CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Common Stock [Member]	Capital in Excess of Par Value [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Treasury Stock [Member]	Total
Balance at beginning of period at Dec. 31, 2018	$ 47,872	$ 102,470	$ 380,113	$ (9,594)	$ (53,660)	$ 467,201
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings	0	0	46,399	0	0	46,399
Other comprehensive income (loss), net of tax	0	0	0	(1,642)	0	(1,642)
Cash dividends paid	0	0	(15,429)	0	0	(15,429)
Purchase of treasury stock	0	0	0	0	(10,738)	(10,738)
Stock-based compensation	0	2,828	0	0	2,914	5,742
Employee Stock Ownership Plan	0	745	0	0		2,519
Employee Stock Ownership Plan					1,774
Balance at end of period at Sep. 30, 2019	47,872	106,043	411,083	(11,236)	(59,710)	494,052
Balance at beginning of period at Jun. 30, 2019	47,872	105,347	401,465	(8,511)	(60,908)	485,265
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings	0	0	14,751	0	0	14,751
Other comprehensive income (loss), net of tax	0	0	0	(2,725)	0	(2,725)
Cash dividends paid	0	0	(5,133)	0	0	(5,133)
Stock-based compensation	0	696	0	0	1,198	1,894
Balance at end of period at Sep. 30, 2019	47,872	106,043	411,083	(11,236)	(59,710)	494,052
Balance at beginning of period at Dec. 31, 2019	47,872	102,742	417,437	(8,589)	(55,234)	504,228
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings	0	0	48,219	0	0	48,219
Other comprehensive income (loss), net of tax	0	0	0	(1,570)	0	(1,570)
Cash dividends paid	0	0	(5,615)	0	0	(5,615)
Purchase of treasury stock	0	0	0	0	(8,726)	(8,726)
Stock-based compensation	0	3,204	0	0	2,863	6,067
Employee Stock Ownership Plan	0	630	0	0		2,301
Employee Stock Ownership Plan					1,671
Balance at end of period at Sep. 30, 2020	47,872	106,576	460,041	(10,159)	(59,426)	544,904
Balance at beginning of period at Jun. 30, 2020	47,872	104,128	431,416	(13,021)	(59,739)	510,656
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings	0	0	28,625	0	0	28,625
Other comprehensive income (loss), net of tax	0	0	0	2,862	0	2,862
Stock-based compensation	0	2,448	0	0	313	2,761
Balance at end of period at Sep. 30, 2020	$ 47,872	$ 106,576	$ 460,041	$ (10,159)	$ (59,426)	$ 544,904