Condensed Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Cash Flows from Operating Activities:
Net income	$ 169,683	$ 147,745
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	57,977	51,227
Amortization on marketable securities, net	727	967
Impairment charge	2,228	8,212
Provision for uncollectible accounts	842	1,808
Provision for (benefit of) deferred income taxes	6,243	(4,649)
Share-based compensation expense	15,021	14,760
Other	1,160	(305)
Tax benefit from share-based compensation arrangements	(10,225)	(10,044)
Changes in assets and liabilities:
Accounts receivable	(16,647)	(51,024)
Inventories	(2,503)	(27,238)
Other assets and liabilities	8,648	16,538
Accounts payable	(2,496)	(2,841)
Deferred revenue	3,798	(2,688)
Net cash provided by operating activities	234,456	142,468
Cash Flows from Investing Activities:
Purchases of property and equipment	(49,956)	(67,517)
Purchase of marketable securities	(178,829)	(231,387)
Proceeds from the sale and maturities of marketable securities	152,277	24,711
Acquisitions of a business, net of cash acquired		(8,200)
Net cash used by investing activities	(76,508)	(282,393)
Cash Flows from Financing Activities:
Repayments on revolving credit facilities, net	(85,000)	(6,500)
Debt issue costs	(57)	(199)
Issuance of long term debt		250,097
Repurchases of common stock	(91,562)	(309,057)
Proceeds from exercises of stock options and employee stock purchase plans	28,815	19,221
Payment of acquisition-related contingent consideration	(3,633)
Tax benefit from share-based compensation arrangements	10,225	10,044
Net cash used by financing activities	(141,212)	(36,394)
Net effect of changes in exchange rates on cash	4,342	(5,067)
Net increase (decrease) in cash and cash equivalents	21,078	(181,386)
Cash and cash equivalents at beginning of period	128,994	322,536
Cash and cash equivalents at end of period	$ 150,072	$ 141,150