Stockholders' Equity Share Repurchases in the Open Market (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		12 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Class of Stock [Line Items]
Total cost		$ 6,891	$ 6,987	$ 2,910
Class A common stock
Class of Stock [Line Items]
Shares repurchased in the open market (in shares)	[1],[2]	77	91
Average repurchase price per share	[2],[3]	$ 89.98	$ 77.05
Total cost	[2]	$ 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 2017. These amounts include repurchases traded but not yet settled on or before September 30, 2016 for fiscal 2017 or September 30, 2015 for fiscal 2016 and exclude repurchases traded but not yet settled on or before September 30, 2017 for fiscal 2017 or September 30, 2016 for fiscal 2016.

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