Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements (Schedule of Derivative Instruments) (Details)

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Fair Value Measurements (Schedule of Derivative Instruments) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Liabilities      
Asset retirement obligation capitalized $ 329 $ 6,779  
Level 1      
Assets      
Derivative Instruments 0   $ 0
Liabilities      
Derivative Instruments 0   0
Level 2      
Assets      
Derivative Instruments 66,675   87,532
Liabilities      
Derivative Instruments 40,069   35,523
Level 3      
Assets      
Derivative Instruments 0   0
Liabilities      
Derivative Instruments 0   $ 0
Asset retirement obligation capitalized $ 400