Condensed Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash Flows from Operating Activities:
Net income	$ 198,156	$ 154,465
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	41,696	40,893
Benefit of deferred income taxes	8,638	2,691
Share-based compensation expense	12,352	11,742
Other	1,613	1,031
Changes in assets and liabilities:
Accounts receivable	(32,872)	(32,400)
Inventories	(16,825)	(18,850)
Other assets and liabilities	(55,781)	(21,426)
Accounts payable	3	1,422
Deferred revenue	(3,252)	1,898
Net cash provided by operating activities	153,728	141,466
Cash Flows from Investing Activities:
Purchases of property and equipment	(51,377)	(38,566)
Purchase of marketable securities	(87)	(175,522)
Proceeds from the sale and maturities of marketable securities	284,125	155,903
Acquisitions of a business, net of cash acquired	0	(14,529)
Net cash provided (used) by investing activities	232,661	(72,714)
Cash Flows from Financing Activities:
(Repayments) borrowings on revolving credit facilities, net	(218,000)	93,000
Payment of acquisition-related contingent consideration	(1,000)	0
Repurchases of common stock	(189,884)	(170,798)
Proceeds from exercises of stock options and employee stock purchase plans	21,905	23,170
Shares withheld for statutory tax withholding on restricted stock	(8,720)	(7,459)
Net cash used by financing activities	(395,699)	(62,087)
Net effect of changes in exchange rates on cash	(3,806)	4,409
Net (decrease) increase in cash and cash equivalents	(13,116)	11,074
Cash and cash equivalents at beginning of period	187,675	154,901
Cash and cash equivalents at end of period	$ 174,559	$ 165,975