Condensed Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Capital In Excess of Par Value [Member]	Accumulated other comprehensive income (loss) [Member]	Retained earnings [Member]	Non-controlling interests in consolidated subsidiaries [Member]
Cumulative effect adjustment for adoption of ASUs	$ (1,261)	$ 0	$ 0	$ 13	$ (1,274)	$ 0
Stockholders' equity, beginning balance at Dec. 31, 2017	1,345,560	10,280	535,815	63	799,402	0
Net Income (loss)	37,138	0	0	0	32,552	4,586
Reclassification adjustment for write-down of securities, net of tax	0
Other comprehensive income (loss)	(179)	0	0	(179)	0	0
Total comprehensive income (loss)	36,959
Contributions	300,000	0	102,958	0	0	197,042
Transaction costs associated with sale of non-controlling interest	(2,303)	0	(2,303)	0	0	0
Tax effect on sale of non-controlling interest	(24,300)	0	(24,300)	0	0	0
Activity in employee compensation plans	14,710	134	14,576	0	0	0
Stockholders' equity, ending balance at Sep. 30, 2018	1,669,365	10,414	626,746	(103)	830,680	201,628
Stockholders' equity, beginning balance at Jun. 30, 2018	1,643,654	10,414	622,120	(65)	811,781	199,404
Net Income (loss)	21,123	0	0	0	18,899	2,224
Reclassification adjustment for write-down of securities, net of tax	0
Other comprehensive income (loss)	(38)	0	0	(38)	0	0
Total comprehensive income (loss)	21,085
Transaction costs associated with sale of non-controlling interest	(49)	0	(49)	0	0	0
Activity in employee compensation plans	4,675	0	4,675	0	0	0
Stockholders' equity, ending balance at Sep. 30, 2018	1,669,365	10,414	626,746	(103)	830,680	201,628
Cumulative effect adjustment for adoption of ASUs	174	0	0	0	174	0
Stockholders' equity, beginning balance at Dec. 31, 2018	1,593,444	10,414	628,108	(481)	752,840	202,563
Net Income (loss)	(218,088)	0	0	0	(218,899)	811
Reclassification adjustment for write-down of securities, net of tax	481	0	0	481	0	0
Other comprehensive income (loss)	0
Total comprehensive income (loss)	(217,607)
Distributions to non-controlling interest	(918)	0	0	0	0	(918)
Activity in employee compensation plans	16,140	176	15,934	0	0	30
Stockholders' equity, ending balance at Sep. 30, 2019	1,391,233	10,590	644,042	0	534,115	202,486
Stockholders' equity, beginning balance at Jun. 30, 2019	1,593,232	10,590	638,769	(487)	741,001	203,359
Net Income (loss)	(207,789)	0	0	0	(206,886)	(903)
Reclassification adjustment for write-down of securities, net of tax	487	0	0	487	0	0
Other comprehensive income (loss)	0
Total comprehensive income (loss)	(207,302)
Activity in employee compensation plans	5,303	0	5,273	0	0	30
Stockholders' equity, ending balance at Sep. 30, 2019	$ 1,391,233	$ 10,590	$ 644,042	$ 0	$ 534,115	$ 202,486