Stockholders' Equity - Number of Shares of Class A Common Shares Outstanding on an As-Converted Basis (Detail) shares in Millions		Dec. 31, 2018 shares	Sep. 30, 2018 shares
Schedule of Common Stock as Converted [Line Items]
As-converted Class A Common Stock	[1],[2]	2,277	2,291
U.K.& I preferred stock
Schedule of Common Stock as Converted [Line Items]
Preferred stock, shares outstanding	[2]	2	2
Preferred stock, conversion rate into Class A Common Stock		12.9390	12.9550
As-converted Class A Common Stock	[1],[2],[3]	32	32
Europe preferred stock
Schedule of Common Stock as Converted [Line Items]
Preferred stock, shares outstanding	[2]	3	3
Preferred stock, conversion rate into Class A Common Stock		13.8860	13.8880
As-converted Class A Common Stock	[1],[2],[3]	44	44
Class A common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding	[2],[4]	1,754	1,768
As-converted Class A Common Stock	[1],[2],[4]	1,754	1,768
Class B common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding	[2]	245	245
Common stock, conversion rate	[5]	1.6298	1.6298
As-converted Class A Common Stock	[1],[2]	400	400
Class C common stock
Schedule of Common Stock as Converted [Line Items]
Common stock, shares outstanding	[2]	12	12
Common stock, conversion rate		4.0000	4.0000
As-converted Class A Common Stock	[1],[2]	47	47

[1] As-converted class A common stock is calculated based on unrounded numbers.

[2] Figures in the table may not recalculate exactly due to rounding.

[3] The reduction in equivalent number of shares of class A common stock was less than one million shares during the three months ended December 31, 2018.

[4] Class A common stock shares outstanding reflect repurchases settled on or before December 31, 2018 and September 30, 2018.

[5] 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.