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Prepaid Expenses and Other Assets (Tables)
12 Months Ended
Dec. 31, 2019
Prepaid Expense and Other Assets [Abstract]  
Schedule of prepaid expenses and other assets

   December 31,
2019
   September 30,
2018
 
   (in millions) 
Prepaid expenses and other assets  $7.9   $5.0 
Unbilled accounts receivable   15.3    10.8 
Total prepaid expenses and other assets  $23.2   $15.8