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, 2013	Dec. 31, 2012	Dec. 31, 2011
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	$ 1,418,395,000	$ 1,041,036,000	$ 748,786,000
Net change in sales prices and production costs related to future production	(232,361,000)	(5,530,000)	263,477,000
Net change in estimated future development costs	(10,602,000)	91,404,000	(4,653,000)
Net change due to revisions in quantity estimates	205,686,000	(124,722,000)	(51,782,000)
Accretion of discount	185,389,000	144,904,000	100,624,000
Changes in production rates (timing) and other	11,892,000	(9,430,000)	(94,293,000)
Total revisions	160,004,000	96,626,000	213,373,000
Net change due to extensions and discoveries, net of estimated future development and production costs	873,028,000	599,544,000	508,558,000
Net change due to sales of minerals in place	(632,752,000)	(212,910,000)	(150,437,000)
Sales of oil and gas produced, net of production costs	(444,841,000)	(313,354,000)	(173,853,000)
Previously estimated development costs incurred	217,395,000	234,947,000	45,160,000
Net change in income taxes	30,182,000	(27,494,000)	(150,551,000)
Net change in standardized measure of discounted future net cash flows	203,016,000	377,359,000	292,250,000
Standardized measure — end of period	1,621,411,000	1,418,395,000	1,041,036,000
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	1,179,483,000	856,463,000	654,684,000
Net change in sales prices and production costs related to future production	(232,361,000)	(55,249,000)	134,952,000
Net change in estimated future development costs	(10,602,000)	91,404,000	(509,000)
Net change due to revisions in quantity estimates	205,686,000	(77,919,000)	(64,860,000)
Accretion of discount	141,229,000	107,451,000	81,225,000
Changes in production rates (timing) and other	56,052,000	(3,369,000)	(78,199,000)
Total revisions	160,004,000	62,318,000	72,609,000
Net change due to extensions and discoveries, net of estimated future development and production costs	873,028,000	599,544,000	508,558,000
Net change due to sales of minerals in place	(191,155,000)	(212,910,000)	(150,437,000)
Sales of oil and gas produced, net of production costs	(444,841,000)	(313,354,000)	(173,853,000)
Previously estimated development costs incurred	217,395,000	202,187,000	5,381,000
Net change in income taxes	(172,503,000)	(14,765,000)	(60,479,000)
Net change in standardized measure of discounted future net cash flows	441,928,000	323,020,000	201,779,000
Standardized measure — end of period	1,621,411,000	1,179,483,000	856,463,000
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	238,912,000	184,573,000	94,102,000
Net change in sales prices and production costs related to future production	0	49,719,000	128,525,000
Net change in estimated future development costs	0	0	(4,144,000)
Net change due to revisions in quantity estimates	0	(46,803,000)	13,078,000
Accretion of discount	44,160,000	37,453,000	19,399,000
Changes in production rates (timing) and other	(44,160,000)	(6,061,000)	(16,094,000)
Total revisions	0	34,308,000	140,764,000
Net change due to extensions and discoveries, net of estimated future development and production costs	0	0	0
Net change due to sales of minerals in place	(441,597,000)	0	0
Sales of oil and gas produced, net of production costs	0	0	0
Previously estimated development costs incurred	0	32,760,000	39,779,000
Net change in income taxes	202,685,000	(12,729,000)	(90,072,000)
Net change in standardized measure of discounted future net cash flows	(238,912,000)	54,339,000	90,471,000
Standardized measure — end of period	$ 0	$ 238,912,000	$ 184,573,000