Condensed Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash Flows from Operating Activities:
Net income	$ 224,979	$ 169,683
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	61,620	57,977
Impairment charge		2,228
Provision for uncollectible accounts	1,396	842
(Provision for) benefit of deferred income taxes	(438)	6,243
Share-based compensation expense	17,762	15,021
Other	516	1,887
Tax benefit from share-based compensation arrangements (Note 2)		(10,225)
Changes in assets and liabilities:
Accounts receivable	(18,724)	(16,647)
Inventories	(22,966)	(2,503)
Other assets and liabilities	(10,734)	12,380
Accounts payable	(3,540)	(2,496)
Deferred revenue	2,279	3,798
Net cash provided by operating activities	252,150	238,188
Cash Flows from Investing Activities:
Purchases of property and equipment	(54,370)	(49,956)
Purchase of marketable securities	(269,798)	(178,829)
Proceeds from the sale and maturities of marketable securities	224,816	152,277
Acquisitions of intangible assets	(320)
Acquisitions of a business, net of cash acquired	(14,529)
Net cash used by investing activities	(114,201)	(76,508)
Cash Flows from Financing Activities:
Borrowings (repayments) on revolving credit facilities, net	75,250	(85,000)
Debt issue costs		(57)
Repurchases of common stock	(228,693)	(91,562)
Proceeds from exercises of stock options and employee stock purchase plans	31,314	28,815
Payment of acquisition-related contingent consideration		(3,633)
Shares withheld for statutory tax withholding on restricted stock (Note 2)	(7,829)	(3,732)
Tax benefit from share-based compensation arrangements (Note 2)		10,225
Net cash used by financing activities	(129,958)	(144,944)
Net effect of changes in exchange rates on cash	6,127	4,342
Net increase in cash and cash equivalents	14,118	21,078
Cash and cash equivalents at beginning of period	154,901	128,994
Cash and cash equivalents at end of period	$ 169,019	$ 150,072