Stockholders' Equity - Number of Shares of Class A Common Shares Outstanding on an As-Converted Basis (Detail) shares in Millions		Jun. 30, 2018 shares		Sep. 30, 2017 shares
Schedule of Common Stock as Converted [Line Items]
As-converted Class A Common Stock		2,302
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		12.955		13.077
As-converted Class A Common Stock	[1]	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		13.888		13.948
As-converted Class A Common Stock	[1]	44
Class A common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding		1,778	[2]	1,818
As-converted Class A Common Stock	[1],[2]	1,778
Class B common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding		245		245
Common stock, conversion rate	[3]	1.6298
As-converted Class A Common Stock	[1]	400
Class C common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding		12		13
Common stock, conversion rate		4.0000
As-converted Class A Common Stock	[1]	48

[1] Figures in the table may not recalculate exactly due to rounding. As-converted class A common stock is calculated based on unrounded numbers.

[2] Class A common stock shares outstanding exclude repurchases traded but not yet settled on or before June 30, 2018.

[3] The class B to class A common stock conversion rate is presented on a rounded basis. Conversion calculations for dividend payments are based on a conversion rate rounded to the tenth decimal.