Stockholders' Equity - Number of Shares of Class A Common Shares Outstanding on an As-Converted Basis (Detail) shares in Millions	Mar. 31, 2021 shares	Sep. 30, 2020 shares
Schedule of Common Stock as Converted [Line Items]
As-converted Class A Common Stock (in shares)	2,183	2,197
UK&I preferred stock
Schedule of Common Stock as Converted [Line Items]
Preferred stock, shares outstanding (in shares)	2	2
Preferred stock, conversion rate into Class A Common Stock	6.368	6.387
As-converted Class A Common Stock (in shares)	16	16
Europe preferred stock
Schedule of Common Stock as Converted [Line Items]
Preferred stock, shares outstanding (in shares)	3	3
Preferred stock, conversion rate into Class A Common Stock	6.853	6.861
As-converted Class A Common Stock (in shares)	22	22
Class A common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding (in shares)	1,694	1,683
As-converted Class A Common Stock (in shares)	1,694	1,683
Class B common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding (in shares)	245	245
Common stock, conversion rate into Class A Common Stock	1.6228	1.6228
As-converted Class A Common Stock (in shares)	398	398
Class C common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding (in shares)	11	11
Common stock, conversion rate into Class A Common Stock	4.0000	4.0000
As-converted Class A Common Stock (in shares)	43	43
Series A preferred stock
Schedule of Common Stock as Converted [Line Items]
Preferred stock, shares outstanding (in shares)	0	0
Preferred stock, conversion rate into Class A Common Stock	100.0000	100.0000
As-converted Class A Common Stock (in shares)	10	35