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	Cumulative Effect, Period of Adoption, Adjustment [Member] Common Stock [Member] Adjustments Due to Adoption of ASU 2014-09 [Member]	Cumulative Effect, Period of Adoption, Adjustment [Member] Capital In Excess of Par Value [Member] Adjustments Due to Adoption of ASU 2014-09 [Member]	Cumulative Effect, Period of Adoption, Adjustment [Member] Retained Earnings [Member] Adjustments Due to Adoption of ASU 2014-09 [Member]	Cumulative Effect, Period of Adoption, Adjustment [Member] Accumulated Other Comprehensive Income (Loss) [Member] Adjustments Due to Adoption of ASU 2014-09 [Member]	Cumulative Effect, Period of Adoption, Adjustment [Member] Treasury Stock [Member] Adjustments Due to Adoption of ASU 2014-09 [Member]	Cumulative Effect, Period of Adoption, Adjustment [Member] Adjustments Due to Adoption of ASU 2014-09 [Member]
Balance at beginning of period at Dec. 31, 2017	$ 47,872	$ 100,057	$ 357,153	$ (4,109)	$ (47,319)	$ 453,654
Balance at beginning of period (ASC 606 [Member]) at Dec. 31, 2017							$ 0	$ 0	$ (1,189)	$ 0	$ 0	$ (1,189)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings	0	0	43,003	0	0	43,003
Other comprehensive income (loss), net of tax	0	0	0	(5,485)	0	(5,485)
Cash dividends paid	0	0	(18,854)	0	0	(18,854)
Purchase of treasury stock	0	0	0	0	(14,483)	(14,483)
Stock-based compensation	0	1,648	0	0	6,350	7,998
Employee Stock Ownership Plan	0	765	0	0		2,557
Employee Stock Ownership Plan					1,792
Balance at end 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	57,917	0	0	57,917
Other comprehensive income (loss), net of tax	0	0	0	1,005	0	1,005
Cash dividends paid	0	0	(20,593)	0	0	(20,593)
Purchase of treasury stock	0	0	0	0	(10,738)	(10,738)
Stock-based compensation	0	(473)	0	0	7,390	6,917
Employee Stock Ownership Plan	0	745	0	0		2,519
Employee Stock Ownership Plan					1,774
Balance at end 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	57,393	0	0	57,393
Other comprehensive income (loss), net of tax	0	0	0	2,913	0	2,913
Cash dividends paid	0	0	(11,218)	0	0	(11,218)
Purchase of treasury stock	0	0	0	0	(13,482)	(13,482)
Stock-based compensation	0	1,712	0	0	6,389	8,101
Employee Stock Ownership Plan	0	630	0	0		2,301
Employee Stock Ownership Plan					1,671
Balance at end of period at Dec. 31, 2020	$ 47,872	$ 105,084	$ 463,612	$ (5,676)	$ (60,656)	$ 550,236