The capitalized costs incurred during the period (excluded from amortization) in identifying areas that may warrant examination and in examining specific areas that are considered to have prospects of containing oil and gas reserves, including costs of drilling exploratory wells and exploratory-type stratigraphic test wells. Exploration costs may be incurred both before acquiring the related property (sometimes referred to in part as prospecting costs) and after acquiring the property. Principal types of exploration costs, which include depreciation and applicable operating costs of support equipment and facilities and other costs of exploration activities, are: costs of topographical, geographical and geophysical studies, rights of access to properties to conduct those studies, and salaries and other expenses of geologists, geophysical crews, and others conducting those studies. Collectively, these are sometimes referred to as geological and geophysical or "G&G" costs. Exploration costs also include costs of carrying and retaining undeveloped properties, such as delay rentals, ad valorem taxes on properties, legal costs for title defense, the maintenance of land and lease records, dry hole contributions and bottom hole contributions, costs of drilling and equipping exploratory wells and costs of drilling exploratory-type stratigraphic test wells.

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The expense recorded to reduce the value of oil and gas assets consisting of proved properties and unproved properties as the estimate of future successful production from these properties is reduced.

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Property and Equipment (Narrative) (Details) (USD $)	3 Months Ended		6 Months Ended
Property and Equipment (Narrative) (Details) (USD $)	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Amortization period			5 years
Directly related employee costs capitalized	$ 1,800,000	$ 5,300,000	$ 6,800,000	$ 9,800,000
Impairment of oil and gas properties		19,960,000	3,534,000	35,700,000
U.S. [Member]
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Impairment of oil and gas properties		20,000,000
Average price per barrel, oil	91.60	95.54
Average price per barrel, gas	3.46	3.13
U.K. [Member]
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Average price per barrel, oil	107.58	112.40
Average price per barrel, gas	10.24	8.68
U.K. Activities [Member]
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Exploration and development related capitalized costs	$ 7,600,000	$ 6,300,000	$ 13,800,000	$ 11,900,000