Stockholders' Equity Effect of VE Territory Covered Losses Recovery on the Company Repurchasing its Common Stock (Details)		3 Months Ended	6 Months Ended
		Mar. 31, 2017 USD ($) $ / shares shares	Mar. 31, 2017 USD ($) $ / shares shares	Sep. 30, 2016
Conversion of Stock [Line Items]
Recovery through conversion rate adjustment | $			$ (120,000,000)
Conversion rate of preferred stock to class A common stock after adjustment				13.952
As-converted preferred stock after recovery		2,384,000,000	2,384,000,000
U.K.& I preferred stock
Conversion of Stock [Line Items]
Reduction in equivalent number of shares of class A common stock		1,000,000	1,000,000
Effective price per share (in USD per share) | $ / shares	[1]	$ 85.01	$ 85.01
Recovery through conversion rate adjustment | $		$ 119,000,000	$ 119,000,000
Conversion rate of preferred stock to class A common stock after adjustment		13.3880	13.3880
As-converted preferred stock after recovery	[2]	33,000,000	33,000,000
Europe preferred stock
Conversion of Stock [Line Items]
Reduction in equivalent number of shares of class A common stock	[3]	0	0
Effective price per share (in USD per share) | $ / shares	[1]	$ 85.01	$ 85.01
Recovery through conversion rate adjustment | $		$ 1,000,000	$ 1,000,000
Conversion rate of preferred stock to class A common stock after adjustment		13.9480	13.9480
As-converted preferred stock after recovery	[2]	44,000,000	44,000,000

[1] Effective price per share 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.

[2] Figures in the table may not recalculate exactly due to rounding. As-converted class A common stock is calculated based on unrounded numbers.

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