Stockholders' Equity - Number of Shares of Class A Common Shares Outstanding on an As-Converted Basis (Detail) shares in Millions		Dec. 31, 2017 shares		Sep. 30, 2017 shares
Schedule of Common Stock as Converted [Line Items]
As-converted Class A Common Stock		2,335
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		12.9660		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		13.8930		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		1,805	[2]	1,818
As-converted Class A Common Stock	[1],[2]	1,805
Class B common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding		245		245
Common stock, conversion rate into Class A Common Stock	[3]	1.6483
As-converted Class A Common Stock	[1]	405
Class C common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding		12		13
Common stock, conversion rate into Class A Common Stock		4
As-converted Class A Common Stock	[1]	49

[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 December 31, 2017.

[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.