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Revenue and Contracts with Customers Revenue and Contracts with Customers (Schedule of Accounts Receivable) (Details) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Billed accounts receivable $ 12.7 $ 23.0
Unbilled contract revenue 4.2 6.6
Accounts receivable, net $ 16.9 $ 29.6