticut's Public Utilities Regulatory Authority.

No definition available.

Name:	uil_AllowedDistributionReturnOnEquityPer2013FinalDecision
Namespace Prefix:	uil_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

The amount for which PURA will allow to be recovered as a result of the entity's semi-annual Generation Service Charge, bypassable federally mandated congestion charge and nonbypassable federally mandated congestion charge reconciliations.

No definition available.

Name:	uil_AmountApprovedForRecovery
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Represents amount of decrease in annual revenue requirements during the reporting period.

No definition available.

Name:	uil_AmountOfDecreaseInAnnualRevenueRequirements
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Refers to the amount transferred from CL&P associated with the Greater Springfield Reliability Project to UI.

No definition available.

Name:	uil_AmountOfTransmissionAssets
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The entity's approved return on equity, as per the relevant regulatory commission.

No definition available.

Name:	uil_ApprovedReturnOnEquity
Namespace Prefix:	uil_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Represents basis point added to return on equity during the period.

No definition available.

Name:	uil_BasisPointAddedToReturnOnEquity
Namespace Prefix:	uil_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

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

No definition available.

Name:	uil_CumulativeAmountOfDepositsInProjectsToImproveRegionalEnergyReliability
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Current approved base return on equity.

No definition available.

Name:	uil_CurrentApprovedBaseReturnOnEquity
Namespace Prefix:	uil_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Amount of disallowed deferred storm costs and capital costs per PURA final decision in the 2013 rate case.

No definition available.

Name:	uil_DisallowanceOfDeferredStormCostsAndCapitalCostsPerPuraFinalDecisionInRateCase
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

The write-off related to regulatory disallowances.

No definition available.

Name:	uil_DisallowanceOfDeferredStormCostsAndCapitalCostsPerUtilityRegulatoryAgency
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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

No definition available.

Name:	uil_DurationOfApprovedRatePlan
Namespace Prefix:	uil_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

The mechanism used to determine the share earnings ratio.

No definition available.

Name:	uil_EarningsShareMechanismRatio
Namespace Prefix:	uil_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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

No definition available.

Name:	uil_EarningsShareSplitOfProfit
Namespace Prefix:	uil_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Name:	uil_ElectricDistributionAndTransmissionRatesAbstract
Namespace Prefix:	uil_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Entity's share of the second set of Ct Yankee DOE litigation damage claims which will be netted against the entity's storm reserve

No definition available.

Name:	uil_EntitysShareOfSecondSetOfDamageClaims
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Name:	uil_EquityInvestmentInPeakingGenerationAbstract
Namespace Prefix:	uil_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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

No definition available.

Name:	uil_ExpectedAmountOfInvestment
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

This element represent the length of time for the life of a proposed rate amendment.

No definition available.

Name:	uil_LengthOfTimeForProposedRateAmendment
Namespace Prefix:	uil_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Maximum amount of commitment to purchase Renewable Energy Credits (REC's).

No definition available.

Name:	uil_MaximumAmountOfCommitmentToPurchaseRenewableEnergyCreditsRecsFromNewFacilitiesBehindDistributionCustomerMeters
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Maximum annual commitment level obligation that are expected to peak after year six.

No definition available.

Name:	uil_MaximumAnnualCommitmentLevelObligationAfterYearSix
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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

No definition available.

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

Name:	uil_NewRenewableSourceGenerationAbstract
Namespace Prefix:	uil_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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

No definition available.

Name:	uil_NumberOfInterRelatedTransmissionProjects
Namespace Prefix:	uil_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

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

No definition available.

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

This line item represents the number of long term contracts to purchase RECs from existing biomass facilities.

No definition available.

Name:	uil_NumberOfLongTermContractsToPurchaseRecsFromExistingBiomassFacilities
Namespace Prefix:	uil_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

This refers to number of peaking generation plants.

No definition available.

Name:	uil_NumberOfPeakingGenerationPlants
Namespace Prefix:	uil_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

The number of items used in rate making process.

No definition available.

Name:	uil_NumberOfRateMakingMechanisms
Namespace Prefix:	uil_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Represents the number of winning bidders approved by PURA.

No definition available.

Name:	uil_NumberOfWinningBiddersApprovedByPura
Namespace Prefix:	uil_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Name:	uil_OtherProceedingsAbstract
Namespace Prefix:	uil_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Percentage of UI's represented by quantity of energy and RECs to be purchased under power purchase agreements.

No definition available.

Name:	uil_PercentageOfEntitySDistributionLoadRepresentedByQuantityOfEnergyAndRECsToBePurchased
Namespace Prefix:	uil_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

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

No definition available.

Name:	uil_PercentageOfStandardServiceCustomersWithWholesalePowerSupplyAgreementsInPlaceForNextFiscalYear
Namespace Prefix:	uil_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

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.

No definition available.

Name:	uil_PercentageOfStandardServiceCustomersWithWholesalePowerSupplyAgreementsInPlaceForYearTwo
Namespace Prefix:	uil_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Period of commitment to purchase Renewable Energy Credits (REC's).

No definition available.

Name:	uil_PeriodOfCommitmentToPurchaseRenewableEnergyCredits
Namespace Prefix:	uil_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

The period of U.S. Treasury Bond rates.

No definition available.

