Summary Of Significant Accounting Policies (Schedule Of Changes In Asset Retirement Obligations) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended										12 Months Ended					6 Months Ended	12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Sep. 30, 2009	Jun. 30, 2009	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009	Jun. 30, 2010 United Kingdom [Member]	Dec. 31, 2011 United Kingdom [Member]	Dec. 31, 2010 United Kingdom [Member]	Dec. 31, 2011 Asset Retirement Obligation [Member]
Asset Retirement Obligations [Line Items]
Asset retirement obligations beginning balance				$ 234,936				$ 248,128			$ 234,936		$ 248,128
Liability incurred during the period											4,982		18,056
Liability settled during the period											(42,675)		(55,114)
Other revisions in estimated cash flows											42,268	[1]	8,349	[1]
Accretion expense (included in depreciation and amortization)											14,880		15,517
Asset retirement obligations ending balance	254,391				234,936						254,391		234,936		248,128
Impairment charges	$ 107,500	$ 2,400	$ 22,700	$ 0	$ 9,200	$ 900	$ 159,900	$ 11,100	$ 64,600	$ 48,200	$ 132,603		$ 181,083		$ 120,550	$ 5,000	$ 20,000	$ 5,000	$ 41,700

[1] Includes impairment charges totaling $41.7 million, including $20.0 million associated with our only United Kingdom oil and gas property, to increase the estimated asset retirement obligation related to properties that are no longer producing oil or natural gas.