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, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Net income	$ 3,026	$ 3,084	$ 6,152	$ 6,356
Class A common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 2,342	$ 2,360	$ 4,752	$ 4,866
Weighted- Average Shares Outstanding - Basic (in shares)	1,695	1,703	1,695	1,708
Earnings per Share - Basic (in dollars per share)	$ 1.38	$ 1.39	$ 2.80	$ 2.85
Income Allocation - Diluted	$ 3,026	$ 3,084	$ 6,152	$ 6,356
Weighted- Average Shares Outstanding - Diluted (in shares)	2,193	2,228	2,196	2,234
Earnings per Share - Diluted (in dollars per share)	$ 1.38	$ 1.38	$ 2.80	$ 2.85
Class B common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 550	$ 552	$ 1,117	$ 1,135
Weighted- Average Shares Outstanding - Basic (in shares)	245	245	245	245
Earnings per Share - Basic (in dollars per share)	$ 2.24	$ 2.25	$ 4.55	$ 4.62
Income Allocation - Diluted	$ 550	$ 551	$ 1,116	$ 1,134
Weighted- Average Shares Outstanding - Diluted (in shares)	245	245	245	245
Earnings per Share - Diluted (in dollars per share)	$ 2.24	$ 2.25	$ 4.54	$ 4.62
Class C common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 59	$ 61	$ 120	$ 126
Weighted- Average Shares Outstanding - Basic (in shares)	11	11	11	11
Earnings per Share - Basic (in dollars per share)	$ 5.52	$ 5.54	$ 11.22	$ 11.40
Income Allocation - Diluted	$ 59	$ 60	$ 120	$ 125
Weighted- Average Shares Outstanding - Diluted (in shares)	11	11	11	11
Earnings per Share - Diluted (in dollars per share)	$ 5.52	$ 5.54	$ 11.20	$ 11.38
Participating securities
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 75	$ 111	$ 163	$ 229
Income Allocation - Diluted	$ 74	$ 111	$ 163	$ 229