CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - 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 SMP [Member]	Non-controlling Interest [Member]	Total
Balance at beginning of period at Dec. 31, 2020	$ 47,872	$ 105,084	$ 463,612	$ (5,676)	$ (60,656)	$ 550,236	$ 0	$ 550,236
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Noncontrolling interest in business step acquisition	0	0	0	0	0	0	11,504	11,504
Net earnings	0	0	90,886	0	0	90,886	68	90,954
Other comprehensive loss, net of tax	0	0	0	(2,493)	0	(2,493)	15	(2,478)
Cash dividends paid	0	0	(22,179)	0	0	(22,179)	0	(22,179)
Purchase of treasury stock	0	0	0	0	(26,862)	(26,862)	0	(26,862)
Dividends paid to noncontrolling interest	0	0	0	0	0	0	(540)	(540)
Stock-based compensation	0	159	0	0	9,320	9,479	0	9,479
Employee Stock Ownership Plan	0	134	0	0	2,379	2,513	0	2,513
Balance at end of period at Dec. 31, 2021	47,872	105,377	532,319	(8,169)	(75,819)	601,580	11,047	612,627
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings	0	0	55,351	0	0	55,351	84	55,435
Other comprehensive loss, net of tax	0	0	0	(4,301)	0	(4,301)	(113)	(4,414)
Cash dividends paid	0	0	(23,428)	0	0	(23,428)	0	(23,428)
Purchase of treasury stock	0	0	0	0	(29,656)	(29,656)	0	(29,656)
Stock-based compensation	0	(131)	0	0	8,309	8,178	0	8,178
Employee Stock Ownership Plan	0	369	0	0	1,927	2,296	0	2,296
Balance at end of period at Dec. 31, 2022	47,872	105,615	564,242	(12,470)	(95,239)	610,020	11,018	621,038
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Noncontrolling interest in business step acquisition	0	0	0	0	0	0	5,273	5,273
Net earnings	0	0	34,148	0	0	34,148	204	34,352
Other comprehensive loss, net of tax	0	0	0	6,496	0	6,496	14	6,510
Cash dividends paid	0	0	(25,164)	0	0	(25,164)	0	(25,164)
Dividends paid to noncontrolling interest	0	0	0	0	0	0	(700)	(700)
Stock-based compensation	0	(3,880)	0	0	10,478	6,598	0	6,598
Employee Stock Ownership Plan	0	16	0	0	2,950	2,966	0	2,966
Balance at end of period at Dec. 31, 2023	$ 47,872	$ 101,751	$ 573,226	$ (5,974)	$ (81,811)	$ 635,064	$ 15,809	$ 650,873