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, 2014	$ 334,208	$ 120	$ 307,059	$ (70,247)	$ 101,813	$ (4,537)
Net income	96,037				96,037
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	(512)					(512)
Unrealized net gain (loss) on securities available-for-sale, net of tax	(180)					(180)
Stock-based compensation	12,519		12,519
Exercise of stock options	1,824	1	1,823
Withholding tax payments on restricted stock vesting and stock option exercises	(4,496)		(4,496)
Excess tax benefits from stock-based compensation	4,310		4,310
Repurchases of common stock	(23,158)			(23,158)
Cash dividend on common stock	(29,839)				(29,839)
Ending Balance at Dec. 31, 2015	390,713	121	321,215	(93,405)	168,011	(5,229)
Net income	126,172				126,172
Cumulative translation adjustment and foreign currency exchange hedge, net of tax	(7,037)					(7,037)
Unrealized net gain (loss) on securities available-for-sale, net of tax	38					38
Stock-based compensation	14,511		14,511
Exercise of stock options	2,299		2,299
Withholding tax payments on restricted stock vesting and stock option exercises	(5,173)	(1)	(5,172)
Excess tax benefits from stock-based compensation	9,458		9,458
Repurchases of common stock	(23,925)			(23,925)
Cash dividend on common stock	(39,043)				(39,043)
Ending 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)