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Derivative Instruments - Narrative (Details)
9 Months Ended 12 Months Ended
Sep. 30, 2018
USD ($)
Bcf
MBbls
Dec. 31, 2017
Bcf
MBbls
Derivative Instruments, Gain (Loss) [Line Items]    
Maximum additional collateral as percentage of derivative liability 100.00%  
Aggregate fair value of derivative instruments with credit-risk related contingencies $ 109,200,000  
Collateral posted $ 0  
Natural Gas Liquid Instrument    
Derivative Instruments, Gain (Loss) [Line Items]    
Volume of derivative instruments (in Bcf, Mbbls) | MBbls 2,058 1,460
Cash flow hedging | Commodity derivatives    
Derivative Instruments, Gain (Loss) [Line Items]    
Volume of derivative instruments (in Bcf, Mbbls) | Bcf 3,005 2,148