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	64,514	0	0	64,514
Cash dividends	0	0	(30,073)	0	0	(30,073)
Net issuance of shares to employee and other stock plans	0	(4,596)	0	0	4,026	(570)
Stock-based compensation	0	3,290	(95)	0	0	3,195
Other comprehensive (loss) 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
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	83,914	0	0	83,914
Cash dividends	0	0	(31,899)	0	0	(31,899)
Net issuance of shares to employee and other stock plans	0	(2,511)	0	0	1,859	(652)
Stock-based compensation	0	3,457	0	0	0	3,457
Other comprehensive (loss) income	0	0	0	(18,815)	0	(18,815)
Balance at Sep. 30, 2018	$ 497	$ 575,155	$ 603,921	$ (49,112)	$ (136,306)	$ 994,155