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		3 Months Ended	9 Months Ended
		Jun. 30, 2018	Jun. 30, 2018
Conversion of Stock [Line Items]
Recovery Through Conversion Rate Adjustment		$ (6)	$ (56)
U.K.& I preferred stock
Conversion of Stock [Line Items]
Reduction in equivalent number of shares of class A common stock		0	0	[1]
Effective price per share (in USD per share)	[2]	$ 130.50	$ 113.05
Recovery Through Conversion Rate Adjustment		$ 4	$ 35
Europe preferred stock
Conversion of Stock [Line Items]
Reduction in equivalent number of shares of class A common stock		0	0	[1]
Effective price per share (in USD per share)		$ 130.50	$ 112.92	[2]
Recovery Through Conversion Rate Adjustment		$ 2	$ 21

[1] (1) The reduction in equivalent number of shares of class A common stock was less than one million shares for both series of preferred stock.

[2] (2) Effective price per share for the three months ended June 30, 2018 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 the nine months ended June 30, 2018 is calculated using the weighted-average effective prices of the November 2017 and May 2018 adjustments.