Earnings Per Share - Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Schedule of Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Income allocation - basic earnings per share	$ 10,866	$ 12,080	$ 10,301
Dilutive shares of outstanding stock awards included in computation of weighted-average dilutive shares outstanding (in shares)	3,000,000
Antidilutive securities excluded from computation of earnings per share (in shares)	1,000,000
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	$ 8,310	$ 9,273	$ 7,937
Weighted-average shares outstanding - basic (in shares)	1,697,000,000	1,742,000,000	1,792,000,000
Earnings per share - basic (in dollars per share)	$ 4.90	$ 5.32	$ 4.43
Income allocation - diluted	$ 10,866	$ 12,080	$ 10,301
Weighted-average shares outstanding - diluted (in shares)	2,223,000,000	2,272,000,000	2,329,000,000
Earnings per share - diluted (in dollars per share)	$ 4.89	$ 5.32	$ 4.42
Class B 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	$ 1,951	$ 2,130	$ 1,787
Weighted-average shares outstanding - basic (in shares)	245,000,000	245,000,000	245,000,000
Earnings per share - basic (in dollars per share)	$ 7.94	$ 8.68	$ 7.28
Income allocation - diluted	$ 1,948	$ 2,127	$ 1,785
Weighted-average shares outstanding - diluted (in shares)	245,000,000	245,000,000	245,000,000
Earnings per share - diluted (in dollars per share)	$ 7.93	$ 8.66	$ 7.27
Weighted average number shares as-converted basis (in shares)	398,000,000	400,000,000	403,000,000
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	$ 214	$ 247	$ 218
Weighted-average shares outstanding - basic (in shares)	11,000,000	12,000,000	12,000,000
Earnings per share - basic (in dollars per share)	$ 19.58	$ 21.30	$ 17.72
Income allocation - diluted	$ 214	$ 246	$ 217
Weighted-average shares outstanding - diluted (in shares)	11,000,000	12,000,000	12,000,000
Earnings per share - diluted (in dollars per share)	$ 19.56	$ 21.26	$ 17.69
Weighted average number shares as-converted basis (in shares)	44,000,000	46,000,000	49,000,000
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	$ 391	$ 430	$ 359
Income allocation - diluted	$ 391	$ 429	$ 358
Weighted average number shares as-converted basis (in shares)	1,000,000
Series B Preferred Stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Weighted average number shares as-converted basis (in shares)	32,000,000	32,000,000	32,000,000
Series C Preferred Stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Weighted average number shares as-converted basis (in shares)	43,000,000	44,000,000	44,000,000