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, 2016	$ 468,013	$ 120	$ 342,311	$ (117,330)	$ 255,140	$ (12,228)
Net income	148,089				148,089
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	2,236					2,236
Unrealized net gain (loss) on securities available-for-sale, net of tax	(234)					(234)
Stock-based compensation	14,421		14,421
Exercise of stock options	1,972	1	1,971
Withholding tax payments on restricted stock vesting and stock option exercises	(27,691)		(27,691)
Repurchases of common stock	(42,461)			(42,461)
Cumulative effect of change in accounting for employee share-based payments	17		69		(52)
Cash dividend on common stock	(49,594)				(49,594)
Ending 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 gain (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 gain (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)