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, 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	12,216
Balance, end of period at Jun. 30, 2020	299,271	5,463	1,569	8,942	294,378	(11,081)
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)
Balance, beginning of period at Dec. 31, 2020	300,624	5,490	1,568	10,895	294,270	(11,599)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	1,015	92		923
Conversion of Class B to Class A shares	0	1	(1)
Net income	8,961				8,961
Cash dividends on Class A and Class B common stock	(1,374)				(1,374)
Reclassification adjustment to net income, net of tax	143					143
Balance, end of period at Mar. 31, 2021	309,369	5,583	1,567	11,818	301,857	(11,456)
Balance, beginning of period at Dec. 31, 2020	300,624	5,490	1,568	10,895	294,270	(11,599)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	15,466
Balance, end of period at Jun. 30, 2021	315,727	5,595	1,567	12,928	306,950	(11,313)
Balance, beginning of period at Mar. 31, 2021	309,369	5,583	1,567	11,818	301,857	(11,456)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	1,122	12		1,110
Net income	6,505				6,505
Cash dividends on Class A and Class B common stock	(1,412)				(1,412)
Reclassification adjustment to net income, net of tax	143					143
Balance, end of period at Jun. 30, 2021	$ 315,727	$ 5,595	$ 1,567	$ 12,928	$ 306,950	$ (11,313)