Acquisitions and Divestitures (Fair Value of Acquired Assets and Liabilities) (Details) - USD ($) $ in Thousands		Dec. 06, 2018	Apr. 03, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Proved oil and natural gas properties				$ 6,341,582	$ 6,018,568	$ 5,712,813
Unproved properties not being amortized				252,874	330,216	296,764
Asset retirement obligation				$ (66,627)	$ (64,208)	$ (69,444)
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
Excalibur Acquisition
Business Acquisition [Line Items]
Total consideration given		$ 29,633
Proved oil and natural gas properties		14,546
Unproved properties not being amortized		15,502
Total oil and natural gas properties included in the full cost pool	[2]	30,048
Asset retirement obligation		(415)
Fair value of net assets acquired		$ 29,633

[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.

[2] 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