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Revenue Recognition (Customer Accounts Receivable) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Revenue from Contract with Customer [Abstract]    
Customer Accounts Receivable, Billed (Less Reserve) $ 449.9 $ 466.6
Customer Accounts Receivable, Unbilled (Less Reserve) 233.5 $ 346.6
Increase (Decrease) in Customer Accounts Receivable, Billed (Less Reserve) (16.7)  
Increase (Decrease) in Customer Accounts Receivable, Unbilled (Less Reserve) $ (113.1)