Stockholders' Equity - Number of Shares of Class A Common Shares Outstanding on an As-Converted Basis (Detail)	Jun. 30, 2020 shares	Sep. 30, 2019 shares
Schedule of Common Stock as Converted [Line Items]
As-converted Class A Common Stock (in shares)	2,203,000,000	2,237,000,000
U.K.& I preferred stock
Schedule of Common Stock as Converted [Line Items]
Preferred stock, shares outstanding (in shares)	2,000,000	2,000,000
Preferred stock, conversion rate into Class A Common Stock	12.775	12.936
As-converted Class A Common Stock (in shares)	32,000,000	32,000,000
Europe preferred stock
Schedule of Common Stock as Converted [Line Items]
Preferred stock, shares outstanding (in shares)	3,000,000	3,000,000
Preferred stock, conversion rate into Class A Common Stock	13.722	13.884
As-converted Class A Common Stock (in shares)	43,000,000	44,000,000
Class A common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding (in shares)	1,687,000,000	1,718,000,000
As-converted Class A Common Stock (in shares)	1,687,000,000	1,718,000,000
Class B common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding (in shares)	245,000,000	245,000,000
Common stock, conversion rate into Class A Common Stock	1.6228	1.6228
As-converted Class A Common Stock (in shares)	398,000,000	398,000,000
Class C common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding (in shares)	11,000,000	11,000,000
Common stock, conversion rate into Class A Common Stock	4.0000	4.0000
As-converted Class A Common Stock (in shares)	43,000,000	45,000,000