Condensed Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash Flows from Operating Activities:
Net income	$ 154,465	$ 113,231
Adjustments to reconcile net income to net cash provided (used) by operating activities:
Depreciation and amortization	40,893	38,210
Impairment charge		2,228
Provision for uncollectible accounts	825	252
Benefit of deferred income taxes	2,691	1,516
Share-based compensation expense	11,742	9,927
Other	206	1,540
Tax benefit from share-based compensation arrangements (Note 2)		(4,791)
Changes in assets and liabilities:
Accounts receivable	(32,400)	(23,647)
Inventories	(18,850)	214
Other assets and liabilities	(21,426)	(3,994)
Accounts payable	1,422	99
Deferred revenue	1,898	3,088
Net cash provided by operating activities	141,466	137,873
Cash Flows from Investing Activities:
Purchases of property and equipment	(38,566)	(37,868)
Purchase of marketable securities	(175,522)	(123,809)
Proceeds from the sale and maturities of marketable securities	155,903	108,115
Acquisitions of a business, net of cash acquired	(14,529)
Net cash used by investing activities	(72,714)	(53,562)
Cash Flows from Financing Activities:
Borrowings (repayments) on revolving credit facilities, net	93,000	(15,000)
Debt issue costs		(57)
Repurchases of common stock	(170,798)	(76,536)
Proceeds from exercises of stock options and employee stock purchase plans	23,170	17,554
Payment of acquisition-related contingent consideration		(2,717)
Shares withheld for statutory tax withholding on restricted stock (Note 2)	(7,459)	(3,732)
Tax benefit from share-based compensation arrangements (Note 2)		4,791
Net cash used by financing activities	(62,087)	(75,697)
Net effect of changes in exchange rates on cash	4,409	3,531
Net increase in cash and cash equivalents	11,074	12,145
Cash and cash equivalents at beginning of period	154,901	128,994
Cash and cash equivalents at end of period	$ 165,975	$ 141,139