Supplemental Information on Oil and Natural Gas Properties (Unaudited) (Capitalized Costs) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Evaluated properties	$ 4,585,020	$ 3,429,570
Unevaluated properties	1,404,513	1,168,016
Total oil and natural gas properties	5,989,533	4,597,586
Accumulated depreciation, depletion, amortization and impairment	(2,270,675)	(2,084,095)
Total oil and natural gas properties, net	$ 3,718,858	$ 2,513,491