Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Capital In Excess of Par Value	Accumulated Other Comprehensive Loss	Retained Earnings	Non-controlling Interest in Consolidated Subsidiaries
Beginning 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
Reclassification adjustment for write-down of securities	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
Reclassification adjustment for write-down of securities	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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect adjustment for adoption of ASUs	174	0	0	0	174	0
Net income (loss)	(553,828)				(553,879)	51
Total comprehensive income (loss)	(553,347)
Reclassification adjustment for write-down of securities	481	0	0	481	0	0
Distributions to non-controlling interest	(918)	0	0	0	0	(918)
Activity in employee compensation plans	16,282	177	16,044	0	0	61
Ending balance at Dec. 31, 2019	$ 1,055,635	$ 10,591	$ 644,152	$ 0	$ 199,135	$ 201,757