Fair Value Measurements (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Mar. 31, 2010	Jun. 30, 2011	Jun. 30, 2010
Oil and gas property impairments	$ 22,721	$ 159,862	$ 7,000	$ 22,721	$ 170,974
Aggregate fair value of properties	2,900	62,500	28,200	2,900	62,500
US Impairments [Member]
Oil and gas property impairments					159,900
UK Impairments [Member]
Oil and gas property impairments			$ 7,000
Natural Gas Properties [Member]
Number of properties impaired			3		15