Stockholders' Equity - Effect of VE Territory Covered Losses Through Coversion Rate Adjustments (Details) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended	12 Months Ended
	Jun. 30, 2019	Sep. 30, 2018
U.K.& I preferred stock
Conversion of Stock [Line Items]
Effective price per share (in USD per share)	$ 141.32	$ 113.05
Recovery through conversion rate adjustment	$ 6	$ 35
Europe preferred stock
Conversion of Stock [Line Items]
Effective price per share (in USD per share)	$ 150.26	$ 112.92
Recovery through conversion rate adjustment	$ 2	$ 21