Stockholders' Equity Effect of VE Territory Covered Losses Recovery on the Company Repurchasing its Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		12 Months Ended
		Sep. 30, 2018		Sep. 30, 2017
Conversion of Stock [Line Items]
Recovery through conversion rate adjustment		$ 0		$ 0
Series B Preferred Stock
Conversion of Stock [Line Items]
Reduction in equivalent number of as-converted shares of class A common stock (in shares)		0	[1]	2
Effective price per share (in dollars per share)	[2]	$ 113.05		$ 88.70
Recovery through conversion rate adjustment		$ 35		$ 190
Series C Preferred Stock
Conversion of Stock [Line Items]
Reduction in equivalent number of as-converted shares of class A common stock (in shares)	[1]	0		0
Effective price per share (in dollars per share)	[2]	$ 112.92		$ 85.01
Recovery through conversion rate adjustment		$ 21		$ 1

[1] The reduction in equivalent number of shares of class A common stock was less than one million shares.

[2] Effective price per share for each adjustment made during the year is calculated using the volume-weighted average price of the Company’s class A common stock over a pricing period in accordance with the Company’s current certificates of designations for its series B and C convertible participating preferred stock. Effective price per share for each fiscal year is calculated using the weighted-average effective prices of the respective adjustments made during the year