Number of Shares of Class A Common Shares Outstanding on an As-Converted Basis (Detail) shares in Millions					12 Months Ended
		Sep. 30, 2017 conversion_rate shares		Sep. 30, 2016 shares	Sep. 30, 2017 conversion_rate shares		Mar. 19, 2015 conversion_rate
Class of Stock [Line Items]
As-converted class A common stock	[1]				2,350
Series B Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares outstanding		2		2	2
Preferred stock series B conversion rate		13.0770	[2]	13.952
Shares Outstanding As Converted Basis	[1]	32			32
Series C Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares outstanding		3		3	3
Preferred stock series B conversion rate		13.9480		13.952
Shares Outstanding As Converted Basis	[1]	44			44
Class A common stock
Class of Stock [Line Items]
Common stock, shares, outstanding		1,818	[3]	1,871	1,818	[3]
As-converted class A common stock	[1],[3]				1,818
Class B common stock
Class of Stock [Line Items]
Common stock, shares, outstanding		245		245	245
Common stock, conversion rate | conversion_rate	[2]	1.6483			1.6483		1.6483
As-converted class A common stock	[1]				405
Class C common stock
Class of Stock [Line Items]
Common stock, shares, outstanding		13		17	13
Common stock, conversion rate | conversion_rate		4.0000			4.0000		4.0
As-converted class A common stock	[1]				51

[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] 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.

[3] Class A common stock shares outstanding reflect repurchases settled on or before September 30, 2017.