Supplemental Oil and Gas Disclosures (Reserves Volume) (Details)	12 Months Ended
	Dec. 31, 2014 MBbls	Dec. 31, 2013 MBbls	Dec. 31, 2012 MBbls
Oil Reserves [Member]
Reserve Quantities [Line Items]
Proved reserves at opening balance	12,342,000	13,739,000	4,101,000
Production	(2,572,000)	(3,018,000)	(1,997,000)
Purchases of reserves			11,071,000
Sale of reserves in place			(19,000)
Extensions and discoveries	73,000		1,992,000
Revisions of previous estimates	(3,805,000)	1,621,000	(1,409,000)
Proved reserves closing balance	6,038,000	12,342,000	13,739,000
Proved Developed Reserves	5,819,000	5,661,000	5,267,000
Natural Gas Reserves [Member]
Reserve Quantities [Line Items]
Proved reserves at opening balance	67,173,000	71,591,000	111,701,000
Production	(7,830,000)	(3,830,000)	(5,297,000)
Purchases of reserves			2,407,000
Sale of reserves in place		(4,200,000)	(5,213,000)
Extensions and discoveries	24,285,000		473,000
Revisions of previous estimates	(46,390,000)	3,612,000	(32,480,000)
Proved reserves closing balance	37,238,000	67,173,000	71,591,000
Proved Developed Reserves	35,639,000	28,176,000	17,837,000
United Kingdom [Member] | Oil Reserves [Member]
Reserve Quantities [Line Items]
Proved reserves at opening balance	12,340,000	13,733,000	4,060,000
Production	(2,564,000)	(3,017,000)	(1,994,000)
Purchases of reserves			11,071,000
Extensions and discoveries			1,992,000
Revisions of previous estimates	(3,813,000)	1,624,000	(1,396,000)
Proved reserves closing balance	5,963,000	12,340,000	13,733,000
Proved Developed Reserves	5,744,000	5,659,000	5,261,000
United Kingdom [Member] | Natural Gas Reserves [Member]
Reserve Quantities [Line Items]
Proved reserves at opening balance	55,398,000	56,901,000	50,723,000
Production	(6,253,000)	(1,194,000)	(91,000)
Purchases of reserves			1,409,000
Extensions and discoveries			473,000
Revisions of previous estimates	(24,962,000)	(309,000)	4,387,000
Proved reserves closing balance	24,183,000	55,398,000	56,901,000
Proved Developed Reserves	24,183,000	18,384,000	3,147,000
United States [Member] | Oil Reserves [Member]
Reserve Quantities [Line Items]
Proved reserves at opening balance	2,000	6,000	41,000
Production	(8,000)	(1,000)	(3,000)
Sale of reserves in place			(19,000)
Extensions and discoveries	73,000
Revisions of previous estimates	8,000	(3,000)	(13,000)
Proved reserves closing balance	75,000	2,000	6,000
Proved Developed Reserves	75,000	2,000	6,000
United States [Member] | Natural Gas Reserves [Member]
Reserve Quantities [Line Items]
Proved reserves at opening balance	11,775,000	14,690,000	60,978,000
Production	(1,577,000)	(2,636,000)	(5,206,000)
Purchases of reserves			998,000
Sale of reserves in place		(4,200,000)	(5,213,000)
Extensions and discoveries	24,285,000
Revisions of previous estimates	(21,428,000)	3,921,000	(36,867,000)
Proved reserves closing balance	13,055,000	11,775,000	14,690,000
Proved Developed Reserves	11,456,000	9,792,000	14,690,000