Stockholders' Equity - Number of Shares of Class A Common Shares Outstanding on an As-Converted Basis (Detail) shares in Millions	6 Months Ended	12 Months Ended
	Mar. 31, 2020 shares	Sep. 30, 2019 shares
Schedule of Common Stock as Converted [Line Items]
As-converted Class A Common Stock (in shares)	2,210	2,237
U.K.& I 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	12.936	12.936
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 (in shares)	3	3
Preferred stock, conversion rate into Class A Common Stock	13.884	13.884
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 (in shares)	1,693	1,718
As-converted Class A Common Stock (in shares)	1,693	1,718
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	1.6228	1.6228
As-converted Class A Common Stock (in shares)	398	398
Class C common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding (in shares)	11	11
Common stock, conversion rate into Class A Common Stock	4.0000	4.0000
As-converted Class A Common Stock (in shares)	43	45