CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance (in shares) at Dec. 31, 2015		53,024,972
Balance at Dec. 31, 2015	$ 784,794	$ 530	$ 542,859	$ 249,567	$ (8,162)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units and restricted stock awards (in shares)		397,182
Vesting of restricted stock units and restricted stock awards	(7,160)	$ 4	(7,164)
Employee stock purchase plan (in shares)		242,303
Employee stock purchase plan	7,507	$ 2	7,505
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		185,484
Exercise of common stock options	2,251	$ 2	2,249
Stock Repurchased and Retired During Period, Shares		(1,133,553)
Stock Repurchased and Retired During Period, Value	(43,091)	$ (11)	(11,885)	(31,195)
Tax deficiency from stock-based compensation	2,739		2,739
Stock-based compensation expense	26,559		26,559
Translation adjustments	(1,861)				(1,861)
Net income	97,201			97,201
Balance (in shares) at Dec. 31, 2016		52,716,388
Balance at Dec. 31, 2016	868,939	$ 527	562,862	315,573	(10,023)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units and restricted stock awards (in shares)		413,372
Vesting of restricted stock units and restricted stock awards	(15,801)	$ 4	(15,805)
Employee stock purchase plan (in shares)		214,466
Employee stock purchase plan	7,404	$ 2	7,402
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		36,561
Exercise of common stock options	351		351
Stock Repurchased and Retired During Period, Shares		(1,229,249)
Stock Repurchased and Retired During Period, Value	(58,070)	$ (12)	(13,229)	(44,829)
Stock-based compensation expense	24,509		24,509
Translation adjustments	6,384				6,384
Net income	$ 157,675			157,675
Balance (in shares) at Dec. 31, 2017	52,151,538	52,151,538
Balance at Dec. 31, 2017	$ 991,391	$ 521	566,090	428,419	(3,639)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted stock units and restricted stock awards (in shares)		155,029
Vesting of restricted stock units and restricted stock awards	(4,695)	$ 2	(4,697)
Employee stock purchase plan (in shares)		160,407
Employee stock purchase plan	8,825	$ 2	8,823
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		44,466
Exercise of common stock options	547		547
Stock-based compensation expense	30,991		30,991
Translation adjustments	(2,703)				(2,703)
Net income	$ 157,706			157,706
Balance (in shares) at Dec. 31, 2018	52,511,440	52,511,440
Balance at Dec. 31, 2018	$ 1,182,062	$ 525	$ 601,754	$ 586,125	$ (6,342)