Stockholders' Equity - Share Repurchases in the Open Market (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Class of Stock [Line Items]
Total cost	$ 8,607	$ 7,192	$ 6,891
Class A common stock
Class of Stock [Line Items]
Shares repurchased in the open market (in shares)	56	58	77
Average repurchase price per share (in dollars per share)	$ 154.01	$ 123.76	$ 89.98
Total cost	$ 8,607	$ 7,192	$ 6,891