Consolidated Statement of Changes in Stockholders' Equity - 3 months ended Mar. 31, 2018 - USD ($) $ in Thousands	Total	Common Stock Voting [Member]	Additional Paid-in Capital [Member]	Treasury Stock - Common Stock Voting [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]
Beginning Balance at Dec. 31, 2017	$ 514,768	$ 121	$ 331,081	$ (159,791)	$ 353,583	$ (10,226)
Net income	47,940				47,940
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	268					268
Unrealized net loss on securities available-for-sale, net of tax	(334)					(334)
Stock-based compensation	3,951		3,951
Exercise of stock options	258		258
Withholding tax payments on restricted stock vesting and stock option exercises	(7,621)		(7,621)
Repurchases of common stock	(6,268)			(6,268)
Cash dividend on common stock	(15,798)				(15,798)
Ending Balance at Mar. 31, 2018	$ 537,164	$ 121	$ 327,669	$ (166,059)	$ 385,725	$ (10,292)