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, 2015	$ 497	$ 576,726	$ 462,232	$ (22,418)	$ (135,033)	$ 882,004
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	0	0	78,409	0	0	78,409
Cash dividends	0	0	(38,880)	0	0	(38,880)
Purchase of 675,535 treasury shares	0	0	0	0	(17,193)	(17,193)
Net issuance of shares to employee and other stock plans	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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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	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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect adjustment ASU 2016-01 implementation | ASU 2016-01 [Member]			2,618	(2,645)		(27)
Cumulative effect adjustment for ASU 2018-02 implementation | ASU 2018-02 [Member]			5,575	(5,575)		0
Net income	0	0	112,566	0	0	112,566
Cash dividends	0	0	(43,269)	0	0	(43,269)
Net issuance of shares to employee and other stock plans	0	(2,679)	0	0	2,082	(597)
Stock-based compensation	0	3,936	0	0	0	3,936
Other comprehensive income (loss)	0	0	0	(12,877)	0	(12,877)
Balance at Dec. 31, 2018	$ 497	$ 575,466	$ 621,203	$ (43,174)	$ (136,083)	$ 1,017,909