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Asset Retirement Obligations (Activity Associated With Asset Retirement Obligations) (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Asset Retirement Obligations, Roll Forward Analysis [Roll Forward]    
Asset retirement obligations, January 1 $ 2,800 $ 3,297
Accretion expense 30 43
Liabilities settled (297) (128)
Changes in expected cash flows and timing 185 98
Asset retirement obligations, March 31 $ 2,718 $ 3,310