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
	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	$ 2,341,354	$ 2,811,956	$ 2,925,744
Production	509,408	419,617	583,050
Development and abandonment	509,971	584,623	603,070
Total future costs	1,019,379	1,004,240	1,186,120
Future net cash flows before income taxes	1,321,975	1,807,716	1,739,624
Future income tax expense	330,630	477,630	430,153
Future net cash flows	991,345	1,330,086	1,309,471
Discount at 10% annual rate	153,032	266,954	318,404
Standardized measure of discounted future net cash flows	838,313	1,063,132	991,067
United States [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	2,341,354	2,811,956	2,925,744
Production	509,408	419,617	583,050
Development and abandonment	507,074	557,323	590,870
Total future costs	1,016,482	976,940	1,173,920
Future net cash flows before income taxes	1,324,872	1,835,016	1,751,824
Future income tax expense	330,630	477,630	430,153
Future net cash flows	994,242	1,357,386	1,321,671
Discount at 10% annual rate	153,032	266,954	318,404
Standardized measure of discounted future net cash flows	841,210	1,090,432	1,003,267
United Kingdom [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Development and abandonment	2,897	27,300	12,200
Total future costs	2,897	27,300	12,200
Future net cash flows before income taxes	(2,897)	(27,300)	(12,200)
Future net cash flows	(2,897)	(27,300)	(12,200)
Standardized measure of discounted future net cash flows	$ (2,897)	$ (27,300)	$ (12,200)