Consolidated Statements of Stockholders' Equity (unaudited) - 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
Net income	0	0	58,801	0	0	58,801
Cash dividends	0	0	(28,903)	0	0	(28,903)
Purchase of treasury shares	0	0	0	0	(17,193)	(17,193)
Net issuance of shares to employee benefit plans and other stock plans, including tax benefit	0	(6,164)	0	0	7,270	1,106
Stock-based compensation	0	3,096	0	0	0	3,096
Other comprehensive income	0	0	0	10,438	0	10,438
Balance at Sep. 30, 2016	497	573,658	492,130	(11,980)	(144,956)	909,349
Balance at Dec. 31, 2016	497	575,078	501,761	(21,520)	(142,500)	913,316
Net income	0	0	64,514	0	0	64,514
Cash dividends	0	0	(30,073)	0	0	(30,073)
Net issuance of shares to employee benefit plans and other stock plans, including tax benefit	0	(4,596)	0	0	4,026	(570)
Stock-based compensation	0	3,290	(95)	0	0	3,195
Other comprehensive income	0	0	0	4,856	0	4,856
Balance at Sep. 30, 2017	$ 497	$ 573,772	$ 536,107	$ (16,664)	$ (138,474)	$ 955,238