Supplemental Oil and Gas Disclosures (Reserves Volume) (Details)	12 Months Ended
	Dec. 31, 2012 MBbls	Dec. 31, 2011 MBbls	Dec. 31, 2010 MBbls
[OilReservesMember]
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at opening balance	4,101,000	3,723,000	3,366,000
Production	(1,997,000)	(380,000)	(551,000)
Purchases of reserves	11,071,000	303,000
Sale of reserves in place	(19,000)
Extensions and discoveries	1,992,000		491,000
Revisions of previous estimates	(1,409,000)	455,000	417,000
Proved reserves closing balance	13,739,000	4,101,000	3,723,000
Proved Developed Reserves	5,267,000	1,311,000	1,254,000
[NaturalGasReservesMember]
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at opening balance	111,701,000	87,954,000	89,100,000
Production	(5,297,000)	(5,170,000)	(5,707,000)
Purchases of reserves	2,407,000	90,000	2,657,000
Sale of reserves in place	(5,213,000)		(51,522,000)
Extensions and discoveries	473,000	46,100,000	50,873,000
Revisions of previous estimates	(32,480,000)	(17,273,000)	2,553,000
Proved reserves closing balance	71,591,000	111,701,000	87,954,000
Proved Developed Reserves	17,837,000	23,499,000	13,836,000
[GB] | [OilReservesMember]
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at opening balance	4,060,000	3,664,000	3,348,000
Production	(1,994,000)	(373,000)	(545,000)
Purchases of reserves	11,071,000	303,000
Sale of reserves in place	0
Extensions and discoveries	1,992,000		457,000
Revisions of previous estimates	(1,396,000)	466,000	404,000
Proved reserves closing balance	13,733,000	4,060,000	3,664,000
Proved Developed Reserves	5,261,000	1,270,000	1,240,000
[GB] | [NaturalGasReservesMember]
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at opening balance	50,723,000	56,177,000	78,316,000
Production	(91,000)	(94,000)	(3,071,000)
Purchases of reserves	1,409,000	90,000	0
Sale of reserves in place	0		(51,522,000)
Extensions and discoveries	473,000	0	26,692,000
Revisions of previous estimates	4,387,000	(5,450,000)	5,762,000
Proved reserves closing balance	56,901,000	50,723,000	56,177,000
Proved Developed Reserves	3,147,000	795,000	555,000
[DomesticCountryMember] | [OilReservesMember]
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at opening balance	41,000	59,000	18,000
Production	(3,000)	(7,000)	(6,000)
Purchases of reserves	0	0
Sale of reserves in place	(19,000)
Extensions and discoveries	0		34,000
Revisions of previous estimates	(13,000)	(11,000)	13,000
Proved reserves closing balance	6,000	41,000	59,000
Proved Developed Reserves	6,000	41,000	14,000
[DomesticCountryMember] | [NaturalGasReservesMember]
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at opening balance	60,978,000	31,777,000	10,784,000
Production	(5,206,000)	(5,076,000)	(2,636,000)
Purchases of reserves	998,000	0	2,657,000
Sale of reserves in place	(5,213,000)		0
Extensions and discoveries	0	46,100,000	24,181,000
Revisions of previous estimates	(36,867,000)	(11,823,000)	(3,209,000)
Proved reserves closing balance	14,690,000	60,978,000	31,777,000
Proved Developed Reserves	14,690,000	22,704,000	13,281,000