Unaudited Condensed Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Common Stock Class A Common Stock	Common Stock Class B Common Stock	Capital in Excess of Par Value	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Balance, beginning of period at Dec. 31, 2018	$ 250,704	$ 5,352	$ 1,569	$ 7,042	$ 250,352	$ (13,611)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	897	102		795
Purchase of treasury shares	(1,300)	(36)		(1,264)
Net income	15,018				15,018
Cash dividends on Class A and Class B common stock	(1,153)				(1,153)
Reclassification adjustment to net income, net of tax	101					101
Balance, end of period at Mar. 31, 2019	264,267	5,418	1,569	6,573	264,217	(13,510)
Balance, beginning of period at Dec. 31, 2018	250,704	5,352	1,569	7,042	250,352	(13,611)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	33,257
Balance, end of period at Sep. 30, 2019	283,255	5,425	1,569	9,829	279,801	(13,369)
Balance, beginning of period at Mar. 31, 2019	264,267	5,418	1,569	6,573	264,217	(13,510)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	1,249	5		1,244
Purchase of treasury shares	(85)	(2)		(83)
Net income	7,975				7,975
Cash dividends on Class A and Class B common stock	(1,327)				(1,327)
Reclassification adjustment to net income, net of tax	71					71
Balance, end of period at Jun. 30, 2019	272,150	5,421	1,569	7,734	270,865	(13,439)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	2,124	5		2,119
Purchase of treasury shares	(25)	(1)		(24)
Net income	10,264				10,264
Cash dividends on Class A and Class B common stock	(1,328)				(1,328)
Reclassification adjustment to net income, net of tax	70					70
Balance, end of period at Sep. 30, 2019	283,255	5,425	1,569	9,829	279,801	(13,369)
Balance, beginning of period at Dec. 31, 2019	289,392	5,397	1,569	8,911	284,852	(11,337)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	465	88		377
Purchase of treasury shares	(1,002)	(32)		(970)
Net income	6,166				6,166
Cash dividends on Class A and Class B common stock	(1,339)				(1,339)
Reclassification adjustment to net income, net of tax	155					155
Balance, end of period at Mar. 31, 2020	293,837	5,453	1,569	8,318	289,679	(11,182)
Balance, beginning of period at Dec. 31, 2019	289,392	5,397	1,569	8,911	284,852	(11,337)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	20,238
Balance, end of period at Sep. 30, 2020	306,489	5,477	1,568	9,381	301,045	(10,982)
Balance, beginning of period at Mar. 31, 2020	293,837	5,453	1,569	8,318	289,679	(11,182)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	634	10		624
Net income	6,050				6,050
Cash dividends on Class A and Class B common stock	(1,351)				(1,351)
Reclassification adjustment to net income, net of tax	101					101
Balance, end of period at Jun. 30, 2020	299,271	5,463	1,569	8,942	294,378	(11,081)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	452	13		439
Conversion of Class B to Class A shares	0	1	(1)
Net income	8,022				8,022
Cash dividends on Class A and Class B common stock	(1,355)				(1,355)
Reclassification adjustment to net income, net of tax	99					99
Balance, end of period at Sep. 30, 2020	$ 306,489	$ 5,477	$ 1,568	$ 9,381	$ 301,045	$ (10,982)