Consolidated Statements of Cash Flows - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018	[1]
Operating activities
Net income		$ 704,216	$ 895,073		$ 811,483	[2]
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation		65,181	62,784		52,936
Stock-based compensation		43,384	60,953		69,939
Provision for losses on accounts and other receivables		158,549	74,309		64,377
Amortization of deferred financing costs and discounts		6,486	5,106		5,342
Amortization of intangible assets and premium on receivables		189,620	211,426		221,673
Deferred income taxes		(15,112)	37,883		(2,750)
Loss on extinguishment of debt		0	0		2,098	[2]
Investment (gain) loss, net		(30,008)	3,470		7,147	[2]
Gain on sale of assets/business		0	0		(152,750)
Other non-cash operating income2	[3]	(1,985)	522		8,607
Changes in operating assets and liabilities (net of acquisitions/disposition):
Accounts receivable and other receivables		264,140	(196,028)		(159,024)
Prepaid expenses and other current assets		(14,521)	(185,391)		(27,650)
Other assets		12,656	(6,792)		(25,432)
Accounts payable, accrued expenses and customer deposits		89,983	198,756		27,386
Net cash provided by operating activities		1,472,589	1,162,071		903,382
Investing activities
Acquisitions, net of cash acquired		(80,787)	(448,277)		(20,843)
Purchases of property and equipment		(78,425)	(75,170)		(81,387)
Proceeds from disposal of an asset/business		0	0		98,735
Proceeds from disposal of investment		52,963	0		0
Other		0	(255)		(22,775)
Net cash used in investing activities		(106,249)	(523,702)		(26,270)
Financing activities
Proceeds from issuance of common stock		136,797	168,925		55,680
Repurchase of common stock		(849,910)	(694,909)		(958,696)
(Payments) borrowings on securitization facility, net		(270,973)	84,973		75,000
Deferred financing costs paid and debt discount		(2,637)	(2,868)		(4,927)
Proceeds from issuance of notes payable		0	700,000		363,430
Principal payments on notes payable		(175,285)	(138,500)		(498,305)
Borrowings from revolver		1,243,500	1,811,509		1,493,091
Payments on revolver		(1,496,907)	(2,292,349)		(1,099,040)
(Payments) borrowings on swing line of credit, net		(1,042)	52,996		(4,935)
Other		(344)	52		887
Net cash used in financing activities		(1,416,801)	(310,171)		(577,815)
Effect of foreign currency exchange rates on cash		(148,157)	(17,854)		(65,274)
Net (decrease) increase in cash and cash equivalents and restricted cash		(198,618)	310,344		234,023
Cash and cash equivalents and restricted cash, beginning of year		1,675,237	1,364,893	[1]	1,130,870
Cash and cash equivalents and restricted cash, end of year		1,476,619	1,675,237		1,364,893
Supplemental cash flow information
Cash paid for interest		126,460	178,417		156,749
Cash paid for income taxes		$ 165,315	$ 200,525		$ 207,504
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201602Member

[1] The Company applied the modified retrospective transition method when adopting ASU 2016-02 "Leases", therefore the Company's 2018 prior results were not restated to reflect ASU 2016-02. Refer to footnote 14.

[2] The Company applied the modified retrospective transition method when adopting ASU 2016-02 "Leases", therefore the Company's 2018 prior results were not restated to reflect ASU 2016-02. Refer to footnote 14.

[3] Comparable disclosure provided to conform with 2020 presentation.