U.S. and Europe Retrospective Responsibility Plan - Additional Details (Details) € in Millions, $ in Millions		3 Months Ended			9 Months Ended
	Dec. 27, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2020 EUR (€)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2019
Class of Stock [Line Items]
Return of takedown payment to the litigation escrow account	$ 467				$ 467	$ 0
VE covered loss, maximum amount of loss to allow adjustment of conversion rate during six-month period \| €			€ 20
Recovery through conversion rate adjustment		$ 5		$ 0	5
UK&I preferred stock
Class of Stock [Line Items]
Recovery through conversion rate adjustment					$ 72	6
Preferred stock, conversion rate		12.775	12.775		12.775		12.936
Europe preferred stock
Class of Stock [Line Items]
Recovery through conversion rate adjustment					$ 92	2
Preferred stock, conversion rate		13.722	13.722		13.722		13.884
Right to Recover for Covered Losses
Class of Stock [Line Items]
Recovery through conversion rate adjustment		$ 169		$ 2	$ 169	$ 8