Supplemental Quarterly Info (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Sales	$ 431,660	$ 393,223	$ 393,590	$ 485,019	$ 460,251	$ 267,009	$ 354,410	$ 304,279	$ 1,730,434	$ 1,455,131	$ 759,968
Costs and Expenses	148,480	123,779	140,040	167,785	214,050	171,608	166,296	162,150	631,766	1,336,550	1,345,316
Income from Continuing Operations	129,415	146,756	61,394	545,546	286,035	(21,796)	121,807	(91,007)	883,111	295,039	(550,945)
Income (Loss) from Discontinued Operations	0	0	0	0	(9,391)	(4,645)	47,703	52,041	0	85,708	(297,157)
Net Income (Loss)	$ 101,927	$ 125,029	$ 42,014	$ 527,563	$ 276,644	$ (26,441)	$ 169,510	$ (38,966)	$ 883,111	$ 380,747	$ (848,102)
Basic
(Loss) Income from Continuing Operations (in usd per share)	$ 0.51	$ 0.59	$ 0.19	$ 2.38	$ 1.27	$ (0.09)	$ 0.53	$ (0.40)	$ 3.75	$ 1.29	$ (2.40)
Income (Loss) from Discontinued Operations (in usd per share)	0	0	0	0	(0.04)	(0.02)	0.21	0.23	0	0.37	(1.30)
Total Basic Earnings (Loss) Per Share (in usd per share)	0.51	0.59	0.19	2.38	1.23	(0.11)	0.74	(0.17)	3.75	1.66	(3.70)
Diluted
(Loss) Income from Continuing Operations (in usd per share)	0.50	0.59	0.19	2.35	1.26	(0.09)	0.52	(0.40)	3.71	1.28	(2.40)
Income (Loss) from Discontinued Operations (in usd per share)	0	0	0	0	(0.05)	(0.02)	0.21	0.23	0	0.37	(1.30)
Total Diluted Earnings (Loss) Per Share (in usd per share)	$ 0.50	$ 0.59	$ 0.19	$ 2.35	$ 1.21	$ (0.11)	$ 0.73	$ (0.17)	$ 3.71	$ 1.65	$ (3.70)
Impairment of Exploration and Production Properties			$ 18,650					$ 137,865	$ 0	$ 137,865	$ 0