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)		1,434
Issuance of common stock under stock plans	25,882	$ 3	25,879
Repurchase of common stock (in shares)		(2,750)
Repurchase of common stock	(142,262)	$ (5)		(142,257)
Stock-based compensation expense	31,339		31,339
Payment of dividends	(23,283)			(23,283)
Net income	173,849			173,849
Net unrealized gain (loss) on available-for-sale investments, net of tax of ($82)	(180)				(180)
Reclassification of net realized (gain) loss on the sale of available-for-sale investments	(535)				(535)
Foreign currency translation adjustment	(5,078)				(5,078)
Ending Balance (in shares) at Sep. 30, 2018		172,191
Ending Balance at Sep. 30, 2018	1,149,444	$ 345	518,556	670,935	(40,392)
Beginning Balance (in shares) at Jul. 01, 2018		172,032
Beginning Balance at Jul. 01, 2018	1,079,132	$ 344	498,343	619,212	(38,767)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under stock plans (in shares)		553
Issuance of common stock under stock plans	11,071	$ 1	11,070
Repurchase of common stock (in shares)		(394)
Repurchase of common stock	(20,954)	$ 0		(20,954)
Stock-based compensation expense	9,143		9,143
Payment of dividends	(7,759)			(7,759)
Net income	80,436			80,436
Net unrealized gain (loss) on available-for-sale investments, net of tax of ($82)	522				522
Reclassification of net realized (gain) loss on the sale of available-for-sale investments	(266)				(266)
Foreign currency translation adjustment	(1,881)				(1,881)
Ending Balance (in shares) at Sep. 30, 2018		172,191
Ending Balance at Sep. 30, 2018	$ 1,149,444	$ 345	518,556	670,935	(40,392)
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)		1,475
Issuance of common stock under stock plans	31,780	$ 3	31,777
Repurchase of common stock (in shares)		(1,398)
Repurchase of common stock	(61,690)	$ (3)		(61,687)
Stock-based compensation expense	33,442		33,442
Payment of dividends	(25,682)			(25,682)
Net income	123,538			123,538
Net unrealized gain (loss) on available-for-sale investments, net of tax of ($82)	4,827				4,827
Reclassification of net realized (gain) loss on the sale of available-for-sale investments	(716)				(716)
Foreign currency translation adjustment	$ (1,544)				(1,544)
Ending Balance (in shares) at Sep. 29, 2019	170,897	170,897
Ending Balance at Sep. 29, 2019	$ 1,239,218	$ 342	594,427	682,383	(37,934)
Beginning Balance (in shares) at Jun. 30, 2019		170,661
Beginning Balance at Jun. 30, 2019	1,191,640	$ 341	578,871	649,234	(36,806)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under stock plans (in shares)		236
Issuance of common stock under stock plans	5,363	$ 1	5,362
Stock-based compensation expense	10,194		10,194
Payment of dividends	(8,536)			(8,536)
Net income	41,685			41,685
Net unrealized gain (loss) on available-for-sale investments, net of tax of ($82)	265				265
Reclassification of net realized (gain) loss on the sale of available-for-sale investments	(294)				(294)
Foreign currency translation adjustment	$ (1,099)				(1,099)
Ending Balance (in shares) at Sep. 29, 2019	170,897	170,897
Ending Balance at Sep. 29, 2019	$ 1,239,218	$ 342	$ 594,427	$ 682,383	$ (37,934)