U.S. and Europe Retrospective Responsibility Plans - Preferred Stock Rollforward (Details) € in Millions, $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Jun. 30, 2019 EUR (€)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)
Class of Stock [Line Items]
VE territory covered loss, maximum amount of loss to allow adjustment of conversion rate during six-month period | €	€ 20
Preferred Stock and Right to Recover for Covered Losses [Roll Forward]
Preferred stock, beginning				$ 5,470
Right to recover for covered losses, beginning				7
VE territory covered losses incurred		$ 8	$ 5	170	$ 9
Preferred stock, ending		5,462		5,462		$ 5,470
Right to recover for covered losses, ending		$ 169		169		7
U.K.& I preferred stock
Class of Stock [Line Items]
Recovery through conversion rate adjustment				$ (6)		$ (35)
Preferred stock, conversion ratio	12.9360	12.9360		12.9360		12.9550
Preferred Stock and Right to Recover for Covered Losses [Roll Forward]
Preferred stock, beginning				$ 2,291
VE territory covered losses incurred				0
Recovery through conversion rate adjustment				(6)		$ (35)
Preferred stock, ending		$ 2,285		2,285		2,291
Europe preferred stock
Class of Stock [Line Items]
Recovery through conversion rate adjustment				$ (2)		$ (21)
Preferred stock, conversion ratio	13.884	13.884		13.884		13.8880
Preferred Stock and Right to Recover for Covered Losses [Roll Forward]
Preferred stock, beginning				$ 3,179
VE territory covered losses incurred				0
Recovery through conversion rate adjustment				(2)		$ (21)
Preferred stock, ending		$ 3,177		3,177		$ 3,179
Preferred Stock
Class of Stock [Line Items]
Recovery through conversion rate adjustment		2	6	8	56
Preferred Stock and Right to Recover for Covered Losses [Roll Forward]
Recovery through conversion rate adjustment		2	6	8	56
Right to Recover for Covered Losses
Class of Stock [Line Items]
Recovery through conversion rate adjustment		(2)	(6)	(8)	(56)
Preferred Stock and Right to Recover for Covered Losses [Roll Forward]
VE territory covered losses incurred		8	5	170	9
Recovery through conversion rate adjustment		$ (2)	$ (6)	$ (8)	$ (56)