Supplemental Oil And Gas Disclosures (Standardized Measure Of Discounted Future Net Cash Flows Relating To Proved Oil And Gas Reserves) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended												1 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 United States [Member]	Dec. 31, 2010 United States [Member]	Dec. 31, 2009 United States [Member]	Dec. 31, 2011 United Kingdom [Member]		Dec. 31, 2010 United Kingdom [Member]		Dec. 31, 2009 United Kingdom [Member]		Feb. 28, 2010 Camelot Field [Member]	Dec. 31, 2009 Camelot Field [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	$ 2,811,956	$ 2,925,744	$ 3,227,146	$ 2,811,956	$ 2,925,744	$ 3,166,306					$ 60,840	[1]
Production	(419,617)	(583,050)	(637,466)	(419,617)	(583,050)	(618,391)					(19,075)	[1]
Development and abandonment	(584,623)	(603,070)	(789,533)	(557,323)	(590,870)	(755,726)	(27,300)	[1]	(12,200)	[1]	(33,807)	[1]
Future net cash flows before income taxes	1,807,716	1,739,624	1,800,147	1,835,016	1,751,824	1,792,189	(27,300)	[1]	(12,200)	[1]	7,958	[1]
Future income tax expense	(477,630)	(430,153)	(418,602)	(477,630)	(430,153)	(417,042)					(1,560)	[1]
Future net cash flows	1,330,086	1,309,471	1,381,545	1,357,386	1,321,671	1,375,147	(27,300)	[1]	(12,200)	[1]	6,398	[1]
Discount at 10% annual rate	(266,954)	(318,404)	(390,485)	(266,954)	(318,404)	(387,036)					(3,449)	[1]
Standardized measure of discounted future net cash flows	$ 1,063,132	$ 991,067	$ 991,060	$ 1,090,432	$ 1,003,267	$ 988,111	$ (27,300)	[1]	$ (12,200)	[1]	$ 2,949	[1]
Ownership percentage														50.00%
Additional ownership percentage acquired													50.00%

[1] Reflects 50% ownership in the Camelot field's reserves in 2009. In February 2010 we acquired the other 50% ownership interest in the Camelot field (Note 5).