Earnings Per Share - Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Net income	$ 3,126	$ 3,272
Class A common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 2,410	$ 2,506
Weighted- Average Shares Outstanding - Basic (in shares)	1,694	1,713
Earnings per Share - Basic (in dollars per share)	$ 1.42	$ 1.46
Income Allocation - Diluted	$ 3,126	$ 3,272
Weighted- Average Shares Outstanding - Diluted (in shares)	2,200	2,240
Earnings per Share - Diluted (in dollars per share)	$ 1.42	$ 1.46
Class B common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 567	$ 583
Weighted- Average Shares Outstanding - Basic (in shares)	245	245
Earnings per Share - Basic (in dollars per share)	$ 2.31	$ 2.37
Income Allocation - Diluted	$ 566	$ 582
Weighted- Average Shares Outstanding - Diluted (in shares)	245	245
Earnings per Share - Diluted (in dollars per share)	$ 2.31	$ 2.37
Class C common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 61	$ 65
Weighted- Average Shares Outstanding - Basic (in shares)	11	11
Earnings per Share - Basic (in dollars per share)	$ 5.69	$ 5.85
Income Allocation - Diluted	$ 61	$ 65
Weighted- Average Shares Outstanding - Diluted (in shares)	11	11
Earnings per Share - Diluted (in dollars per share)	$ 5.68	$ 5.84
Participating securities
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 88	$ 118
Income Allocation - Diluted	$ 89	$ 117