Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash Flows from Operating Activities:
Net income	$ 582,131	$ 427,792	$ 377,047
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	95,998	88,011	83,178
Impairment charge	2,501	968	2,629
Provision for uncollectible accounts	4,946	662	1,609
(Benefit of) provision for deferred income taxes	(38,082)	6,162	1,209
Share-based compensation expense	30,951	39,278	25,157
Other	1,379	1,254	895
Changes in assets and liabilities:
Accounts receivable	(60,722)	(22,472)	(18,401)
Inventories	(18,885)	(37,306)	(25,623)
Accounts payable	981	1,957	(166)
Deferred revenue	(13,373)	(12,360)	(7,719)
Other assets and liabilities	60,238	(34,788)	(39,731)
Net cash provided by operating activities	648,063	459,158	400,084
Cash Flows from Investing Activities:
Purchases of property and equipment	(106,958)	(154,969)	(115,751)
Purchases of marketable securities	0	0	(87)
Proceeds from the sale and maturities of marketable securities	0	0	284,125
Acquisitions of intangible assets and equity investments	(918)	(255)	(7,185)
Acquisitions of businesses, net of cash acquired	(1,500)	(50,304)	(22,500)
Net cash (used) provided by investing activities	(109,376)	(205,528)	138,602
Cash Flows from Financing Activities:
Repayment on revolving credit facilities, net	(289,625)	(110,275)	(256,040)
Issuance of senior notes	200,000	100,000	0
Debt issuance costs	(5,025)	(154)	0
Repurchases of common stock, net	(182,815)	(301,658)	(369,319)
Proceeds from exercises of stock options and employee stock purchase plans	51,328	36,106	38,201
Payment of acquisition-related contingent consideration	(1,676)	(2,375)	(1,266)
Shares withheld for statutory tax withholding on restricted stock	(20,603)	(8,053)	(9,375)
Net cash used by financing activities	(248,416)	(286,409)	(597,799)
Net effect of changes in exchange rates on cash	3,331	(689)	(4,768)
Net increase (decrease) in cash and cash equivalents	293,602	(33,468)	(63,881)
Cash and cash equivalents at beginning of period	90,326	123,794	187,675
Cash and cash equivalents at end of period	$ 383,928	$ 90,326	$ 123,794