Earnings Per Share - Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Net income	$ 2,373	$ 3,101	$ 8,729	$ 9,055
Class A common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 1,814	$ 2,379	$ 6,679	$ 6,956
Weighted- Average Shares Outstanding - Basic (in shares)	1,690	1,735	1,702	1,748
Earnings per Share - Basic (in dollars per share)	$ 1.07	$ 1.37	$ 3.92	$ 3.98
Income Allocation - Diluted	$ 2,373	$ 3,101	$ 8,729	$ 9,055
Weighted- Average Shares Outstanding - Diluted (in shares)	2,214	2,265	2,227	2,278
Earnings per Share - Diluted (in dollars per share)	$ 1.07	$ 1.37	$ 3.92	$ 3.97
Class B common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 428	$ 549	$ 1,564	$ 1,592
Weighted- Average Shares Outstanding - Basic (in shares)	245	245	245	245
Earnings per Share - Basic (in dollars per share)	$ 1.74	$ 2.23	$ 6.37	$ 6.49
Income Allocation - Diluted	$ 427	$ 548	$ 1,561	$ 1,590
Weighted- Average Shares Outstanding - Diluted (in shares)	245	245	245	245
Earnings per Share - Diluted (in dollars per share)	$ 1.74	$ 2.23	$ 6.36	$ 6.48
Class C common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 46	$ 63	$ 172	$ 186
Weighted- Average Shares Outstanding - Basic (in shares)	11	12	11	12
Earnings per Share - Basic (in dollars per share)	$ 4.29	$ 5.48	$ 15.70	$ 15.92
Income Allocation - Diluted	$ 47	$ 63	$ 172	$ 185
Weighted- Average Shares Outstanding - Diluted (in shares)	11	12	11	12
Earnings per Share - Diluted (in dollars per share)	$ 4.29	$ 5.48	$ 15.68	$ 15.90
Participating securities
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 85	$ 110	$ 314	$ 321
Income Allocation - Diluted	$ 85	$ 110	$ 314	$ 321