Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Capital In Excess of Par Value	Accumulated Other Comprehensive Income	Retained Earnings
Beginning balances at Dec. 31, 2014	$ 2,332,394	$ 9,732	$ 468,123	$ 0	$ 1,854,539
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(1,037,361)	0	0	0	(1,037,361)
Other comprehensive income	0
Total comprehensive income	(1,037,361)
Activity in employee compensation plans	18,547	99	18,448	0	0
Ending balances at Dec. 31, 2015	1,313,580	9,831	486,571	0	817,178
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(135,624)	0	0	0	(135,624)
Other comprehensive income	0
Total comprehensive income	(135,624)
Activity in employee compensation plans	16,114	185	15,929	0	0
Ending balances at Dec. 31, 2016	1,194,070	10,016	502,500	0	681,554
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	117,848	0	0	0	117,848
Other comprehensive income	63	0	0	63	0
Total comprehensive income	117,911
Proceeds from sale of stock	18,623	158	18,465	0	0
Activity in employee compensation plans	14,956	106	14,850	0	0
Ending balances at Dec. 31, 2017	$ 1,345,560	$ 10,280	$ 535,815	$ 63	$ 799,402