v3.21.2
Stockholders' Equity - Effect of VE Territory Covered Losses Recovery on the Company Repurchasing its Common Stock (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Conversion of Stock [Line Items]      
Recovery through conversion rate adjustment $ (2) $ 5  
Fourth anniversary release   $ (5)  
Series B Preferred Stock      
Conversion of Stock [Line Items]      
Reduction in equivalent number of as-converted shares of class A common stock (in shares) 0 16 0
Effective price per share (in dollars per share) $ 220.84 $ 194.31 $ 141.32
Recovery through conversion rate adjustment $ 35 $ 72 $ 6
Fourth anniversary release $ 0 $ 3,084 $ 0
Series C Preferred Stock      
Conversion of Stock [Line Items]      
Reduction in equivalent number of as-converted shares of class A common stock (in shares) 0 22 0
Effective price per share (in dollars per share) $ 220.71 $ 194.33 $ 150.26
Recovery through conversion rate adjustment $ 20 $ 92 $ 2
Fourth anniversary release $ 0 $ 4,216 $ 0