CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Common Stock [Member]	Common Stock [Member] Adjustments Due to Adoption of ASU 2014-09 [Member]	Capital in Excess of Par Value [Member]	Capital in Excess of Par Value [Member] Adjustments Due to Adoption of ASU 2014-09 [Member]	Retained Earnings [Member]	Retained Earnings [Member] Adjustments Due to Adoption of ASU 2014-09 [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Other Comprehensive Income (Loss) [Member] Adjustments Due to Adoption of ASU 2014-09 [Member]	Treasury Stock [Member]	Treasury Stock [Member] Adjustments Due to Adoption of ASU 2014-09 [Member]	Total	Adjustments Due to Adoption of ASU 2014-09 [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect adjustment (Note 2) | ASC 606 [Member]		$ 0		$ 0		$ (1,189)		$ 0		$ 0		$ (1,189)
Balance at beginning of period at Dec. 31, 2017	$ 47,872		$ 100,057		$ 357,153		$ (4,109)		$ (47,319)		$ 453,654
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings	0		0		23,934		0		0		23,934
Other comprehensive income, net of tax	0		0		0		(3,721)		0		(3,721)
Cash dividends paid	0		0		(9,437)		0		0		(9,437)
Purchase of treasury stock	0		0		0		0		(7,324)		(7,324)
Stock-based compensation	0		2,581		0		0		1,315		3,896
Employee stock ownership plan	0		765		0		0		1,792		2,557
Balance at end of period at Jun. 30, 2018	47,872		103,403		370,461		(7,830)		(51,536)		462,370
Balance at beginning of period at Mar. 31, 2018	47,872		102,646		359,232		(1,895)		(48,346)		459,509
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings	0		0		15,945		0		0		15,945
Other comprehensive income, net of tax	0		0		0		(5,935)		0		(5,935)
Cash dividends paid	0		0		(4,716)		0		0		(4,716)
Purchase of treasury stock	0		0		0		0		(4,412)		(4,412)
Stock-based compensation	0		757		0		0		1,222		1,979
Balance at end of period at Jun. 30, 2018	47,872		103,403		370,461		(7,830)		(51,536)		462,370
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		31,648		0		0		31,648
Other comprehensive income, net of tax	0		0		0		1,083		0		1,083
Cash dividends paid	0		0		(10,296)		0		0		(10,296)
Purchase of treasury stock	0		0		0		0		(10,738)		(10,738)
Stock-based compensation	0		2,132		0		0		1,716		3,848
Employee stock ownership plan	0		745		0		0		1,774		2,519
Balance at end of period at Jun. 30, 2019	47,872		105,347		401,465		(8,511)		(60,908)		485,265
Balance at beginning of period at Mar. 31, 2019	47,872		104,467		387,170		(8,904)		(57,565)		473,040
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings	0		0		19,432		0		0		19,432
Other comprehensive income, net of tax	0		0		0		393		0		393
Cash dividends paid	0		0		(5,137)		0		0		(5,137)
Purchase of treasury stock	0		0		0		0		(4,411)		(4,411)
Stock-based compensation	0		880		0		0		1,068		1,948
Employee stock ownership plan	0		0		0		0		0		0
Balance at end of period at Jun. 30, 2019	$ 47,872		$ 105,347		$ 401,465		$ (8,511)		$ (60,908)		$ 485,265