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Asset Retirement Obligations (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]      
Asset retirement obligations at beginning of period $ 6,159 $ 8,324  
Liabilities incurred 3,348 1,573  
Liabilities settled (498) (1,666)  
Reduction due to sales of oil and gas properties (2,473) (3,272)  
Accretion expense 471 372 235
Revisions of previous estimates 349 828  
Asset retirement obligations at end of period 7,356 6,159 8,324
Asset retirement obligations due within one year included in “Other accrued liabilities” (780) (1,670)  
Long-term asset retirement obligations $ 6,576 $ 4,489