Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash Flows from Operating Activities:
Net income	$ 427,792	$ 377,047	$ 263,269
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	88,011	83,178	83,140
Impairment charge	968	2,629	0
Provision for uncollectible accounts	662	1,609	1,881
Benefit of (provision for) deferred income taxes	6,162	1,209	(7,918)
Share-based compensation expense	39,278	25,157	23,517
Other	1,254	895	1,007
Changes in assets and liabilities:
Accounts receivable	(22,472)	(18,401)	(24,918)
Inventories	(37,306)	(25,623)	(19,062)
Accounts payable	1,957	(166)	1,391
Deferred revenue	(12,360)	(7,719)	3,551
Other assets and liabilities	(34,788)	(39,731)	47,418
Net cash provided by operating activities	459,158	400,084	373,276
Cash Flows from Investing Activities:
Purchases of property and equipment	(154,969)	(115,751)	(74,384)
Purchases of marketable securities	0	(87)	(334,164)
Proceeds from the sale and maturities of marketable securities	0	284,125	286,759
Acquisitions of intangible assets and equity investment	(255)	(7,185)	(2,320)
Acquisitions of businesses	(50,304)	(22,500)	(14,579)
Net cash (used) provided by investing activities	(205,528)	138,602	(138,688)
Cash Flows from Financing Activities:
(Repayment) borrowings on revolving credit facilities, net	(110,275)	(256,040)	44,000
Issuance of senior notes	100,000	0	0
Debt issuance costs	(154)	0	0
Repurchases of common stock, net	(301,658)	(369,319)	(282,565)
Proceeds from exercises of stock options and employee stock purchase plans	36,106	38,201	38,622
Payment of acquisition-related contingent consideration	(2,375)	(1,266)	0
Shares withheld for statutory tax withholding on restricted stock	(8,053)	(9,375)	(8,073)
Net cash used by financing activities	(286,409)	(597,799)	(208,016)
Net effect of changes in exchange rates on cash	(689)	(4,768)	6,202
Net (decrease) increase in cash and cash equivalents	(33,468)	(63,881)	32,774
Cash and cash equivalents at beginning of period	123,794	187,675	154,901
Cash and cash equivalents at end of period	$ 90,326	$ 123,794	$ 187,675