Annual report pursuant to Section 13 and 15(d)

Supplemental Information On Oil And Gas Exploration And Production Activities (Oil And Gas Reserves) (Details)

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Supplemental Information On Oil And Gas Exploration And Production Activities (Oil And Gas Reserves) (Details)
12 Months Ended
Dec. 31, 2011
MBbls
Dec. 31, 2010
MBbls
Dec. 31, 2009
MBbls
Oil [Member]
     
Reserve Quantities [Line Items]      
Beginning of the period 19,704 17,488 21,771
Purchases in oil and gas reserves in place 2 3,913 1,728
Extensions and discoveries 3,940 5,574 2,614
Sales of oil and gas reserves in place     (736)
Revisions of prior reserve estimates (4,714) (5,426) (6,294)
Current production (2,187) (1,845) (1,595)
End of period 16,745 19,704 17,488
Proved developed reserves 7,485 7,230 6,165
Proved undeveloped reserves 9,260 12,474 11,323
Gas [Member]
     
Reserve Quantities [Line Items]      
Beginning of the period 16,158 14,332 22,235
Purchases in oil and gas reserves in place 19 3,482 1,135
Extensions and discoveries 2,091 5,303 2,874
Sales of oil and gas reserves in place     (282)
Revisions of prior reserve estimates (1,662) (6,171) (11,139)
Current production (878) (788) (491)
End of period 15,728 16,158 14,332
Proved developed reserves 6,152 6,068 4,325
Proved undeveloped reserves 9,576 10,090 10,007