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Per Common Share Amounts - Additional Information (Details)
shares in Millions
3 Months Ended 6 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Mar. 31, 2021
shares
Mar. 31, 2020
Class of Stock [Line Items]        
Effective tax rate 30.50% 30.30% 29.30% 30.00%
Class A membership units        
Class of Stock [Line Items]        
Antidilutive securities (in shares)     15  
Conversion ratio     1