Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Common Stock [Member]	Additional Paid-in-Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive (Loss) Income [Member]	Common Stock in Treasury [Member]	Total
Balance at Dec. 31, 2014	$ 497	$ 576,504	$ 423,956	$ (17,027)	$ (119,749)	$ 864,181
Net income	0	0	76,425	0	0	76,425
Cash dividends	0	0	(38,149)	0	0	(38,149)
Purchase of treasury shares	0	0	0	0	(26,797)	(26,797)
Net issuance of shares to employee and other stock plans, including tax benefit for 2016 and 2015	0	(3,864)	0	0	11,513	7,649
Stock-based compensation	0	4,086	0	0	0	4,086
Other comprehensive income (loss)	0	0	0	(5,391)	0	(5,391)
Balance at Dec. 31, 2015	497	576,726	462,232	(22,418)	(135,033)	882,004
Net income	0	0	78,409	0	0	78,409
Cash dividends	0	0	(38,880)	0	0	(38,880)
Purchase of treasury shares	0	0	0	0	(17,193)	(17,193)
Net issuance of shares to employee and other stock plans, including tax benefit for 2016 and 2015	0	(6,026)	0	0	9,726	3,700
Stock-based compensation	0	4,378	0	0	0	4,378
Other comprehensive income (loss)	0	0	0	898	0	898
Balance at Dec. 31, 2016	497	575,078	501,761	(21,520)	(142,500)	913,316
Net income	0	0	82,151	0	0	82,151
Cash dividends	0	0	(40,104)	0	0	(40,104)
Net issuance of shares to employee and other stock plans, including tax benefit for 2016 and 2015	0	(4,608)	0	0	4,335	(273)
Stock-based compensation	0	3,739	(95)	0	0	3,644
Other comprehensive income (loss)	0	0	0	(557)	0	(557)
Balance at Dec. 31, 2017	$ 497	$ 574,209	$ 543,713	$ (22,077)	$ (138,165)	$ 958,177