Supplemental Oil and Gas Disclosures (Capitalized Costs) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Capitalized Costs Relating To Oil And Gas Producing Activities By Geographic Area [Line Items]
Proved	$ 989,758	$ 1,084,669
Unproved	10,784	368,660
Total capitalized costs	1,000,542	1,453,329
Accumulated depreciation, depletion and amortization	(531,073)	(382,795)
Net capitalized costs	469,469	1,070,534
United Kingdom [Member]
Capitalized Costs Relating To Oil And Gas Producing Activities By Geographic Area [Line Items]
Proved	935,557	1,043,927
Unproved		295,083
Total capitalized costs	935,557	1,339,010
Accumulated depreciation, depletion and amortization	(501,894)	(355,366)
Net capitalized costs	433,663	983,644
United States [Member]
Capitalized Costs Relating To Oil And Gas Producing Activities By Geographic Area [Line Items]
Proved	54,201	40,742
Unproved	10,784	73,577
Total capitalized costs	64,985	114,319
Accumulated depreciation, depletion and amortization	(29,179)	(27,429)
Net capitalized costs	$ 35,806	$ 86,890