Supplemental Quarterly Infomation (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplemental Quarterly Information [Abstract]
Sales and other operating revenues	$ 483,000	$ 486,300	$ 411,200	$ 429,100	$ 653,700	$ 665,600	$ 718,600	$ 749,200	$ 1,809,575	$ 2,787,116	$ 5,288,933
Income (loss) from continuing operations before income taxes	(80,100)	(16,700)	(131,300)	(265,000)	(646,500)	(2,408,000)	(110,100)	(117,700)	(493,115)	(3,282,262)	1,252,270
Income (loss) from continuing operations	(62,800)	(14,600)	2,900	(199,500)	(583,200)	(1,587,100)	(89,000)	3,500	(273,943)	(2,255,772)	1,024,973
Net income (loss)	$ (63,900)	$ (16,200)	$ 2,900	$ (198,800)	$ (587,100)	$ (1,595,400)	$ (73,800)	$ (14,500)	$ (275,970)	$ (2,270,833)	$ 905,611
Income (loss) from continuing operations per Common share, Basic	$ (0.36)	$ (0.08)	$ 0.02	$ (1.16)	$ (3.39)	$ (9.22)	$ (0.51)	$ 0.02	$ (1.59)	$ (12.94)	$ 5.73
Income (loss) from continuing operations per Common share, Diluted	(0.36)	(0.08)	0.02	(1.16)	(3.39)	(9.22)	(0.51)	0.02	(1.59)	(12.94)	5.69
Net income (loss) per Common share, Basic	(0.37)	(0.08)	0.02	(1.16)	(3.41)	(9.26)	(0.42)	(0.08)	(1.60)	(13.03)	5.06
Net income (loss) per Common share, Diluted	(0.37)	(0.08)	0.02	(1.16)	(3.41)	(9.26)	(0.42)	(0.08)	(1.60)	(13.03)	5.03
Cash dividend per Common share	0.25	0.25	0.35	0.35	0.35	0.35	0.35	0.35	1.20	1.40	$ 1.325
Market price of Common Stock, High	34.30	32.66	36.24	26.69	31.03	41.42	50.56	51.77	36.24	51.77
Market price of Common Stock, Low	$ 25.00	$ 25.14	$ 23.49	$ 15.76	$ 21.71	$ 23.76	$ 41.42	$ 43.40	$ 15.76	$ 21.71