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	$ 194,820	$ 146,527	$ 147,462
Adjustments to reconcile net income to cash provided by operations:
Depreciation and amortization	81,657	76,672	61,759
Share-based compensation expense	67,365	59,967	52,740
Provision for losses and sales adjustments - accounts receivable	3,517	1,636	(569)
Operating lease right-of-use assets - non cash	5,782	5,397	0
Deferred income tax benefit	(7,936)	(6,088)	(5,069)
Changes in operating assets and liabilities, exclusive of effects of    acquired companies:
Accounts receivable	(10,733)	(65,738)	(50,916)
Income tax receivable	(15,117)	(1,925)	6,642
Prepaid expenses and other current assets	(8,304)	(8,976)	(588)
Accounts payable	(967)	7,403	(2,416)
Operating lease liabilities	(6,549)	(6,113)	0
Accrued liabilities	2,870	1,516	(2,445)
Deferred revenue	48,684	44,442	43,603
Net cash provided by operating activities	355,089	254,720	250,203
Cash flows from investing activities:
Additions to property and equipment	(22,690)	(37,236)	(27,424)
Purchase of marketable security investments	(156,618)	(54,742)	(115,625)
Proceeds from marketable security investments	82,742	70,796	81,205
Purchase of equity investment in common shares	(10,000)	0	0
Proceeds from the sale of equity investment in preferred shares	15,000	0	0
Capitalized software development costs	(5,776)	(4,804)	0
Cost of acquisitions, net of cash acquired	(1,292)	(218,734)	(178,093)
Decrease (increase) in other	314	(295)	1,682
Net cash used by investing activities	(98,320)	(245,015)	(238,255)
Cash flows from financing activities:
Decrease in net borrowings on revolving line of credit	0	0	0
Purchase of treasury shares	(15,484)	(17,786)	(146,553)
Payment of contingent consideration	(5,619)	0	0
Proceeds from exercise of stock options	124,363	96,908	74,907
Contributions from employee stock purchase plan	10,912	9,576	8,051
Net cash provided (used) by financing activities	114,172	88,698	(63,595)
Net increase (decrease) in cash and cash equivalents	370,941	98,403	(51,647)
Cash and cash equivalents at beginning of period	232,682	134,279	185,926
Cash and cash equivalents at end of period	$ 603,623	$ 232,682	$ 134,279