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INCOME TAXES (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Deferred income tax assets    
Tax credit carryforwards 116.7us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsOther $ 113.5us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsOther
Price risk management 0teg_DeferredTaxAssetsHedgingAndDerivativeTransactions 13.0teg_DeferredTaxAssetsHedgingAndDerivativeTransactions
Other 77.4us-gaap_DeferredTaxAssetsOther 98.5us-gaap_DeferredTaxAssetsOther
Total deferred income tax assets 194.1us-gaap_DeferredTaxAssetsGross 225.0us-gaap_DeferredTaxAssetsGross
Valuation Allowance 3.6us-gaap_DeferredTaxAssetsValuationAllowance 8.2us-gaap_DeferredTaxAssetsValuationAllowance
Net deferred income tax assets 190.5us-gaap_DeferredTaxAssetsNet 216.8us-gaap_DeferredTaxAssetsNet
Deferred income tax liabilities    
Plant-related 1,584.1us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment 1,373.8us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment
Regulatory deferrals 55.6us-gaap_DeferredTaxLiabilitiesRegulatoryAssetsAndLiabilities 78.8us-gaap_DeferredTaxLiabilitiesRegulatoryAssetsAndLiabilities
Employee benefits 45.4teg_DeferredTaxLiabilitiesEmployeeBenefits 79.6teg_DeferredTaxLiabilitiesEmployeeBenefits
Other 23.0us-gaap_DeferredTaxLiabilitiesOther 43.5us-gaap_DeferredTaxLiabilitiesOther
Total deferred income tax liabilities 1,708.1us-gaap_DeferredIncomeTaxLiabilities 1,575.7us-gaap_DeferredIncomeTaxLiabilities
Balance sheet presentation    
Current deferred income tax assets 52.4us-gaap_DeferredTaxAssetsLiabilitiesNetCurrent 31.4us-gaap_DeferredTaxAssetsLiabilitiesNetCurrent
Long-term deferred income tax liabilities 1,570.0us-gaap_DeferredTaxLiabilitiesNoncurrent 1,390.3us-gaap_DeferredTaxLiabilitiesNoncurrent
Net deferred income tax liabilities 1,517.6us-gaap_DeferredTaxLiabilities 1,358.9us-gaap_DeferredTaxLiabilities
Expiration of deferred tax assets [Abstract]    
Deferred Tax Assets Expiring Between 2020 and 2025 7.6teg_DeferredTaxAssetsExpiringBetween2020and2025  
Deferred Tax Assets Expiring Between 2026 and 2031 2.9teg_DeferredTaxAssetsExpiringBetween2026and2031  
Deferred Tax Assets Expiring 2032 through 2033 35.7teg_DeferredTaxAssetsExpiring2032through2033  
Income taxes, additional disclosures    
Deferred tax credit carryforwards of alternative minimum tax credits 73.9us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax  
Deferred tax credit carryforwards of general business credits 32.5us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsGeneralBusiness  
Deferred tax credit carryforwards of foreign tax credits 6.2us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign  
Deferred tax credit carryforwards of state tax credits 4.2us-gaap_TaxCreditCarryforwardAmount  
Deferred income tax assets reflecting federal operating loss carryforwards 27.0us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsDomestic  
Deferred income tax assets reflecting state operating loss carryforwards 19.2us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsStateAndLocal  
Deferred tax assets expiring between 2020 and 2033    
Deferred Tax Assets Expiring Between 2020 and 2025 7.6teg_DeferredTaxAssetsExpiringBetween2020and2025  
Deferred Tax Assets Expiring Between 2026 and 2031 2.9teg_DeferredTaxAssetsExpiringBetween2026and2031  
Deferred Tax Assets Expiring 2032 through 2033 35.7teg_DeferredTaxAssetsExpiring2032through2033  
Net regulatory asset for regulatory tax effects 58.8teg_NetRegulatoryAssetForRegulatoryTaxEffects $ 51.6teg_NetRegulatoryAssetForRegulatoryTaxEffects
Foreign carryforwards    
Income taxes, additional disclosures    
Carryback period of tax credits 1 year  
Carryforward period of tax credits 10 years  
State carryforwards    
Income taxes, additional disclosures    
Carryforward period of tax credits 5 years  
Carryforward period of operating loss carryforwards 20 years  
Federal carryforwards    
Income taxes, additional disclosures    
Carryback period of operating loss carryforwards 2 years  
Carryforward period of operating loss carryforwards 20 years  
General business credits    
Income taxes, additional disclosures    
Carryback period of tax credits 1 year  
Carryforward period of tax credits 20 years