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Derivative Instruments - Narrative (Details)
9 Months Ended 12 Months Ended
Sep. 30, 2017
USD ($)
Bcf
MBbls
Dec. 31, 2016
USD ($)
Bcf
MBbls
Derivative Instruments and Hedging Activities Disclosure [Abstract]    
Deferred net gains in accumulated other comprehensive income (loss), net of tax $ 5,600,000 $ 9,600,000
Net gains on derivative commodity instruments $ 1,900,000  
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 $ 12,800,000  
Collateral posted $ 0  
Natural Gas Liquid Instrument    
Derivative Instruments, Gain (Loss) [Line Items]    
Volume of derivative instruments (in Bcf, Mbbls) | MBbls 1,189 1,095
Cash flow hedging | Commodity derivatives    
Derivative Instruments, Gain (Loss) [Line Items]    
Volume of derivative instruments (in Bcf, Mbbls) | Bcf 470 646