Supplemental Oil and Gas Disclosures (Costs Incurred) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Costs Incurred Oil And Gas Property Acquisition Exploration And Development Activities [Line Items]
Proved			$ 144,180
Unproved		2,786	48,034
Exploration costs	39,083us-gaap_CostsIncurredExplorationCosts	50,846us-gaap_CostsIncurredExplorationCosts	62,127us-gaap_CostsIncurredExplorationCosts
Development costs	42,454us-gaap_CostsIncurredDevelopmentCosts	152,206us-gaap_CostsIncurredDevelopmentCosts	172,462us-gaap_CostsIncurredDevelopmentCosts
Total costs incurred	81,537	205,838	426,803
United Kingdom [Member]
Costs Incurred Oil And Gas Property Acquisition Exploration And Development Activities [Line Items]
Proved			143,004
Unproved		2,323	46,878
Exploration costs	18,976	39,202	46,730
Development costs	38,711	149,138	164,562
Total costs incurred	57,687	190,663	401,174
United States [Member]
Costs Incurred Oil And Gas Property Acquisition Exploration And Development Activities [Line Items]
Proved			1,176
Unproved		463	1,156
Exploration costs	20,107	11,644	15,397
Development costs	3,743	3,068	7,900
Total costs incurred	$ 23,850	$ 15,175	$ 25,629

Cost incurred, including capitalized costs and costs charged to expense, in acquisition of oil and gas properties.

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Costs incurred, including capitalized costs and costs charged to expense, associated with the acquisition of oil and gas properties that have proved reserves.

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Costs incurred, including capitalized costs and costs charged to expense, associated with the acquisition of oil and gas properties that have unproved reserves.

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Development costs incurred, including capitalized costs and costs charged to expense, in oil and gas activities.

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Exploration costs incurred, including capitalized costs and costs charged to expense, in oil and gas activities.

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