Accounts Receivable - Company's Accounts Receivable and Securitized Accounts Receivable (Details) - USD ($) $ in Thousands	Sep. 30, 2018		Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross receivables	$ 2,794,361		$ 2,277,042
Gross domestic securitized accounts receivable	931,000	[1]	811,000
Less allowance for doubtful accounts	(52,022)		(46,031)	$ (47,779)	$ (32,506)
Net accounts and securitized accounts receivable	2,742,339		2,231,011
Gross domestic accounts receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross receivables	951,331		661,677
Gross foreign receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total gross receivables	$ 912,030		$ 804,365

[1] Reflects the January 1, 2018 adoption of Accounting Standards Update 2014-09, "Revenue from Contracts with Customers (Topic 606)" and related cost capitalization guidance using the modified retrospective transition method. The adoption of Topic 606 resulted in an adjustment to retained earnings in our consolidated balance sheet for the cumulative effect of applying the standard, which included costs incurred to obtain a contract, as well as presentation changes in our statements of income, including the classification of certain amounts previously classified as merchant commissions and processing expense net with revenues. As a result of the application of the modified retrospective transition method, the Company's prior period results within its annual report on Form 10-K and quarterly reports on Form 10-Q will not be restated to reflect the impact of Topic 606. See footnote 1.