Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Cash Provided by (Used in) Operating Activities [Abstract]
Net income	$ 377,047	$ 263,269	$ 222,054
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	83,178	83,140	78,218
Amortization on marketable securities, net	(5)	38	843
Impairment charge	2,629	0	2,228
Provision for uncollectible accounts	1,609	1,881	1,170
Benefit of (provision for) deferred income taxes	1,209	(7,918)	20,881
Share-based compensation expense	25,157	23,517	19,891
Other	900	969	986
Tax benefit from share-based compensation arrangements	0	0	(14,702)
Changes in assets and liabilities:
Accounts receivable	(18,401)	(24,918)	(22,554)
Inventories	(25,623)	(19,062)	7,648
Accounts payable	(166)	1,391	2,117
Deferred revenue	(7,719)	3,551	7,672
Other assets and liabilities	(39,731)	47,418	12,491
Net cash provided by operating activities	400,084	373,276	338,943
Cash Flows from Investing Activities:
Purchases of property and equipment	(115,751)	(74,384)	(64,787)
Purchases of marketable securities	(87)	(334,164)	(227,894)
Proceeds from the sale and maturities of marketable securities	284,125	286,759	203,859
Acquisitions of intangible assets and equity investment	(7,185)	(2,320)	0
Acquisitions of businesses, net of cash acquired	(22,500)	(14,579)	(1,964)
Net cash provided (used) by investing activities	138,602	(138,688)	(90,786)
Cash Flows from Financing Activities:
(Repayment) borrowings on revolving credit facilities, net	(256,040)	44,000	38,000
Debt issue costs	0	0	(56)
Repurchases of common stock	(369,319)	(282,565)	(304,086)
Proceeds from exercises of stock options and employee stock purchase plans	38,201	38,622	38,344
Payment of acquisition-related contingent consideration	(1,266)	0	(4,728)
Shares withheld for statutory tax withholding on restricted stock (Note 2)	(9,375)	(8,073)	(4,372)
Tax benefit from share-based compensation arrangements	0	0	14,702
Net cash used by financing activities	(597,799)	(208,016)	(222,196)
Net effect of changes in exchange rates on cash	(4,768)	6,202	(54)
Net increase (decrease) in cash and cash equivalents	(63,881)	32,774	25,907
Cash and cash equivalents at beginning of period	187,675	154,901	128,994
Cash and cash equivalents at end of period	$ 123,794	$ 187,675	$ 154,901