Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended		6 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Net income		$ 2,977	$ 2,605	$ 5,954	$ 5,127
Class A common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	[1]	$ 2,287	$ 2,007	$ 4,577	$ 3,952
Weighted- Average Shares Outstanding - Basic		1,748	1,798	1,754	1,805
Earnings per Share - Basic	[2]	$ 1.31	$ 1.12	$ 2.61	$ 2.19
Income Allocation - Diluted	[1]	$ 2,977	$ 2,605	$ 5,954	$ 5,127
Weighted- Average Shares Outstanding - Diluted	[3]	2,279	2,337	2,285	2,345
Earnings per Share - Diluted	[2]	$ 1.31	$ 1.11	$ 2.61	$ 2.19
Class B common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	[1]	$ 523	$ 452	$ 1,044	$ 886
Weighted- Average Shares Outstanding - Basic		245	245	245	245
Earnings per Share - Basic	[2]	$ 2.13	$ 1.84	$ 4.25	$ 3.61
Income Allocation - Diluted	[1]	$ 523	$ 451	$ 1,043	$ 885
Weighted- Average Shares Outstanding - Diluted		245	245	245	245
Earnings per Share - Diluted	[2]	$ 2.13	$ 1.84	$ 4.25	$ 3.60
Class C common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	[1]	$ 61	$ 55	$ 122	$ 110
Weighted- Average Shares Outstanding - Basic		12	12	12	13
Earnings per Share - Basic	[2]	$ 5.23	$ 4.46	$ 10.44	$ 8.76
Income Allocation - Diluted	[1]	$ 61	$ 55	$ 122	$ 109
Weighted- Average Shares Outstanding - Diluted		12	12	12	13
Earnings per Share - Diluted	[2]	$ 5.23	$ 4.46	$ 10.42	$ 8.74
Participating securities
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation - Basic	[1],[4]	$ 106	$ 91	$ 211	$ 179
Income Allocation - Diluted	[1],[4]	$ 106	$ 91	$ 211	$ 178

[1] Net income is allocated based on proportional ownership on an as-converted basis. The weighted-average number of shares of as-converted class B common stock used in the income allocation was 400 million for the three and six months ended March 31, 2019, and 405 million for the three and six months ended March 31, 2018. The weighted-average number of shares of as-converted class C common stock used in the income allocation was 47 million for the three and six months ended March 31, 2019, and 49 million and 50 million for the three and six months ended March 31, 2018, respectively. The weighted-average number of shares of preferred stock included within participating securities was 32 million of as-converted UK&I preferred stock for the three and six months ended March 31, 2019 and 2018, and 44 million of as-converted Europe preferred stock for the three and six months ended March 31, 2019 and 2018.

[2] Figures in the table may not recalculate exactly due to rounding. Earnings per share is calculated based on unrounded numbers.

[3] Weighted-average diluted shares outstanding are calculated on an as-converted basis and include incremental common stock equivalents, as calculated under the treasury stock method. The computation includes approximately 3 million common stock equivalents for the three and six months ended March 31, 2019 and 4 million common stock equivalents for the three and six months ended March 31, 2018, because their effect would have been dilutive. The computation excludes 1 million of common stock equivalents for the three and six months ended March 31, 2019, and 2 million of common stock equivalents for the three and six months ended March 31, 2018, because their effect would have been anti-dilutive.

[4] Participating securities include preferred stock outstanding and unvested share-based payment awards that contain non-forfeitable rights to dividends or dividend equivalents, such as the Company’s UK&I and Europe preferred stock, restricted stock awards, restricted stock units and earned performance-based shares. Participating securities’ income is allocated based on the weighted-average number of shares of as-converted stock.