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Derivative Instruments - Narrative (Details)
MMcf in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2014
USD ($)
Jun. 30, 2015
USD ($)
MMcf
Jun. 30, 2014
USD ($)
Dec. 31, 2014
USD ($)
MMcf
Derivative Instruments, Gain (Loss) [Line Items]        
Deferred net gains in accumulated other comprehensive income (loss), net of tax   $ 133,800,000   $ 217,100,000
Net unrealized gains on derivative commodity instruments   $ 98,300,000   $ 153,200,000
Maximum additional collateral as percentage of derivative liability   100.00%    
Aggregate fair value of derivative instruments with credit-risk related contingencies   $ 24,700,000    
Collateral posted   $ 0    
Cash Flow Hedging | Commodity Derivatives        
Derivative Instruments, Gain (Loss) [Line Items]        
Pre-tax gain recorded within gain on sale / exchange of assets $ 28,000,000   $ 28,000,000  
Absolute quantities of derivative commodity instruments that have been designated and qualify as cash flow or fair value hedges (in Bcf) | MMcf   616   624