Supplemental Oil and Gas Disclosures (Reserves MBBoe) (Details)	12 Months Ended
	Dec. 31, 2014 MBoe	Dec. 31, 2013 MBoe	Dec. 31, 2012 MBoe
Reserve Quantities [Line Items]
Proved reserves at opening balance	23,538,000	25,671,000	22,718,000
Production	(3,877,000)	(3,656,000)	(2,880,000)
Extensions and discoveries	4,120,000		2,071,000
Purchases of proved reserves, in place			11,472,000
Sale of reserves		(700,000)	(888,000)
Revisions of previous estimates	(11,536,000)	2,223,000	(6,822,000)
Proved reserves closing balance	12,245,000	23,538,000	25,671,000
Proved Developed Oil and Gas Reserves (MMBoe):	11,759,000	10,357,000	8,239,000
United Kingdom [Member]
Reserve Quantities [Line Items]
Proved reserves at opening balance	21,573,000	23,217,000	12,514,000
Production	(3,606,000)	(3,216,000)	(2,009,000)
Extensions and discoveries			2,071,000
Purchases of proved reserves, in place			11,306,000
Revisions of previous estimates	(7,973,000)	1,572,000	(665,000)
Proved reserves closing balance	9,994,000	21,573,000	23,217,000
Proved Developed Oil and Gas Reserves (MMBoe):	9,775,000	8,723,000	5,785,000
United States [Member]
Reserve Quantities [Line Items]
Proved reserves at opening balance	1,965,000	2,454,000	10,204,000
Production	(271,000)	(440,000)	(871,000)
Extensions and discoveries	4,120,000
Purchases of proved reserves, in place			166,000
Sale of reserves		(700,000)	(888,000)
Revisions of previous estimates	(3,563,000)	651,000	(6,157,000)
Proved reserves closing balance	2,251,000	1,965,000	2,454,000
Proved Developed Oil and Gas Reserves (MMBoe):	1,984,000	1,634,000	2,454,000