Annual report pursuant to Section 13 and 15(d)

Asset Retirement Obligations (Details)

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Asset Retirement Obligations (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Estimated fair value of the Company's asset retirement obligations      
Balances at January1, $ 14,528 $ 13,425 $ 10,666
Accretion Expense 814 1,014 825
Additions 770 96 2,016
Revisions (5,744) (7) (82)
Balance December 31, $ 10,368 $ 14,528 $ 13,425