Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands		3 Months Ended			12 Months Ended
		Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Income (Loss) from Equity Method Investments	[2]				$ 7,147	[1]	$ 53,164	$ 36,356
Assets:
Foreign exchange contracts			$ 39,000		68,800		39,000
Total assets			517,723		723,163		517,723
Cash collateral obligation for foreign exchange contracts			12,500		9,600		12,500
Liabilities:
Foreign exchange contracts			26,900		72,100		26,900
Total liabilities			26,888		72,125		26,888
Cash collateral obligation for foreign exchange contracts			10,900		73,100		10,900
Reclassification of foreign currency translation loss to investment, net of tax					0		31,381	0
Repurchase agreements
Assets:
Cash and cash equivalents			420,838		581,293		420,838
Money market
Assets:
Cash and cash equivalents			50,423		50,644		50,423
Certificates of deposit
Assets:
Cash and cash equivalents			7,417		22,412		7,417
Level 1
Assets:
Total assets			10		21		10
Liabilities:
Total liabilities			67		0		67
Level 2
Assets:
Total assets			517,713		723,141		517,713
Liabilities:
Total liabilities			26,821		72,125		26,821
Level 2 | Repurchase agreements
Assets:
Cash and cash equivalents			420,838		581,293		420,838
Level 2 | Money market
Assets:
Cash and cash equivalents			50,423		50,644		50,423
Level 2 | Certificates of deposit
Assets:
Cash and cash equivalents			7,417		22,412		7,417
Foreign exchange contracts
Assets:
Cash and cash equivalents			39,045				39,045
Foreign exchange contracts					68,814
Cash collateral obligation for foreign exchange contracts			12,540		9,644		12,540
Liabilities:
Foreign exchange contracts			26,888		72,125		26,888
Cash collateral obligation for foreign exchange contracts			10,882		73,140		10,882
Foreign exchange contracts | Level 1
Assets:
Foreign exchange contracts			10		21		10
Liabilities:
Foreign exchange contracts			67		0		67
Foreign exchange contracts | Level 2
Assets:
Cash and cash equivalents			39,035				39,035
Foreign exchange contracts					68,793
Liabilities:
Foreign exchange contracts			26,821		$ 72,125		26,821
Qui
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Income (Loss) from Equity Method Investments		$ 7,100
Masternaut
Liabilities:
Non-cash impairment charge on equity method investment			$ 36,100	$ 44,600			44,600	$ 36,100
Reclassification of foreign currency translation loss to investment, net of tax				$ 31,400			$ 31,400

[1] Reflects the impact of the Company's adoption of Accounting Standards Update 2014-09, Revenue from Contracts with Customers (Topic 606) ("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 Form 10-K and quarterly reports on Form 10-Q will not be restated to reflect ASC 606.

[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.