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Nonconsolidated Partially-Owned Affiliates - (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Jan. 31, 2020
Sep. 30, 2020
Sep. 30, 2018
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Schedule of Equity Method Investments [Line Items]            
Investments in partially-owned affiliates   $ 707   $ 707 $ 1,399  
Impairment of nonconsolidated partially owned affiliate       $ 231 $ 0 $ 358
Significant Unobservable Inputs (Level 3) | Valuation, income approach | Measurement input, cap rate            
Schedule of Equity Method Investments [Line Items]            
Reporting unit, measurement input     12.50%     13.00%
YFAI            
Schedule of Equity Method Investments [Line Items]            
% ownership 30.00% 0.00%   0.00% 30.00%  
Payments to acquire equity method investments $ 369          
Impairment of nonconsolidated partially owned affiliate     $ 358 $ 231   $ 358
YFAI | Significant Unobservable Inputs (Level 3) | Valuation, income approach | Measurement input, cap rate            
Schedule of Equity Method Investments [Line Items]            
Reporting unit, measurement input   15.00%