CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) shares in Thousands		Total	Common Stock [Member]		Additional Paid-in Capital [Member]	[1]	Retained Earnings [Member]	[1]	Accumulated Other Comprehensive Loss [Member]
Beginning Balance at Dec. 31, 2016		$ 963,385,000	$ 344,000	[1]	$ 374,847,000		$ 644,622,000		$ (56,428,000)
Beginning Balance, shares at Dec. 31, 2016	[1]		171,878
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under stock option plans		54,557,000	$ 8,000	[1]	54,549,000
Issuance of common stock under stock option plans, shares	[1]		4,162
Repurchase of common stock		(123,715,000)	$ (5,000)	[1]			(123,710,000)
Repurchase of common stock, shares	[1]		(2,533)
Stock-based compensation expense		31,942,000			31,942,000
Payment of dividends		(29,037,000)					(29,037,000)
Net income		176,712,000					176,712,000
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) loss 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		1,095,673,000	$ 347,000	[1]	461,338,000		668,587,000		(34,599,000)
Balance, shares at Dec. 31, 2017	[1]		173,507
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adjustment as a result of the adoption of ASU 2016-06 Income Taxes - Intra-Entity Transfers Other than Inventory (Note 18)		(5,961,000)					(5,961,000)
Issuance of common stock under stock option plans		26,783,000	$ 3,000	[1]	26,780,000
Issuance of common stock under stock option plans, shares	[1]		1,493
Repurchase of common stock		(203,822,000)	$ (8,000)	[1]			(203,814,000)
Repurchase of common stock, shares	[1]		(4,180)
Stock-based compensation expense		41,090,000			41,090,000
Payment of dividends		(31,865,000)					(31,865,000)
Net income		219,267,000					219,267,000
Net unrealized gain (loss) on cash flow hedges, net of tax		0
Reclassification of net realized (gain) loss into current operations		0
Net unrealized gain (loss) on available-for-sale investments, net of tax		(1,185,000)							(1,185,000)
Reclassification of net realized (gain) loss on the sale of available-for-sale investments		(501,000)							(501,000)
Foreign currency translation adjustments, net of tax		(4,216,000)							(4,216,000)
Balance at Dec. 31, 2018		$ 1,135,263,000	$ 342,000	[1]	529,208,000		646,214,000		(40,501,000)
Balance, shares at Dec. 31, 2018		170,820	170,820	[1]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under stock option plans		$ 64,581,000	$ 6,000	[1]	64,575,000
Issuance of common stock under stock option plans, shares		3,018	3,018	[1]
Repurchase of common stock		$ (61,690,000)	$ (3,000)	[1]			(61,687,000)
Repurchase of common stock, shares	[1]		(1,398)
Stock-based compensation expense		45,589,000			45,589,000
Payment of dividends		(35,124,000)					(35,124,000)
Net income		203,865,000					203,865,000
Net unrealized gain (loss) on cash flow hedges, net of tax		0
Reclassification of net realized (gain) loss into current operations		0
Net unrealized gain (loss) on available-for-sale investments, net of tax		5,219,000							5,219,000
Reclassification of net realized (gain) loss on the sale of available-for-sale investments		(1,452,000)							(1,452,000)
Foreign currency translation adjustments, net of tax		(541,000)							(541,000)
Balance at Dec. 31, 2019		$ 1,355,710,000	$ 345,000	[1]	$ 639,372,000		$ 753,268,000		$ (37,275,000)
Balance, shares at Dec. 31, 2019		172,440	172,440	[1]

[1] Prior period amounts have been adjusted to reflect the two-for-one stock split effected in the form of a stock dividend which occurred in the fourth quarter of 2017.