Earnings Per Share - Additional Information (Detail) - shares shares 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]
Common stock equivalents included in the computation of diluted shares outstanding (in shares)	2	3	3	3
Antidilutive securities excluded from computation of earnings per share (in shares)	1	1	1	2
Class B common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Weighted-average as-converted common stock used in income allocation (in shares)	398	398	398	398
Class C common stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Weighted-average as-converted common stock used in income allocation (in shares)	43	44	43	44
Series A preferred stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Weighted-average as-converted common stock used in income allocation (in shares)	12		17
UK&I preferred stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Weighted-average as-converted common stock used in income allocation (in shares)	16	32	16	32
Europe preferred stock
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Weighted-average as-converted common stock used in income allocation (in shares)	22	44	22	44