Name:	uil_PeriodOfUSTreasuryBondRates
Namespace Prefix:	uil_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Name:	uil_PowerSupplyArrangementsAbstract
Namespace Prefix:	uil_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Pre-tax income earned on each deposit in New England East-West Solution (NEEWS) Project.

No definition available.

Name:	uil_PreTaxIncomeEarnedOnEachDeposits
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Name:	uil_ProjectsToImproveRegionalEnergyReliabilityAbstract
Namespace Prefix:	uil_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Element represents the amount of proposed budget to develop initial renewal energy by the entity which has been approved by PURA.

No definition available.

Name:	uil_ProposedBudgetAmountToDevelopInitialRenewalEnergy
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

This refers to ration in between joint venture entity for equity investment.

No definition available.

Name:	uil_RatioOfEquityInvestmentInPeakingGenerationForJointVenture
Namespace Prefix:	uil_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Reasonable base return on equity as per FERC order.

No definition available.

Name:	uil_ReasonableBaseReturnOnEquity
Namespace Prefix:	uil_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

The period during which the entity would refund customers as a result of the complaint.

No definition available.

Name:	uil_RegulatoryNoncurrentAssetAmortization
Namespace Prefix:	uil_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The amount of UI's remaining CTA regulatory liability which was approved for application against the storm reserve.

No definition available.

Name:	uil_RemainingCTARegulatoryLiability
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Number of megawatts of renewable energy to be developed by the entity in initial stage which has been approved by the PURA.

No definition available.

Name:	uil_RenewableEnergyGenerationCapacityInInitialStage
Namespace Prefix:	uil_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

Number of megawatts of grid connected, renewable energy allowed to be developed by the entity.

No definition available.

Name:	uil_RenewableGenerationCapacityForRecovery
Namespace Prefix:	uil_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Number of megawatts renewable energy to be developed from biomass plant in New England as directed by DEEP.

No definition available.

Name:	uil_RenewableGenerationCapacityFromBiomassFacility
Namespace Prefix:	uil_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

The amount for which UI requested recovery in its semi-annual Generation Service Charge, bypassable federally mandated congestion charge and nonbypassable federally mandated congestion charge reconciliations.

No definition available.

Name:	uil_RequestedAmountOfRecovery
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Reserve for refund period based upon the Company's assessment of the ultimate outcome of the proceeding.

No definition available.

Name:	uil_ReserveRecordedForRefund
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The restoration of previously disallowed capital cost.

No definition available.

Name:	uil_RestorationOfDisallowedCapitalCost
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The restoration of deferred storm cost that was previously disallowed.

No definition available.

Name:	uil_RestorationOfDisallowedDeferredStorm
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Base return on equity, per initial decision by administrative law judge, for the period after a final opinion is issued by FERC.

No definition available.

Name:	uil_ReturnOnEquityPerInitialDecisionForPeriodAfterFinalOpinionIssuedByRegulatoryAgency
Namespace Prefix:	uil_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Base return on equity per initial decision by administrative law judge for the period October 1, 2011 through December 31, 2012.

No definition available.

Name:	uil_ReturnOnEquityPerInitialDecisionForRefundPeriodOne
Namespace Prefix:	uil_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Percentage return on equity sought by complainants.

No definition available.

Name:	uil_ReturnOnEquitySoughtByComplainants
Namespace Prefix:	uil_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

This refers to revenue requirements for equity investment in peaking generation in next financial year.

No definition available.

Name:	uil_RevenueRequirementsForEquityInvestmentInPeakingGeneration
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

The reversal of write-off related to deferred storm costs and capital costs per PURA final reconsideration decision.

No definition available.

Name:	uil_ReversalOfStormDamageProvision
Namespace Prefix:	uil_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Percentage of revised return on equity sought by complainants.

No definition available.

Name:	uil_RevisedReturnOnEquitySoughtByComplainants
Namespace Prefix:	uil_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Current maximum revised return on equity sought by complainants.

No definition available.

Name:	uil_RevisedReturnOnEquitySoughtByComplainantsInThirdComplaintMaximum
Namespace Prefix:	uil_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Current minimum revised return on equity sought by complainants.

No definition available.

Name:	uil_RevisedReturnOnEquitySoughtByComplainantsInThirdComplaintMinimum
Namespace Prefix:	uil_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Solicitation period that obligation will phase-in.

No definition available.

Name:	uil_SolicitationPeriodObligationsWillPhaseIn
Namespace Prefix:	uil_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

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.

No definition available.

Name:	uil_ThresholdPercentageOfCostsForConnecticutPortionsOfProjects
Namespace Prefix:	uil_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Name:	uil_TransmissionReturnOfEquityRoeAbstract
Namespace Prefix:	uil_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Weighted average return on equity.

No definition available.

Name:	uil_WeightedAverageReturnOnEquity
Namespace Prefix:	uil_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

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.

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=35706170&loc=SL5624258-113959

Name:	us-gaap_AdditionalCollateralAggregateFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Carrying amount as of the balance sheet date of capitalized storm costs of a regulated entity that are expected to be recoverable through rate adjustments within one year or the normal operating cycle, if longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 340

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6499739&loc=d3e43603-110378

Name:	us-gaap_DeferredStormAndPropertyReserveDeficiencyCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Name:	us-gaap_EquityMethodInvestmentsAndJointVenturesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration