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, 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)