Stockholders' Equity - Number of Shares of Class A Common Shares Outstanding on an As-Converted Basis (Detail) shares in Millions	Dec. 31, 2021 shares	Sep. 30, 2021 shares
Schedule of Common Stock as Converted [Line Items]
As-converted Class A Common Stock (in shares)	2,142	2,161
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)	6	7
Series B 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	6.271	6.321
As-converted Class A Common Stock (in shares)	16	16
Series C 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	6.829	6.834
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)	1,661	1,677
As-converted Class A Common Stock (in shares)	1,661	1,677
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	1.6181	1.6228
As-converted Class A Common Stock (in shares)	397	398
Class C common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding (in shares)	10	10
Common stock, conversion rate into Class A Common Stock	4.0000	4.0000
As-converted Class A Common Stock (in shares)	40	41