Annual report pursuant to Section 13 and 15(d)

Oil and Natural Gas Properties (Oil and Gas Properties Activities) (Details)

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Oil and Natural Gas Properties (Oil and Gas Properties Activities) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Evaluated Properties-      
Beginning of period balance $ 1,421,640 $ 1,316,677 $ 1,593,884
Deconsolidation of Subsidiary January 1, 2010 0 0 (364,589)
Capitalized G&A 12,148 11,205 10,676
Property acquisition costs 2,075 0 0
Exploration costs 22,703 5,473 14,739
Development costs 38,444 88,285 61,967
End of period balance 1,497,010 1,421,640 1,316,677
Unevaluated Properties (excluded from amortization):      
Beginning of period balance 2,603 8,106 25,442
Acquisitions 29,590 2,422 1,374
Exploration 34,674 1,372 2,187
Capitalized interest 2,109 573 2,000
Transfers to evaluated (200) (9,870) (22,897)
End of period balance 68,776 2,603 8,106
Accumulated depreciation, depletion and amortization:      
Beginning of period Balance 1,208,331 1,155,915 1,488,718
Capitalized Costs, Provision Charged to Expense Relating to Oil and Gas Producing Activities 48,524 52,416 31,786
Deconsolidation of Subsidiary January 1, 2010 0 0 (364,589)
Sale of mineral interests 39,410 0 0
End of period balance $ 1,296,265 $ 1,208,331 $ 1,155,915