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Derivative Instruments and Hedging Activities, Amounts Included in OCI, Income and AOCI (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Cash Flow Hedging [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (loss) reclassified from OCI into income (effective portion) $ 8.1us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		 $ (7.6)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		  
Net losses on commodity hedges recorded in OCI that are expected to be reclassified to revenue within twelve months 53.3us-gaap_PriceRiskCashFlowHedgeUnrealizedGainLossToBeReclassifiedDuringNext12Months
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		    
Cash Flow Hedging [Member] | Interest Expense, Net [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (loss) reclassified from OCI into income (effective portion) 0us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_InterestExpenseMember		 (1.3)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_InterestExpenseMember		  
Cash Flow Hedging [Member] | Revenues [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (loss) reclassified from OCI into income (effective portion) 8.1us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesMember		 (6.3)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesMember		  
Cash Flow Hedging [Member] | Commodity Contracts [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (loss) recognized in OCI on derivatives (effective portion) 25.2us-gaap_DerivativeInstrumentsGainRecognizedInOtherComprehensiveIncomeEffectivePortion
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		 (11.9)us-gaap_DerivativeInstrumentsGainRecognizedInOtherComprehensiveIncomeEffectivePortion
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		  
Deferred gains (losses) included in accumulated OCI, before tax 77.4trgp_DeferredGainsLossesInAccumulatedOCIBeforeTax
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		 [1]   60.3trgp_DeferredGainsLossesInAccumulatedOCIBeforeTax
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		 [1]
Not Designated as Hedging Instrument [Member] | Commodity Contracts [Member] | Revenues [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (loss) recognized in income on derivatives $ 7.2us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesMember		 $ (0.2)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesMember		  
[1] Includes deferred net gains of $53.3 million as of March 31, 2015 related to contracts that will be settled and reclassified to revenue over the next 12 months.