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Derivative Instruments - Narrative (Details)
9 Months Ended 12 Months Ended
Sep. 30, 2016
USD ($)
Bcf
MBbls
Dec. 31, 2015
USD ($)
Bcf
Derivative Instruments and Hedging Activities Disclosure [Abstract]    
Deferred net gains in accumulated other comprehensive income (loss), net of tax $ 21,700,000 $ 64,800,000
Net gains on derivative commodity instruments $ 15,600,000  
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 $ 8,000,000  
Collateral posted $ 0  
Propane swap    
Derivative Instruments, Gain (Loss) [Line Items]    
Volume of derivative instruments (in Bcf, Mbbls) | MBbls 913  
Cash flow hedging | Commodity derivatives    
Derivative Instruments, Gain (Loss) [Line Items]    
Volume of derivative instruments (in Bcf, Mbbls) | Bcf 677 676