Earnings Per Share - Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Schedule of Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Income allocation - basic earnings per share	$ 12,080	$ 10,301	$ 6,699
Dilutive shares of outstanding stock awards included in computation of weighted-average dilutive shares outstanding (in shares)	3	3	5
Antidilutive securities excluded from computation of earnings per share (in shares)	1	1	2
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	$ 9,273	$ 7,937	$ 5,170
Weighted-average shares outstanding - basic (in shares)	1,742	1,792	1,845
Earnings per share - basic (in dollars per share)	$ 5.32	$ 4.43	$ 2.80
Income allocation - diluted	$ 12,080	$ 10,301	$ 6,699
Weighted-average shares outstanding - diluted (in shares)	2,272	2,329	2,395
Earnings per share - diluted (in dollars per share)	$ 5.32	$ 4.42	$ 2.80
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	$ 2,130	$ 1,787	$ 1,134
Weighted-average shares outstanding - basic (in shares)	245	245	245
Earnings per share - basic (in dollars per share)	$ 8.68	$ 7.28	$ 4.62
Income allocation - diluted	$ 2,127	$ 1,785	$ 1,132
Weighted-average shares outstanding - diluted (in shares)	245	245	245
Earnings per share - diluted (in dollars per share)	$ 8.66	$ 7.27	$ 4.61
Weighted average number shares as-converted basis (in shares)	400	403	405
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	$ 247	$ 218	$ 163
Weighted-average shares outstanding - basic (in shares)	12	12	14
Earnings per share - basic (in dollars per share)	$ 21.30	$ 17.72	$ 11.21
Income allocation - diluted	$ 246	$ 217	$ 162
Weighted-average shares outstanding - diluted (in shares)	12	12	14
Earnings per share - diluted (in dollars per share)	$ 21.26	$ 17.69	$ 11.19
Weighted average number shares as-converted basis (in shares)	46	49	58
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	$ 430	$ 359	$ 232
Income allocation - diluted	$ 429	$ 358	$ 232
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	32	33
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)	44