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Energy Commodity Price Risk Managment (Details) - Energy commodity derivative contracts(a)
Dec. 31, 2017
MMBbls
Bcf
Designated as Hedging Instrument [Member] | Crude Oil Fixed Price [Member]  
Derivative [Line Items]  
Net Open Position Long/(Short) of Outstanding Commodity Forward Contracts to Hedge our Forecasted Energy Commodity Purchases and Sales (21.0)
Designated as Hedging Instrument [Member] | Crude Oil Basis [Member]  
Derivative [Line Items]  
Net Open Position Long/(Short) of Outstanding Commodity Forward Contracts to Hedge our Forecasted Energy Commodity Purchases and Sales (7.2)
Designated as Hedging Instrument [Member] | Natural Gas Fixed Price [Member]  
Derivative [Line Items]  
Net Open Position Long/(Short) of Outstanding Commodity Forward Contracts to Hedge our Forecasted Energy Commodity Purchases and Sales | Bcf (46.4)
Designated as Hedging Instrument [Member] | Natural Gas Basis [Member]  
Derivative [Line Items]  
Net Open Position Long/(Short) of Outstanding Commodity Forward Contracts to Hedge our Forecasted Energy Commodity Purchases and Sales | Bcf (21.7)
Not Designated as Hedging Instrument [Member] | Crude Oil Fixed Price [Member]  
Derivative [Line Items]  
Net Open Position Long/(Short) of Outstanding Commodity Forward Contracts to Hedge our Forecasted Energy Commodity Purchases and Sales (1.9)
Not Designated as Hedging Instrument [Member] | Crude Oil Basis [Member]  
Derivative [Line Items]  
Net Open Position Long/(Short) of Outstanding Commodity Forward Contracts to Hedge our Forecasted Energy Commodity Purchases and Sales (1.2)
Not Designated as Hedging Instrument [Member] | Natural Gas Fixed Price [Member]  
Derivative [Line Items]  
Net Open Position Long/(Short) of Outstanding Commodity Forward Contracts to Hedge our Forecasted Energy Commodity Purchases and Sales | Bcf (9.0)
Not Designated as Hedging Instrument [Member] | Natural Gas Basis [Member]  
Derivative [Line Items]  
Net Open Position Long/(Short) of Outstanding Commodity Forward Contracts to Hedge our Forecasted Energy Commodity Purchases and Sales | Bcf (23.1)
Not Designated as Hedging Instrument [Member] | NGL and other fixed price  
Derivative [Line Items]  
Net Open Position Long/(Short) of Outstanding Commodity Forward Contracts to Hedge our Forecasted Energy Commodity Purchases and Sales (4.1)