Stockholders' Equity - Number of Shares of Class A Common Shares Outstanding on an As-Converted Basis (Detail) shares in Millions	Sep. 30, 2019 shares	Sep. 30, 2018 shares	Sep. 30, 2016
Class of Stock [Line Items]
As-converted Class A Common Stock	2,237	2,291
Series B Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares outstanding (in shares)	2	2
Preferred stock conversion rate	12.936	12.9550	13.952
As-converted Class A Common Stock	32	32
Series C Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares outstanding (in shares)	3	3
Preferred stock conversion rate	13.884	13.888
As-converted Class A Common Stock	44	44
Class A common stock
Class of Stock [Line Items]
Common stock, shares, outstanding (in shares)	1,718	1,768
As-converted Class A Common Stock	1,718	1,768
Class B common stock
Class of Stock [Line Items]
Common stock, shares, outstanding (in shares)	245	245
Common stock, conversion rate	1.6228	1.6298
As-converted Class A Common Stock	398	400
Class C common stock
Class of Stock [Line Items]
Common stock, shares, outstanding (in shares)	11	12
Common stock, conversion rate	4.0000	4.0000
As-converted Class A Common Stock	45	47