Stockholders' Equity Effect of VE Territory Covered Losses Recovery on the Company Repurchasing its Common Stock (Details) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2017 USD ($) $ / shares shares
Series B Preferred Stock
Conversion of Stock [Line Items]
Stock As Converted Repurchased During Period Through VE Territory Covered Losses Recovery Shares (Less than 1 million Series C) | shares	2
Share Reduction Price Paid Per Share VE Territory Covered Losses Recovery | $ / shares	$ 88.70	[1]
Recovery through conversion rate adjustment | $	$ 190
Series C Preferred Stock
Conversion of Stock [Line Items]
Stock As Converted Repurchased During Period Through VE Territory Covered Losses Recovery Shares (Less than 1 million Series C) | shares	0	[2]
Share Reduction Price Paid Per Share VE Territory Covered Losses Recovery | $ / shares	$ 85.01	[1]
Recovery through conversion rate adjustment | $	$ 1

[1] Effective price per share 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.

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