Quarterly report pursuant to Section 13 or 15(d)

Asset Retirement Obligations (Schedule of Change in Asset Retirement Obligation) (Details)

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Asset Retirement Obligations (Schedule of Change in Asset Retirement Obligation) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Mar. 31, 2016
Dec. 31, 2015
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]        
Asset retirement obligations at beginning of period $ 5,107      
Accretion expense 180 $ 209    
Liabilities incurred 4      
Revisions to estimate (31)      
Asset retirement obligations at end of period $ 5,107   $ 5,260 $ 5,107
Less: current asset retirement obligations     (833) (790)
Long-term asset retirement obligations at the end of the period     4,427 4,317
Restricted Investments        
Restricted investments     $ 3,315 $ 3,309