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	47,940				47,940
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	268					268
Unrealized net gain (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)
Beginning Balance at Dec. 31, 2017	514,768	121	331,081	(159,791)	353,583	(10,226)
Net income	127,021
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	(1,602)
Ending Balance at Sep. 30, 2018	578,618	121	336,079	(178,776)	433,227	(12,033)
Beginning Balance at Mar. 31, 2018	537,164	121	327,669	(166,059)	385,725	(10,292)
Net income	40,487				40,487
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	(1,288)					(1,288)
Unrealized net gain (loss) on securities available-for-sale, net of tax	57					57
Stock-based compensation	3,650		3,650
Exercise of stock options	113		113
Withholding tax payments on restricted stock vesting and stock option exercises	(376)		(376)
Repurchases of common stock	(6,636)			(6,636)
Cash dividend on common stock	(15,793)				(15,793)
Ending Balance at Jun. 30, 2018	557,378	121	331,056	(172,695)	410,419	(11,523)
Net income	38,594				38,594
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	(582)					(582)
Unrealized net gain (loss) on securities available-for-sale, net of tax	72					72
Stock-based compensation	3,950		3,950
Exercise of stock options	1,016		1,016
Withholding tax payments on restricted stock vesting and stock option exercises	57		57
Repurchases of common stock	(6,081)			(6,081)
Cash dividend on common stock	(15,786)				(15,786)
Ending Balance at Sep. 30, 2018	578,618	121	336,079	(178,776)	433,227	(12,033)
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 gain (loss) 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	(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 gain (loss) 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	(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 gain (loss) 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	(19,268)				(19,268)
Ending Balance at Sep. 30, 2019	$ 694,623	$ 122	$ 343,543	$ (196,402)	$ 560,164	$ (12,804)