Acquisitions (Schedule of Assets Acquired and Liabilities Assumed) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Acquired Indefinite-lived Intangible Assets [Line Items]
Asset retirement obligations	$ (227,167)	$ (235,110)
Hilcorp Acquisition [Member]
Acquired Indefinite-lived Intangible Assets [Line Items]
Oil and natural gas properties	675,943
Asset retirement obligations	(128,817)
Net assets acquired	547,126
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