Impairment of Proved and Unproved Properties (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of proved properties	$ 38,523	$ 0	$ 38,523	$ 0
Abandonment and impairment of unproved properties	$ 10,707	$ 1,237	$ 10,849	$ 4,316