Supplemental Quarterly Infomation (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015	[1]
Supplemental Quarterly Information [Abstract]
Sales and other operating revenues		$ 545,000	$ 498,300	$ 509,700	$ 544,700	$ 483,000	$ 486,300	$ 411,200	$ 429,100	$ 2,097,695	$ 1,809,575	[1]	$ 2,787,116
Income (loss) from continuing operations before income taxes		2,400	(63,600)	(21,900)	154,900	(80,100)	(16,700)	(131,300)	(265,000)	71,802	(493,115)	[1]	(3,282,262)
Income (loss) from continuing operations		(284,800)	(66,300)	(17,300)	57,500	(62,800)	(14,600)	2,900	(199,500)	(310,936)	(273,943)	[1]	(2,255,772)
Net income (loss)		$ (286,800)	$ (65,900)	$ (17,600)	$ 58,500	$ (63,900)	$ (16,200)	$ 2,900	$ (198,800)	$ (311,789)	$ (275,970)	[1]	$ (2,270,833)
Income (loss) from continuing operations per Common share, Basic		$ (1.65)	$ (0.38)	$ (0.10)	$ 0.33	$ (0.36)	$ (0.08)	$ 0.02	$ (1.16)	$ (1.81)	$ (1.59)	[1]	$ (12.94)
Income (loss) from continuing operations per Common share, Diluted		(1.65)	(0.38)	(0.10)	0.33	(0.36)	(0.08)	0.02	(1.16)	(1.81)	(1.59)	[1]	(12.94)
Net income (loss) per Common share, Basic		(1.66)	(0.38)	(0.10)	0.34	(0.37)	(0.08)	0.02	(1.16)	(1.81)	(1.60)	[1]	(13.03)
Net income (loss) per Common share, Diluted		(1.66)	(0.38)	(0.10)	0.34	(0.37)	(0.08)	0.02	(1.16)	(1.81)	(1.60)	[1]	(13.03)
Cash dividend per Common share		0.25	0.25	0.25	0.25	0.25	0.25	0.35	0.35	1.00	1.20	[1]	$ 1.40
Market price of Common Stock, High	[2]	31.98	27.43	28.71	32.18	34.30	32.66	36.24	26.69	32.18	36.24
Market price of Common Stock, Low	[2]	$ 25.02	$ 22.63	$ 24.06	$ 25.76	$ 25.00	$ 25.14	$ 23.49	$ 15.76	$ 22.63	$ 15.76

[1] Reclassified to conform to current presentation (see Note A).

[2] Prices are as quoted on the New York Stock Exchange.