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)	16,015	0	0	0	13,653	2,362
Other comprehensive income (loss)	(141)	0	0	(141)	0	0
Total comprehensive income (loss)	15,874
Contributions	300,000	0	102,958	0	0	197,042
Transaction costs associated with sale of non-controlling interest	(2,254)	0	(2,254)	0	0	0
Tax effect on sale of non-controlling interest	(24,300)	0	(24,300)	0	0	0
Activity in employee compensation plans	10,035	134	9,901	0	0	0
Stockholders' equity, ending balance at Jun. 30, 2018	1,643,654	10,414	622,120	(65)	811,781	199,404
Stockholders' equity, beginning balance at Mar. 31, 2018	1,357,300	10,403	541,004	(100)	805,993	0
Net Income (loss)	8,150	0	0	0	5,788	2,362
Other comprehensive income (loss)	35	0	0	35	0	0
Total comprehensive income (loss)	8,185
Contributions	300,000	0	102,958	0	0	197,042
Transaction costs associated with sale of non-controlling interest	(2,254)	0	(2,254)	0	0	0
Tax effect on sale of non-controlling interest	(24,300)	0	(24,300)	0	0	0
Activity in employee compensation plans	4,723	11	4,712	0	0	0
Stockholders' equity, ending balance at Jun. 30, 2018	1,643,654	10,414	622,120	(65)	811,781	199,404
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)	(10,299)	0	0	0	(12,013)	1,714
Other comprehensive income (loss)	(6)	0	0	(6)	0	0
Total comprehensive income (loss)	(10,305)
Distributions to non-controlling interest	(918)	0	0	0	0	(918)
Activity in employee compensation plans	10,837	176	10,661	0	0	0
Stockholders' equity, ending balance at Jun. 30, 2019	1,593,232	10,590	638,769	(487)	741,001	203,359
Stockholders' equity, beginning balance at Mar. 31, 2019	1,595,859	10,578	633,361	(457)	749,510	202,867
Net Income (loss)	(8,017)	0	0	0	(8,509)	492
Other comprehensive income (loss)	(30)	0	0	(30)	0	0
Total comprehensive income (loss)	(8,047)
Activity in employee compensation plans	5,420	12	5,408	0	0	0
Stockholders' equity, ending balance at Jun. 30, 2019	$ 1,593,232	$ 10,590	$ 638,769	$ (487)	$ 741,001	$ 203,359