v3.21.2
U.S. and Europe Retrospective Responsibility Plans - Changes in Preferred Stock and Right to Recover Covered Losses (Details)
$ / shares in Units, shares in Millions, $ in Millions
 12 Months Ended
Sep. 30, 2021
USD ($)
$ / shares
shares
Sep. 30, 2020
USD ($)
$ / shares
shares
Sep. 30, 2021
USD ($)
$ / shares
shares
Sep. 30, 2020
USD ($)
$ / shares
shares
Sep. 30, 2019
USD ($)
Increase (Decrease) in Stockholders' Equity [Roll Forward]          
Beginning Balance     $ 36,210 $ 34,684 $ 34,006
VE territory covered losses incurred     (147) (37) (172)
Recovery through conversion rate adjustment     2 (5)  
Ending Balance $ 37,589 $ 36,210 37,589 36,210 34,684
As-converted Value of Preferred Stock 8,299 7,499 8,299 7,499  
Book Value of Preferred Stock 2,594 2,649 2,594 2,649  
Right to recover for covered losses (133) (39) (133) (39)  
As-converted Value of Preferred Stock 8,166 7,460 8,166 7,460  
Book Value of Preferred Stock $ 2,461 2,610 $ 2,461 2,610  
Share price (in dollars per share) | $ / shares $ 222.75   $ 222.75    
Series B Preferred Stock          
Increase (Decrease) in Stockholders' Equity [Roll Forward]          
Recovery through conversion rate adjustment     $ (35) (72) (6)
As-converted Value of Preferred Stock $ 3,493 3,168 3,493 3,168  
Book Value of Preferred Stock $ 1,071 $ 1,106 $ 1,071 $ 1,106  
Preferred stock, shares outstanding (in shares) | shares 2 2 2 2  
Preferred stock, conversion rate 6.3210 6.3870      
Series C Preferred Stock          
Increase (Decrease) in Stockholders' Equity [Roll Forward]          
Recovery through conversion rate adjustment     $ (20) $ (92) (2)
As-converted Value of Preferred Stock $ 4,806 $ 4,331 4,806 4,331  
Book Value of Preferred Stock $ 1,523 $ 1,543 $ 1,523 $ 1,543  
Preferred stock, shares outstanding (in shares) | shares 3 3 3 3  
Preferred stock, conversion rate 6.8340 6.8610      
Class A common stock          
Increase (Decrease) in Stockholders' Equity [Roll Forward]          
Share price (in dollars per share) | $ / shares $ 222.75 $ 199.97 $ 222.75 $ 199.97  
Preferred Stock          
Increase (Decrease) in Stockholders' Equity [Roll Forward]          
Beginning Balance     $ 5,086 $ 5,462 5,470
Recovery through conversion rate adjustment     55 164 8
Ending Balance $ 3,080 $ 5,086 3,080 5,086 5,462
Preferred Stock | Series B Preferred Stock          
Increase (Decrease) in Stockholders' Equity [Roll Forward]          
Beginning Balance     1,106    
VE territory covered losses incurred     0    
Recovery through conversion rate adjustment     (35)    
Ending Balance 1,071 1,106 1,071 1,106  
Preferred Stock | Series C Preferred Stock          
Increase (Decrease) in Stockholders' Equity [Roll Forward]          
Beginning Balance     1,543    
VE territory covered losses incurred     0    
Recovery through conversion rate adjustment     (20)    
Ending Balance 1,523 1,543 1,523 1,543  
Right to Recover for Covered Losses          
Increase (Decrease) in Stockholders' Equity [Roll Forward]          
Beginning Balance     (39) (171) (7)
VE territory covered losses incurred     (147) (37) (172)
Recovery through conversion rate adjustment     (53) (169) (8)
Ending Balance $ (133) $ (39) $ (133) $ (39) $ (171)