Condensed Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash Flows from Operating Activities:
Net income	$ 89,476	$ 69,058
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	20,804	20,307
Benefit of deferred income taxes	3,005	1,941
Share-based compensation expense	5,960	5,655
Other	1,091	860
Changes in assets and liabilities:
Accounts receivable	(21,800)	(19,429)
Inventories	(8,070)	(5,369)
Other assets and liabilities	(52,302)	(38,531)
Accounts payable	(1,939)	(3,687)
Deferred revenue	(1,327)	469
Net cash provided by operating activities	34,898	31,274
Cash Flows from Investing Activities:
Purchases of property and equipment	(23,726)	(23,647)
Purchase of marketable securities	(87)	(90,492)
Proceeds from the sale and maturities of marketable securities	284,125	87,476
Acquisitions of a business, net of cash acquired		(2,349)
Net cash provided (used) by investing activities	260,312	(29,012)
Cash Flows from Financing Activities:
(Repayments) borrowings on revolving credit facilities, net	(247,500)	60,000
Repurchases of common stock	(83,487)	(63,910)
Proceeds from exercises of stock options and employee stock purchase plans	14,551	12,526
Shares withheld for statutory tax withholding on restricted stock	(8,555)	(7,303)
Net cash (used) provided by financing activities	(324,991)	1,313
Net effect of changes in exchange rates on cash	1,335	1,932
Net (decrease) increase in cash and cash equivalents	(28,446)	5,507
Cash and cash equivalents at beginning of period	187,675	154,901
Cash and cash equivalents at end of period	$ 159,229	$ 160,408