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, 2018	$ 607,878	$ 122	$ 341,860	$ (184,962)	$ 463,252	$ (12,394)
Net income	52,522				52,522
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	1,118					1,118
Unrealized net (loss) gain on securities available-for-sale, net of tax	459					459
Stock-based compensation	5,196		5,196
Exercise of stock options	172		172
Withholding tax payments on restricted stock vesting and stock option exercises	(11,803)		(11,803)
Repurchases of common stock	(5,184)			(5,184)
Cash dividend on common stock ($0.60 per share)	(19,212)				(19,212)
Ending Balance at Mar. 31, 2019	631,146	122	335,425	(190,146)	496,562	(10,817)
Beginning Balance at Dec. 31, 2018	607,878	122	341,860	(184,962)	463,252	(12,394)
Net income	154,629
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	(1,398)
Ending Balance at Sep. 30, 2019	694,623	122	343,543	(196,402)	560,164	(12,804)
Beginning Balance at Mar. 31, 2019	631,146	122	335,425	(190,146)	496,562	(10,817)
Net income	48,105				48,105
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	(1,100)					(1,100)
Unrealized net (loss) gain on securities available-for-sale, net of tax	459					459
Stock-based compensation	5,929		5,929
Exercise of stock options	350		350
Withholding tax payments on restricted stock vesting and stock option exercises	(258)		(258)
Repurchases of common stock	(3,486)			(3,486)
Cash dividend on common stock ($0.60 per share)	(19,237)				(19,237)
Ending Balance at Jun. 30, 2019	661,908	122	341,446	(193,632)	525,430	(11,458)
Net income	54,002				54,002
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	(1,416)					(1,416)
Unrealized net (loss) gain on securities available-for-sale, net of tax	70					70
Stock-based compensation	7,944		7,944
Exercise of stock options	363		363
Withholding tax payments on restricted stock vesting and stock option exercises	(6,210)		(6,210)
Repurchases of common stock	(2,770)			(2,770)
Cash dividend on common stock ($0.60 per share)	(19,268)				(19,268)
Ending Balance at Sep. 30, 2019	694,623	122	343,543	(196,402)	560,164	(12,804)
Beginning Balance at Dec. 31, 2019	770,091	122	342,541	(153,388)	591,086	(10,270)
Net income	74,816				74,816
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	(3,859)					(3,859)
Unrealized net (loss) gain on securities available-for-sale, net of tax	(491)					(491)
Stock-based compensation	6,677		6,677
Exercise of stock options	80		80
Withholding tax payments on restricted stock vesting and stock option exercises	(21,243)		(21,243)
Repurchases of common stock	(5,415)			(5,415)
Cash dividend on common stock ($0.60 per share)	(22,773)				(22,773)
Ending Balance at Mar. 31, 2020	797,883	122	328,055	(158,803)	643,129	(14,620)
Beginning Balance at Dec. 31, 2019	770,091	122	342,541	(153,388)	591,086	(10,270)
Net income	226,446
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	213
Ending Balance at Sep. 30, 2020	897,539	122	327,646	(168,841)	749,211	(10,599)
Beginning Balance at Mar. 31, 2020	797,883	122	328,055	(158,803)	643,129	(14,620)
Net income	83,854				83,854
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	(351)					(351)
Unrealized net (loss) gain on securities available-for-sale, net of tax	820					820
Stock-based compensation	5,791		5,791
Exercise of stock options	659		659
Withholding tax payments on restricted stock vesting and stock option exercises	(6,535)		(6,535)
Repurchases of common stock	(5,873)			(5,873)
Cash dividend on common stock ($0.60 per share)	(22,764)				(22,764)
Ending Balance at Jun. 30, 2020	853,484	122	327,970	(164,676)	704,219	(14,151)
Net income	67,776				67,776
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	4,423					4,423
Unrealized net (loss) gain on securities available-for-sale, net of tax	(871)					(871)
Stock-based compensation	6,451		6,451
Exercise of stock options	9		9
Withholding tax payments on restricted stock vesting and stock option exercises	(6,784)		(6,784)
Repurchases of common stock	(4,165)			(4,165)
Cash dividend on common stock ($0.60 per share)	(22,784)				(22,784)
Ending Balance at Sep. 30, 2020	$ 897,539	$ 122	$ 327,646	$ (168,841)	$ 749,211	$ (10,599)