Acquisitions and Dispositions (Schedule of Assets Acquired and Liabilities Assumed) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Acquired Indefinite-lived Intangible Assets [Line Items]
Asset retirement obligations	$ (255,450)	$ (235,110)	$ (99,347)
Hilcorp Acquisition [Member]
Acquired Indefinite-lived Intangible Assets [Line Items]
Oil and natural gas properties	698,660
Asset retirement obligations	(150,959)
Net assets acquired	547,701
South Timbalier Acquisition [Member]
Acquired Indefinite-lived Intangible Assets [Line Items]
Oil and natural gas properties	33,206
Asset retirement obligations	(1,878)
Net assets acquired	31,328
ASOP Acquisition [Member]
Acquired Indefinite-lived Intangible Assets [Line Items]
Oil and natural gas properties		221,751
Asset retirement obligations		(24,858)
Net assets acquired		196,893
Main Pass Acquisition [Member]
Acquired Indefinite-lived Intangible Assets [Line Items]
Oil and natural gas properties		40,826
Asset retirement obligations		(2,991)
Net assets acquired		37,835
Nexen Acquisition [Member]
Acquired Indefinite-lived Intangible Assets [Line Items]
Oil and natural gas properties	93,932
Asset retirement obligations	(27,133)
Net assets acquired	66,799
West Delta 29 Acquisition [Member]
Acquired Indefinite-lived Intangible Assets [Line Items]
Oil and natural gas properties	16,696
Asset retirement obligations	(1,398)
Net assets acquired	$ 15,298