Stockholders' Equity - Share Repurchases in the Open Market (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended		6 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Equity, Class of Treasury Stock [Line Items]
Total cost		$ 1,938	$ 2,072	$ 4,331	$ 3,850
Class A common stock
Equity, Class of Treasury Stock [Line Items]
Shares repurchased in the open market	[1],[2]	14	17	31	33
Average repurchase price per share	[2],[3]	$ 144.94	$ 120.26	$ 141.08	$ 115.41
Total cost	[2]	$ 1,938	$ 2,072	$ 4,331	$ 3,850

[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 and six months ended March 31, 2019 and 2018. These amounts include repurchases traded but not yet settled on or before September 30, 2018 and 2017 for six months, respectively, and December 31, 2018 and 2017 for three months, respectively. Also, these amounts exclude repurchases traded but not yet settled on or before March 31, 2019 and 2018, respectively.

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