Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]
Balance at Dec. 31, 2014	$ 788,689	$ 497	$ 568,896	$ (113,992)	$ (7,357)	$ 340,645
Balance, shares treasury stock at Dec. 31, 2014				(2,736)
Balance, shares common stock at Dec. 31, 2014		49,671
Comprehensive income (loss), net of tax	22,181				(10,018)	32,199
Exercise of common stock options	$ 1,902	$ 1	1,901
Exercise of common stock options, shares	442	123
Restricted stock vested		$ 9	(9)
Restricted stock vested, shares		925
Common stock issued in connection with business acquisitions	$ 36,567	$ 8	36,559
Common stock issued in connection with business acquisitions, shares		787
Stock-based compensation, net of cash settlements	33,741		33,741
Non-cash settlement of vacation liabilities by issuing RSUs	1,353		1,353
Non-cash settlement of employee-related liabilities by issuing RSUs	1,353
Stock repurchases	(76,447)			$ (76,447)
Stock repurchases, shares				(1,740)
Stock issued pursuant to ESPP	9,547	$ 3	9,544
Stock issued pursuant to ESPP, shares		302
Tax benefit from employee stock plans	5,369		5,369
Balance at Dec. 31, 2015	822,902	$ 518	657,354	$ (190,439)	(17,375)	372,844
Balance, shares treasury stock at Dec. 31, 2015				(4,476)
Balance, shares common stock at Dec. 31, 2015		51,808
Comprehensive income (loss), net of tax	37,749				(7,200)	44,949
Exercise of common stock options	$ 1,345	$ 1	1,344
Exercise of common stock options, shares	115	116
Restricted stock vested		$ 8	(8)
Restricted stock vested, shares		787
Common stock issued in connection with business acquisitions	$ 73		73
Common stock issued in connection with business acquisitions, shares		30
Stock-based compensation, net of cash settlements	31,726		31,726
Non-cash settlement of vacation liabilities by issuing RSUs	3,059		3,059
Non-cash settlement of employee-related liabilities by issuing RSUs	3,059
Stock repurchases	(83,291)			$ (83,291)
Stock repurchases, shares				(1,981)
Stock issued pursuant to ESPP	9,759	$ 3	9,756
Stock issued pursuant to ESPP, shares		297
Balance at Dec. 31, 2016	$ 826,015	$ 530	705,901	$ (273,730)	(24,575)	417,889
Balance, shares treasury stock at Dec. 31, 2016	(6,457)			(6,457)
Balance, shares common stock at Dec. 31, 2016		53,038
Cumulative effect adjustment upon adoption of ASU 2016-09	$ 2,839		2,743			96
Comprehensive income (loss), net of tax	17,368				32,713	(15,345)
Exercise of common stock options	$ 2,066	$ 2	2,064
Exercise of common stock options, shares	165	166
Restricted stock vested		$ 7	(7)
Restricted stock vested, shares		761
Stock-based compensation, net of cash settlements	$ 26,532		26,532
Non-cash settlement of employee-related liabilities by issuing RSUs	1,166		1,166
Stock repurchases	(101,844)			$ (101,844)
Stock repurchases, shares				(2,613)
Stock issued pursuant to ESPP	10,008	$ 3	10,005
Stock issued pursuant to ESPP, shares		284
Balance at Dec. 31, 2017	$ 781,311	$ 542	$ 745,661	$ (375,574)	$ 8,138	$ 402,544
Balance, shares treasury stock at Dec. 31, 2017	(9,070)			(9,070)
Balance, shares common stock at Dec. 31, 2017		54,249