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
Cumulative effect adjustment for ASU implementation | ASU 2016-01 [Member]	$ 0	$ 0	$ 2,618	$ (2,645)	$ 0	$ (27)
Cumulative effect adjustment for 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 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
Cumulative effect adjustment for ASU implementation | ASU 2016-01 [Member]	0	0	1,143	(17)	0	1,126
Balance at Mar. 31, 2018	497	574,626	555,783	(40,991)	(137,185)	952,730
Net income	0	0	28,121	0	0	28,121
Cash dividends	0	0	(3)	0	0	(3)
Net issuance of shares to employee and other stock plans	0	(385)	0	0	588	203
Stock-based compensation	0	500	0	0	0	500
Other comprehensive (loss) income	0	0	0	(3,748)	0	(3,748)
Balance at Jun. 30, 2018	497	574,741	585,044	(44,756)	(136,597)	978,929
Balance at Dec. 31, 2018	497	575,466	621,203	(43,174)	(136,083)	1,017,909
Net income	0	0	59,682	0	0	59,682
Cash dividends	0	0	(22,778)	0	0	(22,778)
Net issuance of shares to employee and other stock plans	0	(2,812)	0	0	1,544	(1,268)
Stock-based compensation	0	3,140	0	0	0	3,140
Other comprehensive (loss) income	0	0	0	18,138	0	18,138
Balance at Jun. 30, 2019	497	575,794	658,107	(25,036)	(134,539)	1,074,823
Balance at Mar. 31, 2019	497	575,944	627,556	(34,932)	(135,010)	1,034,055
Net income	0	0	30,555	0	0	30,555
Cash dividends	0	0	(4)	0	0	(4)
Net issuance of shares to employee and other stock plans	0	(713)	0	0	471	(242)
Stock-based compensation	0	563	0	0	0	563
Other comprehensive (loss) income	0	0	0	9,896	0	9,896
Balance at Jun. 30, 2019	$ 497	$ 575,794	$ 658,107	$ (25,036)	$ (134,539)	$ 1,074,823