Supplemental Disclosures About Oil And Gas Producing Activities (Schedule Of Costs Incurred In Oil And Gas Property Acquisition, Exploration And Development Activities) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Unproved property acquisition costs	$ 150,479	$ 109,216	$ 127,589
Exploration costs	557,523	374,366	134,487
Development costs	60,981	58,544	68,327
Asset retirement obligations	3,437	6,018	1,031
Total costs incurred	772,420	548,144	331,434
U.S. [Member]
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Unproved property acquisition costs	139,344	108,212	126,783
Exploration costs	557,523	374,366	134,487
Development costs	25,756	19,769	62,952
Asset retirement obligations	2,401	3,369	1,031
Total costs incurred	725,024	505,716	325,253
U.K.
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Unproved property acquisition costs	11,135	1,004	806
Exploration costs	0	0	0
Development costs	35,225	38,775	5,375
Asset retirement obligations	1,036	2,649	0
Total costs incurred	$ 47,396	$ 42,428	$ 6,181