Stockholders' Equity - Number of Shares of Class A Common Shares Outstanding on an As-Converted Basis (Detail)		Mar. 31, 2017 shares		Sep. 30, 2016 shares
Schedule of Common Stock as Converted [Line Items]
Preferred stock, conversion ratio				13.952
As-converted Class A Common Stock		2,384,000,000
U.K.& I preferred stock
Schedule of Common Stock as Converted [Line Items]
Preferred stock, shares outstanding		2,000,000		2,000,000
Preferred stock, conversion ratio		13.3880
As-converted Class A Common Stock	[1]	33,000,000
Europe preferred stock
Schedule of Common Stock as Converted [Line Items]
Preferred stock, shares outstanding		3,000,000		3,000,000
Preferred stock, conversion ratio		13.9480
As-converted Class A Common Stock	[1]	44,000,000
Class A common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding		1,847,000,000	[2]	1,871,000,000
As-converted Class A Common Stock	[1],[2]	1,847,000,000
Class B common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding		245,000,000		245,000,000
Common stock, conversion rate into Class A Common Stock	[3]	1.6483
As-converted Class A Common Stock	[1]	405,000,000
Class C common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding		14,000,000		17,000,000
Common stock, conversion rate into Class A Common Stock		4
As-converted Class A Common Stock	[1]	55,000,000

[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 March 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.