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Derivative Instruments - Narrative (Details)
MMcf in Thousands
12 Months Ended
Dec. 31, 2016
USD ($)
Dec. 31, 2016
USD ($)
MBbls
Dec. 31, 2016
USD ($)
MMcf
Dec. 31, 2015
USD ($)
MMcf
Dec. 31, 2014
USD ($)
Derivative Instruments, Gain (Loss) [Line Items]          
Deferred net gains $ 9,600,000 $ 9,600,000 $ 9,600,000 $ 64,800,000  
Net gains on derivative commodity instruments $ 4,900,000 4,900,000 4,900,000    
Percentage of derivative liability (up to) 100.00%        
Aggregate fair value of derivative instruments with credit-risk related contingencies $ 236,500,000 236,500,000 236,500,000    
Collateral posted $ 0 $ 0 $ 0    
Commodity Derivatives | Cash flow Hedges          
Derivative Instruments, Gain (Loss) [Line Items]          
Pre-tax gain         $ 28,000,000
Absolute quantities of derivative commodity instruments (in MMcf)   1,095 703 676