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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Net income	$ 3,101	$ 2,329	$ 9,055	$ 7,456
Class A common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 2,379	$ 1,793	$ 6,956	$ 5,746
Weighted- Average Shares Outstanding - Basic (in shares)	1,735	1,784	1,748	1,798
Earnings per Share - Basic (in dollars per share)	$ 1.37	$ 1.00	$ 3.98	$ 3.20
Income Allocation - Diluted	$ 3,101	$ 2,329	$ 9,055	$ 7,456
Weighted- Average Shares Outstanding - Diluted (in shares)	2,265	2,321	2,278	2,337
Earnings per Share - Diluted (in dollars per share)	$ 1.37	$ 1.00	$ 3.97	$ 3.19
Class B common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 549	$ 406	$ 1,592	$ 1,293
Weighted- Average Shares Outstanding - Basic (in shares)	245	245	245	245
Earnings per Share - Basic (in dollars per share)	$ 2.23	$ 1.66	$ 6.49	$ 5.27
Income Allocation - Diluted	$ 548	$ 406	$ 1,590	$ 1,291
Weighted- Average Shares Outstanding - Diluted (in shares)	245	245	245	245
Earnings per Share - Diluted (in dollars per share)	$ 2.23	$ 1.65	$ 6.48	$ 5.26
Class C common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 63	$ 49	$ 186	$ 159
Weighted- Average Shares Outstanding - Basic (in shares)	12	12	12	12
Earnings per Share - Basic (in dollars per share)	$ 5.48	$ 4.02	$ 15.92	$ 12.78
Income Allocation - Diluted	$ 63	$ 49	$ 185	$ 158
Weighted- Average Shares Outstanding - Diluted (in shares)	12	12	12	12
Earnings per Share - Diluted (in dollars per share)	$ 5.48	$ 4.01	$ 15.90	$ 12.76
Participating securities
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	$ 110	$ 81	$ 321	$ 258
Income Allocation - Diluted	$ 110	$ 81	$ 321	$ 259

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

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Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities, and addition from assumption of issuance of common shares for dilutive potential common shares; of income (loss) available to common shareholders.

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