Condensed Consolidated Statements of Changes in Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Capital In Excess of Par Value [Member]	Accumulated other comprehensive income [Member]	Retained earnings [Member]	Non-controlling interests in consolidated subsidiaries [Member]
Stockholders' equity, beginning balance at Dec. 31, 2016	$ 1,194,070	$ 10,016	$ 502,500	$ 0	$ 681,554	$ 0
Net Income	28,693	0	0	0	28,693	0
Other comprehensive income (loss)	53	0	0	53	0	0
Total comprehensive income	28,746
Activity in employee compensation plans	29,089	261	28,828	0	0	0
Stockholders' equity, ending balance at Sep. 30, 2017	1,251,905	10,277	531,328	53	710,247	0
Cumulative effect adjustment for adoption of ASUs (Notes 1 and 2)	(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	37,138	0	0	0	32,552	4,586
Other comprehensive income (loss)	(179)	0	0	(179)	0	0
Total comprehensive income	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 of sale on 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