Consolidated Statement of Changes in Stockholders' Equity - 6 months ended Jun. 30, 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	88,427				88,427
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	(1,020)					(1,020)
Unrealized net loss on securities available-for-sale, net of tax	(277)					(277)
Stock-based compensation	7,601		7,601
Exercise of stock options	371		371
Withholding tax payments on restricted stock vesting and stock option exercises	(7,997)		(7,997)
Repurchases of common stock	(12,904)			(12,904)
Cash dividend on common stock	(31,591)				(31,591)
Ending Balance at Jun. 30, 2018	$ 557,378	$ 121	$ 331,056	$ (172,695)	$ 410,419	$ (11,523)