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Consolidated Statements of (Deficit) Equity (Parenthetical) - $ / shares
 12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Accounting Standards Update [Extensible List]     us-gaap:AccountingStandardsUpdate201409Member
Common Class A      
Common stock, dividends (in usd per share) $ 0.12    
Common Class B      
Common stock, dividends (in usd per share) $ 0.27 $ 0.59 $ 1.84