Stockholders' Equity - Number of Shares of Class A Common Shares Outstanding on an As-Converted Basis (Detail) shares in Millions	Sep. 30, 2020 shares	Sep. 30, 2019 shares
Class of Stock [Line Items]
As-converted Class A Common Stock (in shares)	2,197	2,237
Series A Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares outstanding (in shares)	0	0
Preferred stock conversion rate	100.0000	0
As-converted Class A Common Stock (in shares)	35	0
Series B Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares outstanding (in shares)	2	2
Preferred stock conversion rate	6.3870	12.9360
As-converted Class A Common Stock (in shares)	16	32
Series C Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares outstanding (in shares)	3	3
Preferred stock conversion rate	6.861	13.8840
As-converted Class A Common Stock (in shares)	22	44
Class A common stock
Class of Stock [Line Items]
Common stock, shares, outstanding (in shares)	1,683	1,718
As-converted Class A Common Stock (in shares)	1,683	1,718
Class B common stock
Class of Stock [Line Items]
Common stock, shares, outstanding (in shares)	245	245
Common stock, conversion rate	1.6228	1.6228
As-converted Class A Common Stock (in shares)	398	398
Class C common stock
Class of Stock [Line Items]
Common stock, shares, outstanding (in shares)	11	11
Common stock, conversion rate	4.0000	4.0000
As-converted Class A Common Stock (in shares)	43	45