Consolidated Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Cash flows from operating activities:
Net income	$ 101,066	$ 126,540	$ 210,308	$ 240,521
Adjustments to reconcile net income to net cash provided by operating activities (See Note 1)			53,824	(22,876)
Net cash provided by operating activities			264,132	217,645
Cash flows from investing activities:
Additions to property and equipment			(34,442)	(18,239)
Additions to capitalized software			(12,533)	(19,188)
Purchases of marketable securities			(60,764)	(77,891)
Prepayments and maturities of marketable securities			71,782	85,012
Sales of marketable securities			64	0
Other investing activities			(1,500)	0
Net cash used in investing activities			(37,393)	(30,306)
Cash flows from financing activities:
Payment of contingent consideration			(633)	0
Purchase and retirement of common stock			(219,404)	(183,396)
Proceeds from issuance of common stock			24,587	24,272
Payment of dividends			(103,914)	(100,745)
Net cash used in financing activities			(299,364)	(259,869)
Effect of exchange rate changes on cash, cash equivalents and restricted cash			(10,545)	277
Net decrease in cash, cash equivalents and restricted cash			(83,170)	(72,253)
Cash, cash equivalents and restricted cash, beginning of period			844,547	758,039	$ 758,039
Cash, cash equivalents and restricted cash, end of period	$ 761,377	$ 685,786	761,377	685,786	$ 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