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Prepaid expenses and other assets (Tables)
12 Months Ended
Dec. 31, 2020
Prepaid Expenses And Other Assets [Abstract]  
Schedule of prepaid expenses and other assets
   December 31,
2020
   December 31,
2019
 
   (in millions) 
Prepaid expenses and other assets  $8.6   $7.9 
Unbilled accounts receivable   8.2    15.3 
Total prepaid expenses and other assets  $16.8   $23.2