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EMPLOYEE BENEFIT PLANS - MISCELLANEOUS (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jul. 31, 2014
Jun. 30, 2014
Employee Benefit Plans          
Rabbi trust funding related to potential change in control $ 2.4us-gaap_AssetsHeldInTrustNoncurrent     $ 64.8us-gaap_AssetsHeldInTrustNoncurrent $ 65.0us-gaap_AssetsHeldInTrustNoncurrent
Defined Contribution Benefit Plans          
Shares of the entity's common stock held under employee stock ownership plan (ESOP) 3.9us-gaap_EmployeeStockOwnershipPlanESOPSharesInESOP        
Market value of shares of the entity's common stock held under employee stock ownership plan (ESOP) 303.5us-gaap_EmployeeStockOwnershipPlanESOPDeferredSharesFairValue        
Total costs incurred for defined contribution benefit plans 35.0us-gaap_DefinedContributionPlanCostRecognized 36.4us-gaap_DefinedContributionPlanCostRecognized 19.1us-gaap_DefinedContributionPlanCostRecognized    
Deferred compensation plans          
Percentage of director fees that can be deferred by nonemployee directors 100.00%teg_DeferredCompensationArrangementWithIndividualPercentageOfCompensationOrFeesThatCanBeDeferred        
Deferred compensation obligation classified as an equity instrument 24.3us-gaap_DeferredCompensationArrangementWithIndividualRecordedLiability 24.8us-gaap_DeferredCompensationArrangementWithIndividualRecordedLiability      
Deferred compensation obligation associated with deferrals that allow for distribution in cash 64.4us-gaap_DeferredCompensationLiabilityClassifiedNoncurrent 53.4us-gaap_DeferredCompensationLiabilityClassifiedNoncurrent      
Costs incurred under deferred compensation arrangement 9.5us-gaap_DeferredCompensationArrangementWithIndividualCompensationExpense 6.5us-gaap_DeferredCompensationArrangementWithIndividualCompensationExpense 3.1us-gaap_DeferredCompensationArrangementWithIndividualCompensationExpense    
Total costs of the entity's common stock held in the Rabbi Trust 20.9teg_DeferredCompensationArrangementWithIndividualCostsOfEntitysCommonStockHeldInRabbiTrust 23.0teg_DeferredCompensationArrangementWithIndividualCostsOfEntitysCommonStockHeldInRabbiTrust      
Pension Benefits          
Employee Benefit Plans          
Expected contributions to the plans during the next fiscal year 9.1us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
       
Contributions to be funded through a transfer of assets from the rabbi trust 7.0teg_DefinedBenefitPlanEstimatedFutureEmployerContributionsfromRabbitrust
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
       
Expected payments, reflecting expected future service          
2015 124.6us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
       
2016 122.3us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
       
2017 127.6us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
       
2018 126.0us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
       
2019 136.5us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
       
2020 through 2024 644.4us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
       
Other Benefits          
Employee Benefit Plans          
Expected contributions to the plans during the next fiscal year 8.7us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
       
Expected payments, reflecting expected future service          
2015 23.7us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
       
2016 26.0us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
       
2017 28.4us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
       
2018 30.6us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
       
2019 33.3us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
       
2020 through 2024 $ 195.8us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember