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Summary of Significant Accounting Policies, Accounts Receivable (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Accounts Receivable, after Allowance for Credit Loss [Abstract]      
Unbilled contracts receivable $ 460.1 $ 586.6  
Allowance For Doubtful Accounts, Current. [Member]      
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]      
Allowance at beginning of the year 68.1 52.0 $ 51.3
Charged to income 24.3 30.8 18.8
Write-off of uncollectible receivables (29.5) (13.8) (13.9)
Impact of exchange rate movements and other 0.0 (0.9) (4.2)
Allowance at end of the year $ 66.5 $ 68.1 $ 52.0