Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows from Operating Activities:
Net income	$ 263,269	$ 222,054	$ 192,135
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	83,140	78,218	68,956
Amortization on marketable securities, net	38	843	1,432
Impairment charge		2,228	8,212
Provision for uncollectible accounts	1,881	1,170	2,200
Provision for deferred income taxes	(7,918)	20,881	5,143
Share-based compensation expense	23,517	19,891	19,884
Other	969	986	(472)
Tax benefit from share-based compensation arrangements		(14,702)	(11,315)
Changes in assets and liabilities:
Accounts receivable	(24,918)	(22,554)	(50,142)
Inventories	(19,062)	7,648	(34,969)
Accounts payable	1,391	2,117	(2,468)
Deferred revenue	3,551	7,672	(319)
Other assets and liabilities	47,418	12,491	23,525
Net cash provided by operating activities	373,276	338,943	221,802
Cash Flows from Investing Activities:
Purchases of property and equipment	(74,384)	(64,787)	(82,921)
Purchases of marketable securities	(334,164)	(227,894)	(271,958)
Proceeds from the sale and maturities of marketable securities	286,759	203,859	56,775
Acquisitions of intangible assets	(2,320)
Acquisitions of a businesses, net of cash acquired	(14,579)	(1,964)	(10,302)
Net cash used by investing activities	(138,688)	(90,786)	(308,406)
Cash Flows from Financing Activities:
Borrowings on revolving credit facilities, net	44,000	38,000	24,000
Issuance of senior notes			250,097
Debt issue costs		(56)	(1,380)
Repurchases of common stock	(282,565)	(304,086)	(401,981)
Proceeds from exercises of stock options and employee stock purchase plans	38,622	38,344	22,397
Payment of acquisition-related contingent consideration		(4,728)
Shares withheld for statutory tax withholding on restricted stock (Note 2)	(8,073)	(4,372)	(5,438)
Tax benefit from share-based compensation arrangements		14,702	11,315
Net cash used by financing activities	(208,016)	(222,196)	(100,990)
Net effect of changes in exchange rates on cash	6,202	(54)	(5,948)
Net increase (decrease) in cash and cash equivalents	32,774	25,907	(193,542)
Cash and cash equivalents at beginning of period	154,901	128,994	322,536
Cash and cash equivalents at end of period	$ 187,675	$ 154,901	$ 128,994