Acquisitions and Divestitures - Fair Value of Acquired Assets and Liabilities (Details) (Noble Energy, Inc. [Member], USD $) In Thousands, unless otherwise specified	Sep. 17, 2012
Noble Energy, Inc. [Member]
Preliminary Purchase Price [Abstract]
Total consideration given	$ 594,463
Oil and natural gas properties included in the full cost pool:
Proved oil and natural gas properties	260,798
Undeveloped oil and natural gas properties	355,180
Total oil and natural gas properties included in the full cost pool	615,978	[1]
Equipment and facilities	25,163
Asset retirement obligation	(46,678)
Fair value of net assets acquired	$ 594,463

[1] We used a discounted cash flow model and made market assumptions as to future commodity prices, projections of estimated quantities of oil and natural gas reserves, expectations for timing and amount of future development and operating costs, projections of future rates of production, expected recovery rates and risk adjusted discount rates.