Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended			6 Months Ended
		Mar. 31, 2017	Mar. 31, 2016		Mar. 31, 2017		Mar. 31, 2016
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Net income	[1]	$ 430	$ 1,707		$ 2,500		$ 3,648
Class A common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation (A)	[1]	$ 332	$ 1,360		$ 1,928		$ 2,910
Weighted- Average Shares Outstanding (B)		1,854	1,909		1,857		1,923
Earnings per Share (A)/(B)		$ 0.18	$ 0.71	[2]	$ 1.04	[2]	$ 1.51	[2]
Income Allocation (A)	[1]	$ 430	$ 1,707		$ 2,500		$ 3,648
Weighted- Average Shares Outstanding (B)	[3]	2,406	2,401		2,413		2,416
Earnings per Share (A)/(B)		$ 0.18	$ 0.71	[2]	$ 1.04	[2]	$ 1.51	[2]
Class B common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation (A)	[1]	$ 73	$ 288		$ 420		$ 612
Weighted- Average Shares Outstanding (B)		245	245		245		245
Earnings per Share (A)/(B)		$ 0.30	$ 1.17	[2]	$ 1.71	[2]	$ 2.49	[2]
Income Allocation (A)	[1]	$ 72	$ 288		$ 419		$ 611
Weighted- Average Shares Outstanding (B)		245	245		245		245
Earnings per Share (A)/(B)		$ 0.29	$ 1.17	[2]	$ 1.71	[2]	$ 2.49	[2]
Class C common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation (A)	[1]	$ 10	$ 55		$ 65		$ 118
Weighted- Average Shares Outstanding (B)		15	19		16		19
Earnings per Share (A)/(B)		$ 0.72	$ 2.85	[2]	$ 4.15	[2]	$ 6.05	[2]
Income Allocation (A)	[1]	$ 10	$ 55		$ 65		$ 117
Weighted- Average Shares Outstanding (B)		15	19		16		19
Earnings per Share (A)/(B)		$ 0.72	$ 2.84	[2]	$ 4.14	[2]	$ 6.04	[2]
Participating securities
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Income Allocation (A)	[1],[4]	$ 15	$ 4		$ 87		$ 8
Income Allocation (A)	[1],[4]	$ 15	$ 4		$ 87		$ 8

[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 405 million for the three and six months ended March 31, 2017 and 2016. The weighted-average number of shares of as-converted class C common stock used in the income allocation was 58 million and 63 million for the three and six months ended March 31, 2017, respectively and 77 million and 78 million for the three and six months ended March 31, 2016, respectively. The weighted-average number of shares of preferred stock, included within participating securities, was 34 million of as-converted U.K.&I preferred stock and 44 million of as-converted Europe preferred stock for the three and six months ended March 31, 2017.

[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 5 million common stock equivalents for the three and six months ended March 31, 2017 and 2016, because their effect would be dilutive. The computation excludes 3 million of common stock equivalents for the three and six months ended March 31, 2017 and 1 million of common stock equivalents for the three and six months ended March 31, 2016, 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 U.K.&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.