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	20,279	0	0	20,279
Cash dividends	0	0	(10,020)	0	0	(10,020)
Net issuance of shares to employee benefit plans and other stock plans	0	(3,712)	0	0	2,882	(830)
Stock-based compensation	0	2,261	(95)	0	0	2,166
Other comprehensive income (loss)	0	0	0	1,928	0	1,928
Balance at Mar. 31, 2017	497	573,627	511,925	(19,592)	(139,618)	926,839
ASU implementation | ASU 2016-01 [Member]	0	0	1,475	(2,628)	0	(1,153)
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	25,986	0	0	25,986
Cash dividends	0	0	(20,966)	0	0	(20,966)
Net issuance of shares to employee benefit plans and other stock plans	0	(2,037)	0	0	980	(1,057)
Stock-based compensation	0	2,454	0	0	0	2,454
Other comprehensive income (loss)	0	0	0	(10,711)	0	(10,711)
Balance at Mar. 31, 2018	$ 497	$ 574,626	$ 555,783	$ (40,991)	$ (137,185)	$ 952,730