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, 2016	$ 497	$ 575,078	$ 501,761	$ (21,520)	$ (142,500)	$ 913,316
Net income	0	0	41,638	0	0	41,638
Cash dividends	0	0	(20,044)	0	0	(20,044)
Net issuance of shares to employee benefit plans and other stock plans	0	(4,702)	0	0	3,886	(816)
Stock-based compensation	0	2,733	(95)	0	0	2,638
Other comprehensive (loss) income	0	0	0	3,629	0	3,629
Balance at Jun. 30, 2017	497	573,109	523,260	(17,891)	(138,614)	940,361
ASU implementation | ASU 2016-01 [Member]	0	0	2,618	(2,645)	0	(27)
ASU implementation | ASU 2018-02 [Member]	0	0	5,575	(5,575)	0	0
Balance at Dec. 31, 2017	497	574,209	543,713	(22,077)	(138,165)	958,177
Net income	0	0	54,107	0	0	54,107
Cash dividends	0	0	(20,969)	0	0	(20,969)
Net issuance of shares to employee benefit plans and other stock plans	0	(2,422)	0	0	1,568	(854)
Stock-based compensation	0	2,954	0	0	0	2,954
Other comprehensive (loss) income	0	0	0	(14,459)	0	(14,459)
Balance at Jun. 30, 2018	$ 497	$ 574,741	$ 585,044	$ (44,756)	$ (136,597)	$ 978,929