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RISK MANAGEMENT ACTIVITIES - RISK MANAGEMENT ASSETS AND LIABILITIES (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Derivative Asset [Abstract]    
Risk Management Assets $ 1.8us-gaap_DerivativeAssetFairValueGrossAssetIncludingNotSubjectToMasterNettingArrangement $ 4.5us-gaap_DerivativeAssetFairValueGrossAssetIncludingNotSubjectToMasterNettingArrangement
Derivative Liability [Abstract]    
Risk Management Liabilities 43.3us-gaap_DerivativeLiabilityFairValueGrossLiabilityIncludingNotSubjectToMasterNettingArrangement 49.1us-gaap_DerivativeLiabilityFairValueGrossLiabilityIncludingNotSubjectToMasterNettingArrangement
Designated as hedging instrument    
Derivative, Number of Instruments Held [Abstract]    
Number of derivative instruments held 0us-gaap_DerivativeNumberOfInstrumentsHeld
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember		  
Non-hedge derivatives    
Derivative Asset [Abstract]    
Other current assets from risk management activities 1.2us-gaap_DerivativeInstrumentsAndHedges
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 4.0us-gaap_DerivativeInstrumentsAndHedges
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Other long-term assets from risk management activities 0.6us-gaap_DerivativeInstrumentsAndHedgesNoncurrent
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 0.5us-gaap_DerivativeInstrumentsAndHedgesNoncurrent
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Risk Management Assets 1.8us-gaap_DerivativeAssetFairValueGrossAssetIncludingNotSubjectToMasterNettingArrangement
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 4.5us-gaap_DerivativeAssetFairValueGrossAssetIncludingNotSubjectToMasterNettingArrangement
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Derivative Liability [Abstract]    
Other current liabilities from risk management activities 34.5teg_DerivativeInstrumentsAndHedgesLiabilitiesGrossCurrent
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 42.7teg_DerivativeInstrumentsAndHedgesLiabilitiesGrossCurrent
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Other long-term liabilities from risk management activities 8.8us-gaap_DerivativeInstrumentsAndHedgesLiabilitiesNoncurrent
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 6.4us-gaap_DerivativeInstrumentsAndHedgesLiabilitiesNoncurrent
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Risk Management Liabilities 43.3us-gaap_DerivativeLiabilityFairValueGrossLiabilityIncludingNotSubjectToMasterNettingArrangement
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 49.1us-gaap_DerivativeLiabilityFairValueGrossLiabilityIncludingNotSubjectToMasterNettingArrangement
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Non-hedge derivatives | Natural gas contracts    
Derivative Asset [Abstract]    
Other current assets from risk management activities 0.5us-gaap_DerivativeInstrumentsAndHedges
/ us-gaap_DerivativeInstrumentRiskAxis
= teg_CommodityContractNaturalGasMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 1.8us-gaap_DerivativeInstrumentsAndHedges
/ us-gaap_DerivativeInstrumentRiskAxis
= teg_CommodityContractNaturalGasMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Other long-term assets from risk management activities 0.6us-gaap_DerivativeInstrumentsAndHedgesNoncurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= teg_CommodityContractNaturalGasMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 0.5us-gaap_DerivativeInstrumentsAndHedgesNoncurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= teg_CommodityContractNaturalGasMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Derivative Liability [Abstract]    
Other current liabilities from risk management activities 28.0teg_DerivativeInstrumentsAndHedgesLiabilitiesGrossCurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= teg_CommodityContractNaturalGasMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 37.3teg_DerivativeInstrumentsAndHedgesLiabilitiesGrossCurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= teg_CommodityContractNaturalGasMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Other long-term liabilities from risk management activities 5.7us-gaap_DerivativeInstrumentsAndHedgesLiabilitiesNoncurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= teg_CommodityContractNaturalGasMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 5.3us-gaap_DerivativeInstrumentsAndHedgesLiabilitiesNoncurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= teg_CommodityContractNaturalGasMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Non-hedge derivatives | FTRs    
Derivative Asset [Abstract]    
Other current assets from risk management activities 0.7us-gaap_DerivativeInstrumentsAndHedges
/ us-gaap_DerivativeInstrumentRiskAxis
= teg_CommodityContractFinancialTransmissionRightsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 2.2us-gaap_DerivativeInstrumentsAndHedges
/ us-gaap_DerivativeInstrumentRiskAxis
= teg_CommodityContractFinancialTransmissionRightsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Derivative Liability [Abstract]    
Other current liabilities from risk management activities 0.1teg_DerivativeInstrumentsAndHedgesLiabilitiesGrossCurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= teg_CommodityContractFinancialTransmissionRightsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 0.3teg_DerivativeInstrumentsAndHedgesLiabilitiesGrossCurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= teg_CommodityContractFinancialTransmissionRightsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Non-hedge derivatives | Petroleum product contracts    
Derivative Asset [Abstract]    
Other current assets from risk management activities 0us-gaap_DerivativeInstrumentsAndHedges
/ us-gaap_DerivativeInstrumentRiskAxis
= teg_CommodityContractPetroleumProductsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 0us-gaap_DerivativeInstrumentsAndHedges
/ us-gaap_DerivativeInstrumentRiskAxis
= teg_CommodityContractPetroleumProductsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Other long-term assets from risk management activities   0us-gaap_DerivativeInstrumentsAndHedgesNoncurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= teg_CommodityContractPetroleumProductsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Derivative Liability [Abstract]    
Other current liabilities from risk management activities 2.1teg_DerivativeInstrumentsAndHedgesLiabilitiesGrossCurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= teg_CommodityContractPetroleumProductsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 2.7teg_DerivativeInstrumentsAndHedgesLiabilitiesGrossCurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= teg_CommodityContractPetroleumProductsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Other long-term liabilities from risk management activities   0.1us-gaap_DerivativeInstrumentsAndHedgesLiabilitiesNoncurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= teg_CommodityContractPetroleumProductsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Non-hedge derivatives | Coal contracts    
Derivative Asset [Abstract]    
Other current assets from risk management activities 0us-gaap_DerivativeInstrumentsAndHedges
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CoalContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 0us-gaap_DerivativeInstrumentsAndHedges
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CoalContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Other long-term assets from risk management activities 0us-gaap_DerivativeInstrumentsAndHedgesNoncurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CoalContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 0us-gaap_DerivativeInstrumentsAndHedgesNoncurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CoalContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Derivative Liability [Abstract]    
Other current liabilities from risk management activities 4.3teg_DerivativeInstrumentsAndHedgesLiabilitiesGrossCurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CoalContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 2.4teg_DerivativeInstrumentsAndHedgesLiabilitiesGrossCurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CoalContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Other long-term liabilities from risk management activities $ 3.1us-gaap_DerivativeInstrumentsAndHedgesLiabilitiesNoncurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CoalContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 $ 1.0us-gaap_DerivativeInstrumentsAndHedgesLiabilitiesNoncurrent
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CoalContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember