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, 2015	$ 580,471	$ 560	$ 256,435	$ 696	$ 570,630	$ (247,850)
Beginning Balance (in shares) at Dec. 31, 2015		55,945,245
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	33,248				33,248
Other comprehensive income	14,981			14,981
Cash dividend	(3,556)				(3,556)
Issuance of common stock including proceeds from exercise of common stock options	1,723	$ 2	1,721
Issuance of common stock including proceeds from exercise of common stock options (in shares)		145,276
Stock-based compensation expense	1,363		1,363
Repurchase of common stock	(11,034)					(11,034)
Ending Balance at Jun. 30, 2016	617,196	$ 562	259,519	15,677	600,322	(258,884)
Ending Balance (in shares) at Jun. 30, 2016		56,090,521
Beginning Balance at Mar. 31, 2016				11,454
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	17,478
Other comprehensive income	4,223
Ending Balance at Jun. 30, 2016	617,196	$ 562	259,519	15,677	600,322	(258,884)
Ending Balance (in shares) at Jun. 30, 2016		56,090,521
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	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)
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	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