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, 2012
MBbls
Dec. 31, 2011
MBbls
Dec. 31, 2010
MBbls
Oil [Member]
      
Proved Developed and Undeveloped Reserves [Roll Forward]      
Beginning of the period 16,745 19,704 17,488
Purchases in oil and gas reserves in place 0 2 3,913
Extensions and discoveries 4,880 3,940 5,574
Sales of oil and gas reserves in place (10,604) 0 0
Revisions of prior reserve estimates (382) (4,714) (5,426)
Current production (2,388) (2,187) (1,845)
End of period 8,251 16,745 19,704
Proved developed reserves 5,219 7,485 7,230
Proved undeveloped reserves 3,032 9,260 12,474
Gas [Member]
      
Proved Developed and Undeveloped Reserves [Roll Forward]      
Beginning of the period 15,728 16,158 14,332
Purchases in oil and gas reserves in place 0 19 3,482
Extensions and discoveries 31,265 2,091 5,303
Sales of oil and gas reserves in place (11,757) 0 0
Revisions of prior reserve estimates (357) (1,662) (6,171)
Current production (1,108) (878) (788)
End of period 33,771 15,728 16,158
Proved developed reserves 18,482 6,152 6,068
Proved undeveloped reserves 15,289 9,576 10,090