Earnings Per Share - Schedule of Earnings Per Share, Basic and Diluted (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		12 Months Ended
		Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Schedule of Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Income allocation - basic earnings per share		$ 20,058	$ 19,743	$ 17,273
Class A common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Income allocation - basic earnings per share		$ 17,511	$ 15,790	$ 13,415
Weighted-average shares outstanding - basic (in shares)		1,714	1,621	1,618
Earnings per share - basic (in dollars per share)		$ 10.22	$ 9.74	$ 8.29
Income allocation - diluted earnings per share		$ 20,058	$ 19,743	$ 17,273
Weighted-average shares outstanding - diluted (in shares)		1,966	2,029	2,085
Earnings per share - diluted (in dollars per share)		$ 10.20	$ 9.73	$ 8.28
Class B-1 common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Income allocation - basic earnings per share		$ 77	$ 2,292	$ 3,254
Weighted-average shares outstanding - basic (in shares)		5	148	245
Earnings per share - basic (in dollars per share)		$ 15.97	$ 15.46	$ 13.26
Income allocation - diluted earnings per share		$ 77	$ 2,289	$ 3,251
Weighted-average shares outstanding - diluted (in shares)		5	148	245
Earnings per share - diluted (in dollars per share)		$ 15.95	$ 15.45	$ 13.24
Class B-2 common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Income allocation - basic earnings per share		$ 1,891	$ 752
Weighted-average shares outstanding - basic (in shares)	[1]	120	49	0
Earnings per share - basic (in dollars per share)	[1]	$ 15.72	$ 15.45	$ 0
Income allocation - diluted earnings per share		$ 1,889	$ 751
Weighted-average shares outstanding - diluted (in shares)	[1]	120	49	0
Earnings per share - diluted (in dollars per share)	[1]	$ 15.70	$ 15.43	$ 0
Class C common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Income allocation - basic earnings per share		$ 374	$ 623	$ 320
Weighted-average shares outstanding - basic (in shares)		9	16	10
Earnings per share - basic (in dollars per share)		$ 40.87	$ 38.97	$ 33.17
Income allocation - diluted earnings per share		$ 373	$ 623	$ 319
Weighted-average shares outstanding - diluted (in shares)		9	16	10
Earnings per share - diluted (in dollars per share)		$ 40.82	$ 38.92	$ 33.13
Participating securities
Schedule of Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Income allocation - basic earnings per share		$ 205	$ 286	$ 284
Income allocation - diluted earnings per share		$ 204	$ 286	$ 284

[1] (1)No shares of class B-2 common stock were outstanding prior to the class B-1 common stock exchange offer in May 2024. See Note 15—Stockholders’ Equity for further details.