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Summary of Significant Accounting Policies, Accounts Receivable (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Accounts Receivable, Net [Abstract]      
Unbilled contracts receivable $ 531.4 $ 505.6  
Allowance For Doubtful Accounts, Current. [Member]      
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]      
Allowance at beginning of the year 51.3 37.1 $ 23.2
Charged to income 18.8 26.3 21.3
Write-off of uncollectible receivables (13.9) (14.2) (7.6)
Reserves acquired from acquisitions 0.0 0.0 0.3
Impact of exchange rate movements and other (4.2) 2.1 (0.1)
Allowance at end of the year $ 52.0 $ 51.3 $ 37.1