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	60,076				60,076
Other comprehensive income (loss)	4,494			4,494
Cash dividend	(6,600)				(6,600)
Issuance of common stock including proceeds from exercise of common stock options	2,317	$ 2	2,315
Issuance of common stock including proceeds from exercise of common stock options (in shares)		222,424
Stock-based compensation expense	2,449		2,449
Repurchase of common stock	$ (9,211)					(9,191)
Repurchase of common stock (in shares)	(204,000)	(20,000)
Ending Balance at Sep. 30, 2017	$ 740,861	$ 562	334,221	(3,123)	680,554	(271,353)
Ending Balance (in shares) at Sep. 30, 2017		56,217,643
Beginning Balance at Jun. 30, 2017				(3,858)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	20,569
Other comprehensive income (loss)	735
Ending Balance at Sep. 30, 2017	740,861	$ 562	334,221	(3,123)	680,554	(271,353)
Ending Balance (in shares) at Sep. 30, 2017		56,217,643
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	105,025				105,025
Other comprehensive income (loss)	(23,849)			(23,869)
Cash dividend	(9,797)				(9,797)
Issuance of common stock including proceeds from exercise of common stock options	9,776	$ 4	9,772
Issuance of common stock including proceeds from exercise of common stock options (in shares)		600,808
Stock-based compensation expense	1,857		1,857
Repurchase of common stock	$ (8,515)					(8,515)
Repurchase of common stock (in shares)	(167,000)
Ending Balance at Sep. 30, 2018	$ 798,822	$ 567	347,900	(32,001)	764,765	(282,409)
Ending Balance (in shares) at Sep. 30, 2018		56,880,335
Beginning Balance at Jun. 30, 2018				(25,713)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	38,935
Other comprehensive income (loss)	(6,288)
Ending Balance at Sep. 30, 2018	$ 798,822	$ 567	$ 347,900	$ (32,001)	$ 764,765	$ (282,409)
Ending Balance (in shares) at Sep. 30, 2018		56,880,335