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, 2015		$ 832,331,000		$ 340,000	[1]	$ 310,829,000		$ 573,286,000		$ (52,124,000)
Beginning Balance, shares at Dec. 31, 2015	[1]			169,712
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under stock option plans		43,468,000		$ 8,000	[1]	43,460,000
Issuance of common stock under stock option plans, shares	[1]			3,954
Repurchase of common stock		(47,149,000)		$ (4,000)	[1]			(47,145,000)
Repurchase of common stock, shares	[1]			(1,788)
Stock-based compensation expense		20,558,000				20,558,000
Payment of dividends		(25,213,000)						(25,213,000)
Net income		143,694,000						143,694,000
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) loss 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		963,385,000		$ 344,000	[1]	374,847,000		644,622,000		(56,428,000)
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	[2]	$ 347,000	[1]	461,338,000		668,587,000		(34,599,000)
Balance, shares at Dec. 31, 2017		173,507		173,507	[1]
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 17)		$ (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		1,493		1,493	[1]
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	[2]	$ 342,000	[1]	$ 529,208,000		$ 646,214,000		$ (40,501,000)
Balance, shares at Dec. 31, 2018		170,820		170,820	[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.

[2] Prior period amounts have been adjusted to reflect the two-for-one stock split effected in the form of a stock dividend