Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018		Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017	[2]	Dec. 31, 2017
Assets:
Foreign exchange contracts	$ 28,300			$ 28,300				$ 39,000
Total assets	605,590			605,590				517,723
Cash collateral for foreign exchange contracts	26,500			26,500				12,500
Liabilities:
Foreign exchange contracts	28,600			28,600				26,900
Total liabilities	28,626			28,626				26,888
Cash collateral obligation for foreign exchange contracts	12,300			12,300				10,900
Investment loss	7,147	[1]	$ 47,766	7,147	[1],[2]	$ 52,497
Foreign exchange contracts
Assets:
Foreign exchange contracts	28,293			28,293				39,045
Cash collateral for foreign exchange contracts	26,499			26,499				12,540
Liabilities:
Foreign exchange contracts	28,626			28,626				26,888
Cash collateral obligation for foreign exchange contracts	12,307			12,307				10,882
Repurchase agreements
Assets:
Total assets	526,730			526,730				420,838
Money market
Assets:
Total assets	50,395			50,395				50,423
Certificates of deposit
Assets:
Total assets	172			172				7,417
Level 1
Assets:
Total assets	0			0				10
Liabilities:
Total liabilities	0			0				67
Level 1 | Foreign exchange contracts
Assets:
Foreign exchange contracts	0			0				10
Liabilities:
Foreign exchange contracts	0			0				67
Level 2
Assets:
Total assets	605,590			605,590				517,713
Liabilities:
Total liabilities	28,626			28,626				26,821
Level 2 | Foreign exchange contracts
Assets:
Foreign exchange contracts	28,293			28,293				39,035
Liabilities:
Foreign exchange contracts	28,626			28,626				26,821
Level 2 | Repurchase agreements
Assets:
Total assets	526,730			526,730				420,838
Level 2 | Money market
Assets:
Total assets	50,395			50,395				50,423
Level 2 | Certificates of deposit
Assets:
Total assets	172			$ 172				$ 7,417
Qui
Liabilities:
Investment loss	$ 7,100

[1] Reflects the impact of the Company's adoption of ASC 606 and related cost capitalization guidance, which was adopted by the Company on January 1, 2018 using the modified retrospective transition method. The adoption of ASC 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 ASC 606. See footnote 1, "Summary of Significant Accounting Policies", in the accompanying notes to the unaudited consolidated financial statements.

[2] Reflects the impact of the Company's adoption of Accounting Standards Update 2016-18, "Statement of Cash Flows (Topic 230)", which was adopted by the Company on January 1, 2018 and applied retrospectively to results for 2017. The adoption of Topic 230 resulted in the statement of cash flows presenting the changes in the total of cash, cash equivalents and restricted cash. As a result, the Company will no longer present transfers between cash and cash equivalents and restricted cash in the statement of cash flows.