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Recent Accounting Pronouncements (Details) $ in Thousands	3 Months Ended		9 Months Ended
Recent Accounting Pronouncements (Details) $ in Thousands	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)	Jan. 01, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase (decrease) in revenues	$ 382,690	$ 324,002	$ 1,108,395	$ 919,243
Number of operating segments \| segment			3,000
Difference between Revenue Guidance in Effect before and after Topic 606 \| Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase (decrease) in revenues	10,387		$ 31,598
Fleet Solutions And Travel And Corporate Solutions \| Difference between Revenue Guidance in Effect before and after Topic 606 \| Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase (decrease) in revenues	13,800		44,900
Operating expenses	13,800		44,900
Travel and corporate solutions
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase (decrease) in revenues	82,810	61,026	225,353	163,739
Travel and corporate solutions \| Difference between Revenue Guidance in Effect before and after Topic 606 \| Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase (decrease) in revenues	(3,400)		(13,500)
Operating expenses	(3,400)		(13,500)
Health and Employee Benefit Solutions
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase (decrease) in revenues	$ 50,268	$ 50,898	$ 161,595	$ 152,299
Health and Employee Benefit Solutions \| Difference between Revenue Guidance in Effect before and after Topic 606 \| Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative-effect adjustment					$ 600