v3.26.1
U.S. and Europe Retrospective Responsibility Plans - Schedule of Changes in Preferred Stock and Right to Recover for Covered Losses (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Mar. 31, 2026
Mar. 31, 2025
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Balance as of beginning of period $ 38,777 $ 38,296 $ 37,909 $ 39,137
VE territory covered losses (25) 3 (28) (24)
Recovery through conversion rate adjustments     (1) 0
Balance as of end of period 35,661 38,030 35,661 38,030
Preferred Stock        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Balance as of beginning of period 551 904 745 [1] 1,031 [2]
Recovery through conversion rate adjustments     (109) (8)
Balance as of end of period 528 [1] 880 [2] 528 [1] 880 [2]
Right to Recover for Covered Losses        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Balance as of beginning of period (19) (123) (124) (104)
VE territory covered losses (25) 3 (28) (24)
Recovery through conversion rate adjustments     108 8
Balance as of end of period (44) (120) (44) (120)
Series B preferred stock        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Recovery through conversion rate adjustments     60 5
Series B preferred stock | Preferred Stock        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Balance as of beginning of period     67 104
VE territory covered losses     0 0
Recovery through conversion rate adjustments     (60) (5)
Balance as of end of period 7 99 7 99
Series C preferred stock        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Recovery through conversion rate adjustments     49 3
Series C preferred stock | Preferred Stock        
Increase (Decrease) in Stockholders' Equity [Roll Forward]        
Balance as of beginning of period     165 387
VE territory covered losses     0 0
Recovery through conversion rate adjustments     (49) (3)
Balance as of end of period $ 116 $ 384 $ 116 $ 384
[1] As of March 31, 2026 and September 30, 2025, the book value of series A convertible participating preferred stock (series A preferred stock) was $405 million and $513 million, respectively. See Note 5—U.S. and Europe Retrospective Responsibility Plans for the book value of series B convertible participating preferred stock (series B preferred stock) and series C convertible participating preferred stock (series C preferred stock).
[2] As of March 31, 2025 and September 30, 2024, the book value of series A preferred stock was $397 million and $540 million, respectively. See Note 5—U.S. and Europe Retrospective Responsibility Plans for the book value of series B and series C preferred stock.