Stockholders' Equity Share Repurchases in the Open Market (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Class of Stock [Line Items]
Total cost		$ 7,192	$ 6,891	$ 6,987
Class A common stock
Class of Stock [Line Items]
Shares repurchased in the open market (in shares)	[1],[2]	58	77	91
Average repurchase price per share (in dollars per share)	[2],[3]	$ 123.76	$ 89.98	$ 77.05
Total cost	[2]	$ 7,192	$ 6,891	$ 6,987

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

[2] Shares repurchased in the open market reflect repurchases settled during fiscal 2018, 2017 and 2016. These amounts include repurchases traded but not yet settled on or before September 30, 2017, September 30, 2016 and September 30, 2015 for fiscal 2018, 2017 and 2016, respectively. Also, these exclude repurchases traded but not yet settled on or before September 30, 2018, September 30, 2017 and September 30, 2016 for fiscal 2018, 2017 and 2016, respectively.

[3] Figures in the table may not recalculate exactly due to rounding. Average repurchase price per share is calculated based on unrounded numbers.