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Asset Retirement Obligations (Schedule of Reconciliation of Asset Retirement Obligations) (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2014
Jun. 30, 2013
Jun. 30, 2014
Jun. 30, 2013
Dec. 31, 2013
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]          
Asset retirement obligations at beginning of period     $ 42,954    
Liabilities incurred during the period     355    
Liabilities settled during the period     (17)    
Adjustment to liability due to divestitures     (9)    
Accretion of discount 695 556 1,376 1,246  
Asset retirement obligations at end of period 44,659   44,659    
Less current portion 191   191   191
Long-term portion $ 44,468   $ 44,468