Earnings Per Share - Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Net income	$ 3,084	$ 2,977	$ 6,356	$ 5,954
Class A common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 2,360	$ 2,287	$ 4,866	$ 4,577
Weighted- Average Shares Outstanding - Basic (in shares)	1,703	1,748	1,708	1,754
Earnings per Share - Basic (in dollars per share)	$ 1.39	$ 1.31	$ 2.85	$ 2.61
Income Allocation - Diluted	$ 3,084	$ 2,977	$ 6,356	$ 5,954
Weighted- Average Shares Outstanding - Diluted (in shares)	2,228	2,279	2,234	2,285
Earnings per Share - Diluted (in dollars per share)	$ 1.38	$ 1.31	$ 2.85	$ 2.61
Class B common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 552	$ 523	$ 1,135	$ 1,044
Weighted- Average Shares Outstanding - Basic (in shares)	245	245	245	245
Earnings per Share - Basic (in dollars per share)	$ 2.25	$ 2.13	$ 4.62	$ 4.25
Income Allocation - Diluted	$ 551	$ 523	$ 1,134	$ 1,043
Weighted- Average Shares Outstanding - Diluted (in shares)	245	245	245	245
Earnings per Share - Diluted (in dollars per share)	$ 2.25	$ 2.13	$ 4.62	$ 4.25
Class C common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 61	$ 61	$ 126	$ 122
Weighted- Average Shares Outstanding - Basic (in shares)	11	12	11	12
Earnings per Share - Basic (in dollars per share)	$ 5.54	$ 5.23	$ 11.40	$ 10.44
Income Allocation - Diluted	$ 60	$ 61	$ 125	$ 122
Weighted- Average Shares Outstanding - Diluted (in shares)	11	12	11	12
Earnings per Share - Diluted (in dollars per share)	$ 5.54	$ 5.23	$ 11.38	$ 10.42
Participating securities
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 111	$ 106	$ 229	$ 211
Income Allocation - Diluted	$ 111	$ 106	$ 229	$ 211