CONSOLIDATED STATEMENTS OF SHAREHOLDERS EQUITY - USD ($) shares in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]
Beginning Balance at Dec. 31, 2014	$ 736,437,000	$ 346,000	$ 251,541,000	$ 523,949,000	$ (39,399,000)
Beginning Balance, Shares at Dec. 31, 2014		173,084
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under stock option plans	27,582,000	$ 6,000	27,576,000
Issuance of common stock under stock option plans, shares		3,040
Repurchase of common stock	(126,351,000)	$ 12,000		(126,339,000)
Repurchase of common stock, shares		(6,412)
Stock-based compensation expense	21,274,000		21,274,000
Excess tax benefit from stock option exercises	9,964,000		9,964,000
Tax benefit for research and development credits as a result of stock options	474,000		474,000
Payment of dividends	(18,062,000)			(18,062,000)
Net income	187,074,000			187,074,000
Net unrealized gain (loss) on cash flow hedges, net of tax	(27,000)				(27,000)
Reclassification of net realized (gain) loss into current operations	201,000				201,000
Net unrealized gain (loss) on available-for-sale investments, net of tax	(939,000)				(939,000)
Reclassification of net realized gain on the sale of available-for-sale investments	(344,000)				(344,000)
Foreign currency translation adjustments, net of tax	(11,616,000)				(11,616,000)
Balance at Dec. 31, 2015	825,667,000	$ 340,000	310,829,000	566,622,000	(52,124,000)
Balance, Shares at Dec. 31, 2015		169,712
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under stock option plans	43,468,000	$ 8,000	43,460,000
Issuance of common stock under stock option plans, shares		3,954
Repurchase of common stock	(47,149,000)	$ (4,000)		(47,145,000)
Repurchase of common stock, shares		(1,788)
Stock-based compensation expense	20,558,000		20,558,000
Payment of dividends	(25,213,000)			(25,213,000)
Net income	149,572,000			149,572,000
Net unrealized gain (loss) on cash flow hedges, net of tax	(567,000)				(567,000)
Reclassification of net realized (gain) loss into current operations	398,000				398,000
Net unrealized gain (loss) on available-for-sale investments, net of tax	1,672,000				1,672,000
Reclassification of net realized gain on the sale of available-for-sale investments	(191,000)				(191,000)
Foreign currency translation adjustments, net of tax	(5,616,000)				(5,616,000)
Balance at Dec. 31, 2016	$ 962,599,000	$ 344,000	374,847,000	643,836,000	(56,428,000)
Balance, Shares at Dec. 31, 2016	84,856	171,878
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under stock option plans	$ 54,557,000	$ 8,000	54,549,000
Issuance of common stock under stock option plans, shares	4,162	4,162
Repurchase of common stock	$ (123,715,000)	$ (5,000)		(123,710,000)
Repurchase of common stock, shares		(2,533)
Stock-based compensation expense	31,942,000		31,942,000
Payment of dividends	(29,037,000)			(29,037,000)
Net income	177,178,000			177,178,000
Net unrealized gain (loss) on cash flow hedges, net of tax	4,000				4,000
Reclassification of net realized (gain) loss into current operations	(41,000)				(41,000)
Net unrealized gain (loss) on available-for-sale investments, net of tax	703,000				703,000
Reclassification of net realized gain on the sale of available-for-sale investments	(829,000)				(829,000)
Foreign currency translation adjustments, net of tax	21,992,000				21,992,000
Balance at Dec. 31, 2017	$ 1,095,353,000	$ 347,000	$ 461,338,000	$ 668,267,000	$ (34,599,000)
Balance, Shares at Dec. 31, 2017	85,939	173,507