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Asset Retirement Obligations (Schedule Of Change In Asset Retirement Obligation) (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Dec. 31, 2013
Asset Retirement Obligations [Abstract]      
Asset retirement obligations, Beginning balance $ 155,321    
Asset retirement obligations, Assumed in acquisition 772    
Asset retirement obligations, Revisions to previous estimates 334    
Asset retirement obligations, Liabilities incurred 113    
Asset retirement obligations, Liabilities settled (404)    
Asset retirement obligations, Accretion expense 2,134 1,732  
Asset retirement obligations, Ending balance 158,270    
Less: Current portion of asset retirement obligations (4,461)   (4,310)
Asset retirement obligations - non-current $ 153,809   $ 151,011