Supplemental Oil and Gas Disclosures (Future Cash Inflows) (Details)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
[GB]
Prices Used To Determine Estimate Of Future Cash Inflows Oil	111.13	110.77
Prices Used To Determine Estimate Of Future Cash Inflows Gas	9.34	8.75
[DomesticCountryMember]
Prices Used To Determine Estimate Of Future Cash Inflows Oil	94.71	96.04
Prices Used To Determine Estimate Of Future Cash Inflows Gas	2.75	4.14