Consolidated Statement of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Increase (Decrease) in Stockholders' Equity
Cumulative Effect of New Accounting Principle in Period of Adoption	$ (5,961)			$ (5,961)
Beginning Balance, 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, shares		697
Issuance of common stock under stock plans	11,123	$ 1	11,122
Stock Repurchased During Period, Shares		(1,256)
Stock Repurchased During Period, Value	(69,202)	$ 2		(69,200)
Stock-based compensation expense	13,194		13,194
Payment of dividends	(7,787)			(7,787)
Net income	37,217			37,217
Net unrealized gain (loss) on available-for-sale investments, net of tax of $268	(1,192)				(1,192)
Reclassification of net realized (gain) loss on the sale of available-for-sale investments	(22)				(22)
Foreign currency translation adjustment	4,056				4,056
Ending Balance, Shares at Apr. 01, 2018		172,948
Ending Balance at Apr. 01, 2018	$ 1,077,099	$ 346	485,654	622,856	(31,757)
Beginning Balance, 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, 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 $268	2,251				2,251
Reclassification of net realized (gain) loss on the sale of available-for-sale investments	(40)				(40)
Foreign currency translation adjustment	$ 218				218
Ending Balance, Shares at Mar. 31, 2019	171,537	171,537
Ending Balance at Mar. 31, 2019	$ 1,188,859	$ 343	$ 555,834	$ 670,754	$ (38,072)