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	Non-controlling Interest in Consolidated Subsidiaries
Beginning balance at Dec. 31, 2015	$ 1,313,580	$ 9,831	$ 486,571	$ 0	$ 817,178	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(135,624)	0	0	0	(135,624)	0
Total comprehensive income (loss)	(135,624)
Activity in employee compensation plans	16,114	185	15,929	0	0	0
Ending balance at Dec. 31, 2016	1,194,070	10,016	502,500	0	681,554	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	117,848	0	0	0	117,848	0
Other comprehensive income (loss)	63	0	0	63	0	0
Total comprehensive income (loss)	117,911
Proceeds from sale of stock	18,623	158	18,465	0	0	0
Activity in employee compensation plans	14,956	106	14,850	0	0	0
Ending balance at Dec. 31, 2017	1,345,560	10,280	535,815	63	799,402	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect adjustment for adoption of ASUs	(1,261)	0	0	13	(1,274)	0
Net income (loss)	(39,767)	0	0	0	(45,288)	5,521
Other comprehensive income (loss)	(557)	0	0	(557)	0	0
Total comprehensive income (loss)	(40,324)
Contributions	300,000	0	102,958	0	0	197,042
Transaction costs associated with sale of non-controlling	(2,503)	0	(2,503)	0	0	0
Tax effect on sale of non-controlling interest	(27,453)	0	(27,453)	0	0	0
Activity in employee compensation plans	19,425	134	19,291	0	0	0
Ending balance at Dec. 31, 2018	$ 1,593,444	$ 10,414	$ 628,108	$ (481)	$ 752,840	$ 202,563