Consolidated Statement of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning Balance (in shares) at Dec. 31, 2017		173,507
Beginning Balance at Dec. 31, 2017	$ 1,095,673	$ 347	$ 461,338	$ 668,587	$ (34,599)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under stock plans (in shares)		881
Issuance of common stock under stock plans	14,811	$ 2	14,809
Repurchase of common stock (in shares)		(2,356)
Repurchase of common stock	(121,308)	$ (5)		(121,303)
Stock-based compensation expense	22,196		22,196
Payment of dividends	(15,524)			(15,524)
Net income	93,413			93,413
Net unrealized gain (loss) on available-for-sale investments, net of tax of ($106)	(702)				(702)
Reclassification of net realized (gain) loss on the sale of available-for-sale investments	(269)				(269)
Foreign currency translation adjustment	(3,197)				(3,197)
Ending Balance (in shares) at Jul. 01, 2018		172,032
Ending Balance at Jul. 01, 2018	1,079,132	$ 344	498,343	619,212	(38,767)
Beginning Balance (in shares) at Apr. 01, 2018		172,948
Beginning Balance at Apr. 01, 2018	1,077,099	$ 346	485,654	622,856	(31,757)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under stock plans (in shares)		184
Issuance of common stock under stock plans	3,688	$ 1	3,687
Repurchase of common stock (in shares)		(1,100)
Repurchase of common stock	(52,106)	$ (3)		(52,103)
Stock-based compensation expense	9,002		9,002
Payment of dividends	(7,737)			(7,737)
Net income	56,196			56,196
Net unrealized gain (loss) on available-for-sale investments, net of tax of ($106)	490				490
Reclassification of net realized (gain) loss on the sale of available-for-sale investments	(247)				(247)
Foreign currency translation adjustment	(7,253)				(7,253)
Ending Balance (in shares) at Jul. 01, 2018		172,032
Ending Balance at Jul. 01, 2018	$ 1,079,132	$ 344	498,343	619,212	(38,767)
Beginning Balance (in shares) at Dec. 31, 2018	170,820	170,820
Beginning Balance at Dec. 31, 2018	$ 1,135,263	$ 342	529,208	646,214	(40,501)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under stock plans (in shares)		1,239
Issuance of common stock under stock plans	26,417	$ 2	26,415
Repurchase of common stock (in shares)		(1,398)
Repurchase of common stock	(61,690)	$ (3)		(61,687)
Stock-based compensation expense	23,248		23,248
Payment of dividends	(17,146)			(17,146)
Net income	81,853			81,853
Net unrealized gain (loss) on available-for-sale investments, net of tax of ($106)	4,562				4,562
Reclassification of net realized (gain) loss on the sale of available-for-sale investments	(422)				(422)
Foreign currency translation adjustment	$ (445)				(445)
Ending Balance (in shares) at Jun. 30, 2019	170,661	170,661
Ending Balance at Jun. 30, 2019	$ 1,191,640	$ 341	578,871	649,234	(36,806)
Beginning Balance (in shares) at Mar. 31, 2019		171,537
Beginning Balance at Mar. 31, 2019	1,188,859	$ 343	555,834	670,754	(38,072)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under stock plans (in shares)		522
Issuance of common stock under stock plans	12,071	$ 1	12,070
Repurchase of common stock (in shares)		(1,398)
Repurchase of common stock	(61,690)	$ (3)		(61,687)
Stock-based compensation expense	10,967		10,967
Payment of dividends	(8,582)			(8,582)
Net income	48,749			48,749
Net unrealized gain (loss) on available-for-sale investments, net of tax of ($106)	2,311				2,311
Reclassification of net realized (gain) loss on the sale of available-for-sale investments	(382)				(382)
Foreign currency translation adjustment	$ (663)				(663)
Ending Balance (in shares) at Jun. 30, 2019	170,661	170,661
Ending Balance at Jun. 30, 2019	$ 1,191,640	$ 341	$ 578,871	$ 649,234	$ (36,806)