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Fair Value Measurements (Narrative) (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended9 Months Ended3 Months Ended6 Months Ended3 Months Ended
Sep. 30, 2011
Jun. 30, 2011
Sep. 30, 2010
Jun. 30, 2010
Mar. 31, 2010
Sep. 30, 2011
Sep. 30, 2010
Jun. 30, 2010
US Impairments [Member]
Mar. 31, 2010
UK Impairments [Member]
Mar. 31, 2010
Natural Gas Properties [Member]
Jun. 30, 2010
Natural Gas Properties [Member]
Jun. 30, 2011
Operating Properties [Member]
Jun. 30, 2011
End of Life Properties [Member]
Sep. 30, 2011
Gulf Of Mexico Oil And Gas Properties [Member]
Number of properties impaired 7       315 62
Oil and gas property impairments$ 2,357$ 22,700$ 897$ 159,900$ 7,000$ 25,078$ 171,871$ 159,900$ 7,000    $ 2,400
Aggregate fair value of properties   $ 62,500$ 28,200      $ 2,900$ 0