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	Deferred Gain (Loss) on Equity Securities	Pension and Postretirement Plan Adjustment
Balance, beginning of period at Dec. 31, 2017	$ 219,448	$ 5,282	$ 1,570	$ 4,447	$ 216,490	$ 2,727	$ (11,068)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
ASU 2018-02 adoption	160				2,339		(2,179)
Stock-based compensation	177	87		90
Net income	8,176				8,176
Cash dividends on Class A and Class B common stock	(1,144)				(1,144)
Reclassification adjustment to net income, net of tax	140						140
Balance, end of period at Mar. 31, 2018	224,882	5,369	1,570	4,537	226,513	0	(13,107)
Balance, beginning of period at Dec. 31, 2017	219,448	5,282	1,570	4,447	216,490	2,727	(11,068)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	23,769
Balance, end of period at Sep. 30, 2018	240,201	5,373	1,569	6,341	239,817	0	(12,899)
Balance, beginning of period at Mar. 31, 2018	224,882	5,369	1,570	4,537	226,513	0	(13,107)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	792	7		785
Purchase of treasury shares	(55)	(2)		(53)
Conversion of Class B to Class A shares	0	1	(1)
Net income	6,393				6,393
Cash dividends on Class A and Class B common stock	(1,145)				(1,145)
Reclassification adjustment to net income, net of tax	105						105
Balance, end of period at Jun. 30, 2018	230,972	5,375	1,569	5,269	231,761	0	(13,002)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	1,354	6		1,348
Purchase of treasury shares	(284)	(8)		(276)
Net income	9,200				9,200
Cash dividends on Class A and Class B common stock	(1,144)				(1,144)
Reclassification adjustment to net income, net of tax	103						103
Balance, end of period at Sep. 30, 2018	240,201	5,373	1,569	6,341	239,817	$ 0	(12,899)
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)