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Summary of Significant Accounting Policies, Accounts Receivable (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accounts Receivable, Net [Abstract]      
Unbilled contracts receivable $ 505.6 $ 402.9  
Allowance for Doubtful Accounts, Current [Member]      
Valuation and Qualifying Accounts Disclosure [Line Items]      
Allowance at beginning of the year 37.1 23.2 $ 17.9
Charged to income 26.3 21.3 14.5
Write-off of uncollectible receivables (14.2) (7.6) (8.7)
Reserves acquired from acquisitions 0.0 0.3 0.3
Impact of exchange rate movements and other 2.1 (0.1) (0.8)
Allowance at end of the year $ 51.3 $ 37.1 $ 23.2