Property and Equipment (Schedule of Property and Equipment) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Oil and natural gas properties	$ 8,146,321	$ 6,071,920
Office furniture and fixtures	24,649	21,204
Building	45,204	42,530
Land	5,254	5,252
Total property and equipment	8,221,428	6,140,906
Accumulated depletion, depreciation, amortization and impairment	(3,855,629)	(3,789,780)
Property and equipment, net	$ 4,365,799	$ 2,351,126