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Summary of Significant Accounting Policies (Details) - USD ($)
$ in Millions
12 Months Ended
Feb. 01, 2021
Jan. 14, 2021
Jul. 02, 2020
Dec. 31, 2019
Sep. 30, 2021
Dec. 31, 2020
Sep. 30, 2020
Dec. 31, 2018
Dec. 31, 2017
Sale of Stock [Line Items]                  
Cash and cash equivalents       $ 270.0 $ 319.4 $ 570.8      
Restricted cash       0.3 0.2 0.6      
Total cash, cash equivalents and restricted cash       270.3 $ 319.6 $ 571.4 $ 575.7 $ 102.4 $ 16.9
Offering costs $ 0.4   $ 32.4            
Offering costs - component of gross proceeds     $ 28.9            
Offering costs - component of expense       $ 3.5          
Public Stock Offering                  
Sale of Stock [Line Items]                  
Net proceeds from sale   $ 525.7              
Underwriter's option                  
Sale of Stock [Line Items]                  
Net proceeds from sale $ 114.6 $ 114.6