Summary of Significant Accounting Policies - Effect of Topic 606 on Financial Statements (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2018		Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Consolidated Revenues, net	$ 584,985	[1]	$ 541,237	$ 1,170,484	[1]	$ 1,061,670
Expenses:
Merchant commissions	0	[1]	30,619	0	[1]	55,003
Processing	111,201	[1]	103,322	227,686	[1]	205,146
Selling	44,009	[1]	38,957	91,120	[1]	77,794
General and administrative	96,382	[1]	87,587	186,696	[1]	183,041
Depreciation and amortization	68,610	[1]	64,709	140,112	[1]	129,575
Operating income	264,783	[1]	216,043	524,870	[1]	411,111
Total other expense	33,608	[1]	25,654	64,376	[1]	53,354
Income before income taxes	231,175	[1]	190,389	460,494	[1]	357,757
Provision for income taxes	54,323	[1]	59,402	108,705	[1]	103,077
Net income	$ 176,852	[1]	$ 130,987	$ 351,789	[1],[2]	$ 254,680	[2]
Basic earnings per share (in dollars per share)	$ 1.98	[1]	$ 1.42	$ 3.93	[1]	$ 2.77
Diluted earnings per share (in dollars per share)	$ 1.91	[1]	$ 1.39	$ 3.78	[1]	$ 2.70
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	$ 23,336			$ 47,554
Expenses:
Merchant commissions	26,387			53,290
Processing	(2,713)			(4,784)
Selling	397			2,517
General and administrative	0			0
Depreciation and amortization	0			0
Operating income	(735)			(3,469)
Total other expense	0			0
Income before income taxes	(735)			(3,469)
Provision for income taxes	(91)			(848)
Net income	$ (644)			$ (2,621)
Basic earnings per share (in dollars per share)	$ (0.01)			$ (0.03)
Diluted earnings per share (in dollars per share)	$ (0.01)			$ (0.03)
Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Consolidated Revenues, net	$ 608,321			$ 1,218,038
Expenses:
Merchant commissions	26,387			53,290
Processing	108,488			222,902
Selling	44,406			93,637
General and administrative	96,382			186,696
Depreciation and amortization	68,610			140,112
Operating income	264,048			521,401
Total other expense	33,608			64,376
Income before income taxes	230,440			457,025
Provision for income taxes	54,232			107,857
Net income	$ 176,208			$ 349,168
Basic earnings per share (in dollars per share)	$ 1.98			$ 3.90
Diluted earnings per share (in dollars per share)	$ 1.90			$ 3.76

[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 effective 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.