Supplemental Disclosures About Oil And Gas Producing Activities (Schedule Of 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
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	$ 5,584,365,000	$ 5,452,392,000	$ 3,947,208,000
Future production costs	(1,097,577,000)	(1,307,951,000)	(1,048,930,000)
Future development costs	(993,295,000)	(1,207,331,000)	(675,883,000)
Future income taxes	(764,283,000)	(724,097,000)	(543,639,000)
Future net cash flows	2,729,210,000	2,213,013,000	1,678,756,000
Less 10% annual discount to reflect timing of cash flows	(1,310,815,000)	(1,171,977,000)	(929,970,000)
Standard measure of discounted future net cash flows	1,418,395,000	1,041,036,000	748,786,000
U.S. [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	4,960,687,000	4,834,725,000	3,514,978,000
Future production costs	(1,009,850,000)	(1,212,722,000)	(952,148,000)
Future development costs	(982,101,000)	(1,163,377,000)	(597,444,000)
Future income taxes	(511,790,000)	(477,824,000)	(415,021,000)
Future net cash flows	2,456,946,000	1,980,802,000	1,550,365,000
Less 10% annual discount to reflect timing of cash flows	(1,277,463,000)	(1,124,339,000)	(895,681,000)
Standard measure of discounted future net cash flows	1,179,483,000	856,463,000	654,684,000
U.K.
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	623,678,000	617,667,000	432,230,000
Future production costs	(87,727,000)	(95,229,000)	(96,782,000)
Future development costs	(11,194,000)	(43,954,000)	(78,439,000)
Future income taxes	(252,493,000)	(246,273,000)	(128,618,000)
Future net cash flows	272,264,000	232,211,000	128,391,000
Less 10% annual discount to reflect timing of cash flows	(33,352,000)	(47,638,000)	(34,289,000)
Standard measure of discounted future net cash flows	$ 238,912,000	$ 184,573,000	$ 94,102,000