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Derivative Financial Instruments - Volume of Commodity Derivative Activity (Details) - Not Designated as Hedging Instrument [Member] - Commodity contracts
Mar. 31, 2018
MMBTU
gal
Short  
Derivative [Line Items]  
Nonmonetary Notional Amount of Price Risk Derivatives of Natural Gas Liquids | gal 7,696,503
Long  
Derivative [Line Items]  
Nonmonetary Notional Amount of Price Risk Derivatives Of Natural Gas | MMBTU 745,045