Stockholders' Equity - Effect of VE Territory Covered Losses Recovery on the Company Repurchasing its Common Stock (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Conversion of Stock [Line Items]
Recovery through conversion rate adjustment	$ 8
Series B Preferred Stock
Conversion of Stock [Line Items]
Reduction in equivalent number of as-converted shares of class A common stock	0	0	2
Effective price per share (in dollars per share)	$ 141.32	$ 113.05	$ 88.70
Recovery through conversion rate adjustment	$ 6	$ 35	$ 190
Series C Preferred Stock
Conversion of Stock [Line Items]
Reduction in equivalent number of as-converted shares of class A common stock	0	0	0
Effective price per share (in dollars per share)	$ 150.26	$ 112.92	$ 85.01
Recovery through conversion rate adjustment	$ 2	$ 21	$ 1