Impairment of Proved and Unproved Properties (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of proved properties	$ 0	$ 48,525	$ 38,523	$ 48,525
Abandonment and impairment of unproved properties	$ 447	$ 0	$ 11,296	$ 4,316