Supplemental Disclosures About Oil And Gas Producing Activities (Schedule Of Changes In Standardized Measure Of Discounted Future Net Cash Flows Relating To Proved Oil And Gas Reserves) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward]
Standardized measure — beginning of period	$ 1,041,036,000	$ 748,786,000	$ 382,093,000
Net change in sales prices and production costs related to future production	(5,530,000)	263,477,000	263,663,000
Net change in estimated future development costs	91,404,000	(4,653,000)	83,000
Net change due to revisions in quantity estimates	(124,722,000)	(51,782,000)	(25,451,000)
Accretion of discount	144,904,000	100,624,000	39,833,000
Changes in production rates (timing) and other	(9,430,000)	(94,293,000)	49,806,000
Total revisions	96,626,000	213,373,000	327,934,000
Net change due to extensions and discoveries, net of estimated future development and production costs	599,544,000	508,558,000	351,831,000
Net change due to sales of minerals in place	(212,910,000)	(150,437,000)	0
Sales of oil and gas produced, net of production costs	(313,354,000)	(173,853,000)	(115,800,000)
Previously estimated development costs incurred	234,947,000	45,160,000	43,940,000
Net change in income taxes	(27,494,000)	(150,551,000)	(241,212,000)
Net change in standardized measure of discounted future net cash flows	377,359,000	292,250,000	366,693,000
Standardized measure — end of period	1,418,395,000	1,041,036,000	748,786,000
United States [Member]
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward]
Standardized measure — beginning of period	856,463,000	654,684,000	382,093,000
Net change in sales prices and production costs related to future production	(55,249,000)	134,952,000	263,663,000
Net change in estimated future development costs	91,404,000	(509,000)	83,000
Net change due to revisions in quantity estimates	(77,919,000)	(64,860,000)	(25,451,000)
Accretion of discount	107,451,000	81,225,000	39,833,000
Changes in production rates (timing) and other	(3,369,000)	(78,199,000)	49,806,000
Total revisions	62,318,000	72,609,000	327,934,000
Net change due to extensions and discoveries, net of estimated future development and production costs	599,544,000	508,558,000	157,846,000
Net change due to sales of minerals in place	(212,910,000)	(150,437,000)	0
Sales of oil and gas produced, net of production costs	(313,354,000)	(173,853,000)	(115,800,000)
Previously estimated development costs incurred	202,187,000	5,381,000	43,940,000
Net change in income taxes	(14,765,000)	(60,479,000)	(141,329,000)
Net change in standardized measure of discounted future net cash flows	323,020,000	201,779,000	272,591,000
Standardized measure — end of period	1,179,483,000	856,463,000	654,684,000
United Kingdom [Member]
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward]
Standardized measure — beginning of period	184,573,000	94,102,000	0
Net change in sales prices and production costs related to future production	49,719,000	128,525,000	0
Net change in estimated future development costs	0	(4,144,000)	0
Net change due to revisions in quantity estimates	(46,803,000)	13,078,000	0
Accretion of discount	37,453,000	19,399,000	0
Changes in production rates (timing) and other	(6,061,000)	(16,094,000)	0
Total revisions	34,308,000	140,764,000	0
Net change due to extensions and discoveries, net of estimated future development and production costs	0	0	193,985,000
Net change due to sales of minerals in place	0	0	0
Sales of oil and gas produced, net of production costs	0	0	0
Previously estimated development costs incurred	32,760,000	39,779,000	0
Net change in income taxes	(12,729,000)	(90,072,000)	(99,883,000)
Net change in standardized measure of discounted future net cash flows	54,339,000	90,471,000	94,102,000
Standardized measure — end of period	$ 238,912,000	$ 184,573,000	$ 94,102,000