Unaudited Condensed Consolidated Statement of Changes in Equity - USD ($) $ in Thousands	Total	Total Stockholders' Equity	Common Stock Shares Outstanding Class A	Common Stock Shares Outstanding Class B	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)
Conversion of Stock, Amount Converted	$ 0		1	(1)
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 reclassification		160				2,339		(2,179)
Stock-based compensation		2,323	100		2,223
Purchase of treasury shares		(339)	(10)		(329)
Net income	$ 23,769	23,769				23,769
Cash dividends on Class A and Class B common stock		(3,433)				(3,433)
Reclassification adjustment to net income		348						348
Balance, end of period at Sep. 30, 2018		240,201	$ 5,373	$ 1,569	$ 6,341	239,817	0	$ (12,899)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of new accounting principles on retained earnings | Accounting Standards Update 2014-09		(2,075)				(2,075)
Cumulative effect of new accounting principles on retained earnings | Accounting Standards Update 2016-01		$ 0				$ 2,727	$ (2,727)