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Benefit Plans (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined contribution plans [Line Items]      
Employer contribution expenses $ 46.0 $ 35.0 $ 33.9
Amounts deferred under non-qualified deferred compensation plans 39.9 $ 12.7  
International Financial Data Services U.K.      
Defined Benefit Plan Disclosure [Line Items]      
Net benefit asset $ 0.9