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Oil and Gas Operations (Unaudited) (Costs Incurred) (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Property acquisition:      
Proved $ 214,993 $ 207,161 $ 186,263
Unproved 91,888 201,881 5,100
Exploration 190,854 37,371 16,590
Development 623,775 332,541 226,841
Total costs incurred $ 1,121,510 $ 778,954 $ 434,794