Summary of Significant Accounting Policies - Effect of Topic 606 on Financial Statements (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018		Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Consolidated Revenues, net	$ 619,586	[1]	$ 577,877	$ 1,790,070	[1]	$ 1,639,547
Expenses:
Merchant commissions	0	[1]	27,687	0	[1]	82,690
Processing	128,400	[1]	111,283	356,086	[1]	316,429
Selling	44,806	[1]	45,060	135,926	[1]	122,854
General and administrative	98,023	[1]	92,054	284,718	[1]	275,095
Depreciation and amortization	67,267	[1]	69,156	207,379	[1]	198,731
Operating income	281,090	[1]	232,637	805,961	[1]	643,748
Total other expense (income)	43,522	[1]	(94,865)	107,899	[1]	(41,511)
Income before income taxes	237,568	[1]	327,502	698,062	[1]	685,259
Provision for income taxes	79,874	[1]	124,679	188,579	[1]	227,756
Net income	$ 157,694	[1]	$ 202,823	$ 509,483	[1],[2]	$ 457,503	[2]
Basic earnings per share (in dollars per share)	$ 1.78	[1]	$ 2.23	$ 5.72	[1]	$ 4.99
Diluted earnings per share (in dollars per share)	$ 1.71	[1]	$ 2.18	$ 5.50	[1]	$ 4.87
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Consolidated Revenues, net	$ 27,958			$ 75,513
Expenses:
Merchant commissions	30,909			84,199
Processing	(2,498)			(7,282)
Selling	875			3,392
General and administrative	0			0
Depreciation and amortization	0			0
Operating income	(1,328)			(4,796)
Total other expense (income)	0			0
Income before income taxes	(1,328)			(4,796)
Provision for income taxes	(498)			(1,346)
Net income	$ (830)			$ (3,450)
Basic earnings per share (in dollars per share)	$ (0.01)			$ (0.04)
Diluted earnings per share (in dollars per share)	$ (0.01)			$ (0.04)
Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Consolidated Revenues, net	$ 647,544			$ 1,865,583
Expenses:
Merchant commissions	30,909			84,199
Processing	125,902			348,804
Selling	45,681			139,318
General and administrative	98,023			284,718
Depreciation and amortization	67,267			207,379
Operating income	279,762			801,165
Total other expense (income)	43,522			107,899
Income before income taxes	236,240			693,266
Provision for income taxes	79,376			187,233
Net income	$ 156,864			$ 506,033
Basic earnings per share (in dollars per share)	$ 1.77			$ 5.68
Diluted earnings per share (in dollars per share)	$ 1.70			$ 5.46

[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.