Acquisitions and Divestitures (Fair Value of Acquired Assets and Liabilities) (Details) - USD ($) $ in Thousands		Apr. 03, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Proved oil and natural gas properties			$ 5,712,813	$ 5,446,305	$ 5,401,618
Unproved properties not being amortized			296,764	314,867	337,099
Asset retirement obligation			$ (69,444)	$ (70,170)	$ (98,297)
EOG Acquisition
Business Acquisition [Line Items]
Total consideration given		$ 54,332
Proved oil and natural gas properties		43,745
Unproved properties not being amortized		8,650
Total oil and natural gas properties included in the full cost pool	[1]	52,395
Gas gathering equipment and other		2,340
Asset retirement obligation		(403)
Fair value of net assets acquired		$ 54,332

[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.