Supplemental Oil and Gas Disclosures (Reserves MBBoe) (Details)	12 Months Ended
	Dec. 31, 2012 MMBoe	Dec. 31, 2011 MMBoe	Dec. 31, 2010 MMBoe
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at opening balance	22,718,000	18,382,000	18,216,000
Production	(2,880,000)	(1,242,000)	(1,502,000)
Purchases of reserves	11,472,000	318,000	443,000
Sale of reserves in place	(888,000)		(8,587,000)
Extensions and discoveries	2,071,000	7,683,000	8,970,000
Revisions of previous estimates	(6,822,000)	(2,423,000)	842,000
Proved reserves closing balance	25,671,000	22,718,000	18,382,000
Proved Developed Oil and Gas Reserves (MMBoe):	8,239,000	5,227,000	3,560,000
[GB]
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at opening balance	12,514,000	13,027,000	16,401,000
Production	(2,009,000)	(389,000)	(1,057,000)
Purchases of reserves	11,306,000	318,000	0
Sale of reserves in place	0		(8,587,000)
Extensions and discoveries	2,071,000	0	4,906,000
Revisions of previous estimates	(665,000)	(442,000)	1,364,000
Proved reserves closing balance	23,217,000	12,514,000	13,027,000
Proved Developed Oil and Gas Reserves (MMBoe):	5,785,000	1,402,000	1,333,000
[DomesticCountryMember]
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at opening balance	10,204,000	5,355,000	1,815,000
Production	(871,000)	(853,000)	(445,000)
Purchases of reserves	166,000	0	443,000
Sale of reserves in place	(888,000)		0
Extensions and discoveries	0	7,683,000	4,064,000
Revisions of previous estimates	(6,157,000)	(1,981,000)	(522,000)
Proved reserves closing balance	2,454,000	10,204,000	5,355,000
Proved Developed Oil and Gas Reserves (MMBoe):	2,454,000	3,825,000	2,227,000