Stockholders' Equity - Number of Shares of Class A Common Shares Outstanding on an As-Converted Basis (Detail) shares in Millions	Jun. 30, 2019 shares	Sep. 30, 2018 shares
Schedule of Common Stock as Converted [Line Items]
As-converted Class A Common Stock (in shares)	2,251	2,291
U.K.& I preferred stock
Schedule of Common Stock as Converted [Line Items]
Preferred stock, shares outstanding	2	2
Preferred stock, conversion rate into Class A Common Stock	12.9360	12.9550
As-converted Class A Common Stock (in shares)	32	32
Europe preferred stock
Schedule of Common Stock as Converted [Line Items]
Preferred stock, shares outstanding	3	3
Preferred stock, conversion rate into Class A Common Stock	13.884	13.8880
As-converted Class A Common Stock (in shares)	44	44
Class A common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding	1,729	1,768
As-converted Class A Common Stock (in shares)	1,729	1,768
Class B common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding	245	245
Common stock, conversion rate	1.6298	1.6298
As-converted Class A Common Stock (in shares)	400	400
Class C common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding	12	12
Common stock, conversion rate	4.0000	4.0000
As-converted Class A Common Stock (in shares)	46	47