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	39,507				39,507
Other comprehensive income (loss)	3,759			3,759
Cash dividend	(4,399)				(4,399)
Issuance of common stock including proceeds from exercise of common stock options	885	$ 1	884
Issuance of common stock including proceeds from exercise of common stock options (in shares)		178,583
Stock-based compensation expense	1,564		1,564
Repurchase of common stock	$ (6,029)					(6,029)
Repurchase of common stock (in shares)	133,000
Ending Balance at Jun. 30, 2017	$ 722,623	$ 581	331,905	(3,858)	662,186	(268,191)
Ending Balance (in shares) at Jun. 30, 2017		56,173,802
Beginning Balance at Mar. 31, 2017				(6,787)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	20,570
Other comprehensive income (loss)	2,929
Ending Balance at Jun. 30, 2017	722,623	$ 581	331,905	(3,858)	662,186	(268,191)
Ending Balance (in shares) at Jun. 30, 2017		56,173,802
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification due to the adoption of ASU No. 2016-01	0			20	(20)
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	66,090				66,090
Other comprehensive income (loss)	(17,561)			(17,581)
Cash dividend	(6,298)				(6,298)
Issuance of common stock including proceeds from exercise of common stock options	7,062	$ 3	7,059
Issuance of common stock including proceeds from exercise of common stock options (in shares)		405,857
Stock-based compensation expense	1,420		1,420
Repurchase of common stock	$ (6,061)					(6,061)
Repurchase of common stock (in shares)	120,000
Ending Balance at Jun. 30, 2018	$ 768,977	$ 566	344,750	(25,713)	729,329	(279,955)
Ending Balance (in shares) at Jun. 30, 2018		56,685,384
Beginning Balance at Mar. 31, 2018				(20,820)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	28,740
Other comprehensive income (loss)	(4,893)
Ending Balance at Jun. 30, 2018	$ 768,977	$ 566	$ 344,750	$ (25,713)	$ 729,329	$ (279,955)
Ending Balance (in shares) at Jun. 30, 2018		56,685,384