Stockholders' Equity - Share Repurchases in the Open Market (Detail) - Class A common stock - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Equity, Class of Treasury Stock [Line Items]
Shares repurchased in the open market	[1],[2]	17	16
Average repurchase price per share	[2],[3]	$ 138.11	$ 110.24
Total cost	[2]	$ 2,393	$ 1,778

[1] All shares repurchased in the open market have been retired and constitute authorized but unissued shares.

[2] Figures in the table may not recalculate exactly due to rounding. Shares repurchased in the open market reflect repurchases settled during the three months ended December 31, 2018 and 2017. These amounts include repurchases traded but not yet settled on or before September 30, 2018 and 2017, respectively, and exclude repurchases traded but not yet settled on or before December 31, 2018 and 2017, respectively.

[3] Average repurchase price per share is calculated based on unrounded numbers.