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Asset Retirement Obligations (Schedule Of Changes In Asset Retirement Obligations) (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Asset Retirement Obligations [Abstract]    
Asset retirement obligation - beginning of period $ 7,640 $ 6,511
Liabilities incurred during period 4,100 1,747
Liabilities settled during period (369) (422)
Accretion expense during period 590 414
Revisions to estimates 1,114 (610)
Asset retirement obligation - end of period $ 13,075 $ 7,640