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Revenue Recognition (Customer Accounts Receivable) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2021
Dec. 31, 2020
Revenue from Contract with Customer [Abstract]    
Customer Accounts Receivable, Billed (Less Reserve) $ 507.4 $ 400.0
Customer Accounts Receivable, Unbilled (Less Reserve) 244.3 $ 327.2
Increase (Decrease) in Customer Accounts Receivable, Billed (Less Reserve) 107.4  
Increase (Decrease) in Customer Accounts Receivable, Unbilled (Less Reserve) $ (82.9)