Consolidated Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Cash flows from operating activities:
Net income	$ 111,096	$ 132,168	$ 321,404	$ 372,689
Adjustments to reconcile net income to net cash provided by operating activities (See Note 1)			75,120	8,846
Net cash provided by operating activities			396,524	381,535
Cash flows from investing activities:
Additions to property and equipment			(43,113)	(30,515)
Additions to capitalized software			(18,640)	(26,821)
Purchases of marketable securities			(114,407)	(126,030)
Prepayments and maturities of marketable securities			112,575	137,783
Sales of marketable securities			842	0
Other investing activities			(1,500)	2,538
Net cash used in investing activities			(64,243)	(43,045)
Cash flows from financing activities:
Payment of contingent consideration			(633)	0
Purchase and retirement of common stock			(327,079)	(262,861)
Proceeds from issuance of common stock			29,793	41,864
Payment of dividends			(103,914)	(100,745)
Net cash used in financing activities			(401,833)	(321,742)
Effect of exchange rate changes on cash, cash equivalents and restricted cash			(4,196)	(3,878)
Net (decrease) increase in cash, cash equivalents and restricted cash			(73,748)	12,870
Cash, cash equivalents and restricted cash, beginning of period			844,547	758,039	$ 758,039
Cash, cash equivalents and restricted cash, end of period	$ 770,799	$ 770,909	770,799	770,909	$ 844,547
Non-cash operating activities:
Operating lease right-of-use assets and net lease liabilities recorded upon adoption of ASC 842			$ 0	$ 44,169