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, 2013	$ 497	$ 574,152	$ 385,787	$ (16,765)	$ (127,102)	$ 816,569
Net income	0	0	75,074	0	0	75,074
Cash dividends	0	0	(36,905)	0	0	(36,905)
Purchase of treasury shares	0	0	0	0	(72)	(72)
Net issuance of shares to employee stock plans, including tax benefit	0	(1,169)	0	0	7,425	6,256
Stock-based compensation	0	3,521	0	0	0	3,521
Other comprehensive income (loss)	0	0	0	(262)	0	(262)
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 stock plans, including tax benefit	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 stock plans, including tax benefit	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