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, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Equity, Class of Treasury Stock [Line Items]
Total cost	$ 3,133	$ 1,938	$ 5,503	$ 4,331
Class A common stock
Equity, Class of Treasury Stock [Line Items]
Shares repurchased in the open market (in shares)	18	14	31	31
Average repurchase price per share (in dollars per share)	$ 181.11	$ 144.94	$ 180.31	$ 141.08
Total cost	$ 3,133	$ 1,938	$ 5,503	$ 4,331

Equity impact of the value of stock that has been repurchased and retired during the period. The excess of the purchase price over par value can be charged against retained earnings (once the excess is fully allocated to additional paid in capital).

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