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, 2018		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 ($82)	4,562				4,562
Reclassification of credit (recovery) loss on investments	0
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
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 ($82)	2,311				2,311
Reclassification of credit (recovery) loss on investments	0
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
Ending Balance at Jun. 30, 2019	$ 1,191,640	$ 341	578,871	649,234	(36,806)
Beginning Balance (in shares) at Dec. 31, 2019	172,440	172,440
Beginning Balance at Dec. 31, 2019	$ 1,355,710	$ 345	639,372	753,268	(37,275)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under stock plans (in shares)		1,822
Issuance of common stock under stock plans	48,235	$ 3	48,232
Repurchase of common stock (in shares)		(1,215)
Repurchase of common stock	(51,036)	$ (2)		(51,034)
Stock-based compensation expense	22,808		22,808
Payment of dividends	(18,972)			(18,972)
Net income	19,335			19,335
Net unrealized gain (loss) on available-for-sale investments, net of tax of ($82)	8,590				8,590
Reclassification of credit (recovery) loss on investments	75				75
Reclassification of net realized (gain) loss on the sale of available-for-sale investments	(2,805)				(2,805)
Foreign currency translation adjustment	$ (5,462)				(5,462)
Ending Balance (in shares) at Jun. 28, 2020	173,047	173,047
Ending Balance at Jun. 28, 2020	$ 1,376,478	$ 346	710,412	702,597	(36,877)
Beginning Balance (in shares) at Mar. 29, 2020		171,688
Beginning Balance at Mar. 29, 2020	1,327,493	$ 344	664,132	713,208	(50,191)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under stock plans (in shares)		1,359
Issuance of common stock under stock plans	38,264	$ 2	38,262
Repurchase of common stock (in shares)		0
Repurchase of common stock	0	$ 0		0
Stock-based compensation expense	8,018		8,018
Payment of dividends	(9,469)			(9,469)
Net income	(1,142)			(1,142)
Net unrealized gain (loss) on available-for-sale investments, net of tax of ($82)	12,451				12,451
Reclassification of credit (recovery) loss on investments	(85)				(85)
Reclassification of net realized (gain) loss on the sale of available-for-sale investments	(955)				(955)
Foreign currency translation adjustment	$ 1,903				1,903
Ending Balance (in shares) at Jun. 28, 2020	173,047	173,047
Ending Balance at Jun. 28, 2020	$ 1,376,478	$ 346	$ 710,412	$ 702,597	$ (36,877)