CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Common Stock 2012 Directors Plan	Common Stock Restricted Stock Units (RSUs)	Additional Paid-in Capital	Additional Paid-in Capital Restricted Stock Units (RSUs)	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive (Loss)/ Income
Balance, beginning of period at Dec. 31, 2014	$ 464,050	$ 48			$ 229,501		$ 260,598		$ (22,054)
Balance, beginning of period (in shares) at Dec. 31, 2014		48,303,811
Treasury stock, beginning of period at Dec. 31, 2014								$ (4,043)
Treasury stock, beginning of period (in shares) at Dec. 31, 2014								444,487
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock issued in connection with acquisition (Note 3)	5,081				1,118			$ 3,963
Stock issued in connection with acquisition (Note 3) (in shares)		435,462						435,462
Forfeiture of stock issued in connection with acquisition					13			$ (13)
Forfeiture of stock issued in connection with acquisition (in shares)		(1,482)						1,482
Stock-based compensation expense	43,120				43,120
Stock Issued During Period, Shares, Share-based Compensation, Net of Forfeitures [Abstract]
Stock issued (in shares)			5,295
Stock units vested (in shares)				17,625
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures [Abstract]
Stock units vested	574					$ 574
Proceeds from stock option exercises	20,675	$ 1			20,674
Proceeds from stock options exercises (in shares)		1,405,826
Excess tax benefits	8,363				8,363
Foreign currency translation adjustments	(13,096)								(13,096)
Net income	84,456						84,456
Balance, end of period at Dec. 31, 2015	613,223	$ 49			303,363		345,054		(35,150)
Balance, end of period (in shares) at Dec. 31, 2015		50,166,537
Treasury stock, end of period at Dec. 31, 2015								$ (93)
Treasury stock, end of period (in shares) at Dec. 31, 2015								10,507
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Forfeiture of stock issued in connection with acquisition					84			$ (84)
Forfeiture of stock issued in connection with acquisition (in shares)		(9,228)						9,228
Stock-based compensation expense	46,100				46,100
Stock Issued During Period, Shares, Share-based Compensation, Net of Forfeitures [Abstract]
Stock issued (in shares)			6,510
Stock units vested, net of shares withheld for employee taxes (in shares)				38,064
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures [Abstract]
Stock units vested, net of shares withheld for employee taxes	2,069					2,069
Proceeds from stock option exercises	18,028	$ 1			18,027
Proceeds from stock options exercises (in shares)		895,804
Excess tax benefits	5,264				5,264
Foreign currency translation adjustments	(2,538)								(2,538)
Net income	99,266						99,266
Balance, end of period at Dec. 31, 2016	$ 781,412	$ 50			374,907		444,320		(37,688)
Balance, end of period (in shares) at Dec. 31, 2016	51,097,687	51,097,687
Treasury stock, end of period at Dec. 31, 2016								$ (177)
Treasury stock, end of period (in shares) at Dec. 31, 2016								19,735
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	$ 48,173				48,173
Stock Issued During Period, Shares, Share-based Compensation, Net of Forfeitures [Abstract]
Stock units vested (in shares)				140,043
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures [Abstract]
Stock units withheld for employee taxes	(3,300)					$ (3,300)
Stock units withheld for employee taxes (in shares)				(43,479)
Proceeds from stock option exercises	54,097	$ 3			54,094
Proceeds from stock options exercises (in shares)		1,789,434
Foreign currency translation adjustments	20,065								20,065
Cumulative effect of the adoption of ASU	1,740
Cumulative effect of the adoption of ASU | ASU 2016-09	1,740
Net income	72,760						72,760
Balance, end of period at Dec. 31, 2017	$ 974,947	$ 53			$ 473,874		$ 518,820		$ (17,623)
Balance, end of period (in shares) at Dec. 31, 2017	52,983,685	52,983,685
Treasury stock, end of period at Dec. 31, 2017								$ (177)
Treasury stock, end of period (in shares) at Dec. 31, 2017								19,735