Consolidated Statements of Changes in Stockholders' Equity - 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	172,852				172,852
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	(2,078)					(2,078)
Unrealized net (loss) on securities available-for-sale, net of tax	(90)					(90)
Stock-based compensation	15,850		15,850
Exercise of stock options	2,973	1	2,972
Withholding tax payments on restricted stock vesting and stock option exercises	(8,043)		(8,043)
Repurchases of common stock	(25,171)			(25,171)
Cash dividend on common stock	(63,183)				(63,183)
Ending Balance at Dec. 31, 2018	607,878	122	341,860	(184,962)	463,252	(12,394)
Net income	204,902				204,902
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	1,128					1,128
Unrealized net (loss) on securities available-for-sale, net of tax	996					996
Stock-based compensation	25,294		25,294
Exercise of stock options	1,207		1,207
Withholding tax payments on restricted stock vesting and stock option exercises	(25,820)		(25,820)
Treasury shares used for acquisition	48,830			48,830
Repurchases of common stock	(17,256)			(17,256)
Cash dividend on common stock	(77,068)				(77,068)
Ending Balance at Dec. 31, 2019	770,091	122	342,541	(153,388)	591,086	(10,270)
Net income	299,377				299,377
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	6,164					6,164
Unrealized net (loss) on securities available-for-sale, net of tax	(544)					(544)
Stock-based compensation	25,613		25,613
Exercise of stock options	4,007	1	4,006
Withholding tax payments on restricted stock vesting and stock option exercises	(42,418)		(42,418)
Repurchases of common stock	(16,135)			(16,135)
Cash dividend on common stock	(91,094)				(91,094)
Ending Balance at Dec. 31, 2020	$ 955,061	$ 123	$ 329,742	$ (169,523)	$ 799,369	$ (4,650)