Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Capital in Excess of Par Value [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	Treasury Stock [Member]
Beginning Balance at Dec. 31, 2018	$ 538,898	$ 544	$ 62,239	$ 770,305	$ (137,440)	$ (156,750)
Net income	23,341			23,341
Other comprehensive income (loss)	(8,187)				(8,187)
Comprehensive income	15,154
Cash dividends declared	(17,222)			(17,222)
Share-based compensation expense	2,683		2,683
Delivery of treasury shares
RSUs and PSAs	(324)		(2,421)			2,097
Employee stock options exercised — net	(432)		(1,536)			1,104
Ending Balance at Sep. 30, 2019	538,757	544	60,965	776,424	(145,627)	(153,549)
Beginning Balance at Jun. 30, 2019	543,119	544	59,920	769,940	(133,736)	(153,549)
Net income	12,224			12,224
Other comprehensive income (loss)	(11,891)				(11,891)
Comprehensive income	333
Cash dividends declared	(5,740)			(5,740)
Share-based compensation expense	1,045		1,045
Ending Balance at Sep. 30, 2019	538,757	544	60,965	776,424	(145,627)	(153,549)
Beginning Balance at Dec. 31, 2019	555,959	544	59,900	725,795	(77,896)	(152,384)
Net income	11,517			11,517
Other comprehensive income (loss)	5,190				5,190
Comprehensive income	16,707
Cash dividends declared	(17,740)			(17,740)
Share-based compensation expense	3,994		3,994
Delivery of treasury shares
RSUs and PSAs	(420)		(2,077)			1,657
Employee stock options exercised — net	(75)		(217)			142
Ending Balance at Sep. 30, 2020	558,425	544	61,600	719,572	(72,706)	(150,585)
Beginning Balance at Jun. 30, 2020	540,999	544	59,982	719,035	(87,977)	(150,585)
Net income	6,527			6,527
Other comprehensive income (loss)	15,271				15,271
Comprehensive income	21,798
Cash dividends declared	(5,990)			(5,990)
Share-based compensation expense	1,618		1,618
Ending Balance at Sep. 30, 2020	$ 558,425	$ 544	$ 61,600	$ 719,572	$ (72,706)	$ (150,585)