Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018		Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Foreign exchange gains (losses)	$ (200)		$ 600	$ 200		$ (200)
Retained earnings	3,515,657	[1]		3,515,657	[1]			$ 2,958,921
Prepaid expenses and other current assets	202,102	[1]		202,102	[1]			187,820
Other assets	143,913	[1]		143,913	[1]			114,962
Deferred income taxes	$ 506,310	[1]		$ 506,310	[1]			518,912
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Retained earnings							$ 30,900
Prepaid expenses and other current assets							10,200
Other assets							30,300
Deferred income taxes							$ 9,600
Accounting Standards Update 2016-15
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Retained earnings								17,100
Deferred tax assets								$ 17,100

[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.