CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Capital in Excess of Par Value	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Treasury Stock
Beginning Balance at Dec. 31, 2016	$ 687,336	$ 580	$ 329,457	$ (7,617)	$ 627,078	$ (262,162)
Beginning Balance (in shares) at Dec. 31, 2016		55,995,219
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	18,937				18,937
Other comprehensive income (loss)	830			830
Cash dividend	(2,199)				(2,199)
Issuance of common stock including proceeds from exercise of common stock options	1,094	$ 1	1,093
Issuance of common stock including proceeds from exercise of common stock options (in shares)		129,781
Stock-based compensation expense	821		821
Repurchase of common stock	$ (2,818)	$ 0				(2,818)
Repurchase of common stock (in shares)	62,000
Ending Balance at Mar. 31, 2017	$ 704,001	$ 581	331,371	(6,787)	643,816	(264,980)
Ending Balance (in shares) at Mar. 31, 2017		56,125,000
Beginning Balance at Dec. 31, 2017	724,345	$ 563	336,271	(8,152)	669,557	(273,894)
Beginning Balance (in shares) at Dec. 31, 2017		56,279,527
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	37,350				37,350
Other comprehensive income (loss)	(12,668)			(12,668)
Cash dividend	(2,826)				(2,826)
Issuance of common stock including proceeds from exercise of common stock options	2,613	$ 1	2,612
Issuance of common stock including proceeds from exercise of common stock options (in shares)		115,032
Stock-based compensation expense	946		946
Repurchase of common stock	$ (3,481)	$ 0				(3,481)
Repurchase of common stock (in shares)	70,000	0
Ending Balance at Mar. 31, 2018	$ 746,279	$ 564	$ 339,829	$ (20,820)	$ 704,081	$ (277,375)
Ending Balance (in shares) at Mar. 31, 2018		56,394,559