Acquisitions (Schedule of Assets Acquired and Liabilities Assumed) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jun. 30, 2012 Hilcorp Acquisition [Member]	Mar. 31, 2012 South Timbalier Acquisition [Member]	Dec. 31, 2010 ASOP Acquisition [Member]	Nov. 01, 2011 Main Pass Acquisition [Member]
Acquired Indefinite-lived Intangible Assets [Line Items]
Oil and natural gas properties				$ 671,310	$ 33,587	$ 221,751	$ 40,826
Asset retirement obligations	(235,110)	(99,347)	(71,583)	(128,817)	(1,878)	(24,858)	(2,991)
Net assets acquired				$ 542,493	$ 31,709	$ 196,893	$ 37,835