Quarterly report pursuant to Section 13 or 15(d)

Derivative Financial Instruments - Estimated Fair Value of Derivative Contracts (Details)

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Derivative Financial Instruments - Estimated Fair Value of Derivative Contracts (Details) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2014
Dec. 31, 2013
Derivatives Fair Value [Line Items]    
Fair value of commodity derivative contracts   $ 141
Fair value of commodity derivative contracts 15,543 9,423
Prepaid And Other Assets
   
Derivatives Fair Value [Line Items]    
Fair value of commodity derivative contracts   141
Accrued Liabilities
   
Derivatives Fair Value [Line Items]    
Fair value of commodity derivative contracts $ 15,543 $ 9,423