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)		717
Issuance of common stock under stock plans	14,346	$ 1	14,345
Stock-based compensation expense	12,281		12,281
Payment of dividends	(8,564)			(8,564)
Net income	33,104			33,104
Net unrealized gain (loss) on available-for-sale investments, net of tax of ($82)	2,251				2,251
Reclassification of credit loss on investments	0
Reclassification of net realized (gain) loss on the sale of available-for-sale investments	(40)				(40)
Foreign currency translation adjustment	218				218
Ending Balance (in shares) at Mar. 31, 2019		171,537
Ending Balance at Mar. 31, 2019	$ 1,188,859	$ 343	555,834	670,754	(38,072)
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)		463
Issuance of common stock under stock plans	9,971	$ 1	9,970
Repurchase of common stock (in shares)		(1,215)
Repurchase of common stock	(51,036)	$ (2)		(51,034)
Stock-based compensation expense	14,790		14,790
Payment of dividends	(9,503)			(9,503)
Net income	20,477			20,477
Net unrealized gain (loss) on available-for-sale investments, net of tax of ($82)	(3,861)				(3,861)
Reclassification of credit loss on investments	(160)				160
Reclassification of net realized (gain) loss on the sale of available-for-sale investments	(1,850)				(1,850)
Foreign currency translation adjustment	$ (7,365)				(7,365)
Ending Balance (in shares) at Mar. 29, 2020	171,688	171,688
Ending Balance at Mar. 29, 2020	$ 1,327,493	$ 344	$ 664,132	$ 713,208	$ (50,191)