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Goodwill and Acquired Client Relationships - Schedule of Changes in Goodwill (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Changes in goodwill    
Balance, as of January 1, $ 2,628.1 $ 2,668.4
Foreign currency translation 34.4 (40.3)
Balance, as of December 31, $ 2,662.5 $ 2,628.1