Acquisitions and Divestitures - Fair Value of Acquired Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 17, 2012
Oil and natural gas properties included in the full cost pool:
Asset retirement obligation	$ (100,567)	$ (133,657)	$ (146,159)
Western Oklahoma and Texas Panhandle | Noble Energy, Inc.
Adjusted Purchase Price [Abstract]
Total consideration given				592,627
Oil and natural gas properties included in the full cost pool:
Proved oil and natural gas properties				260,799
Unproved oil and natural gas properties				353,343
Total oil and natural gas properties included in the full cost pool				614,142	[1]
Gas gathering and processing equipment and other				25,163
Asset retirement obligation				(46,678)
Fair value of net assets acquired				$ 592,627

[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.