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Leases - Schedule of Rent Expense (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Operating Leased Assets [Line Items]      
Rent expense $ 35.0 $ 33.0 $ 33.2
Sublease income (0.4) (0.5) (0.5)
Total 34.6 32.5 32.7
Real estate      
Operating Leased Assets [Line Items]      
Rent expense 32.2 31.8 31.5
Office and computer equipment      
Operating Leased Assets [Line Items]      
Rent expense $ 2.8 $ 1.2 $ 1.7