U.S. and Europe Retrospective Responsibility Plans - Preferred Stock Rollforward (Details) € in Millions, $ in Millions		3 Months Ended			6 Months Ended		12 Months Ended
		Mar. 31, 2019 EUR (€)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	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
Recovery through conversion rate adjustment					$ 0	$ 0
Preferred Stock and Right to Recover for Covered Losses [Roll Forward]
Preferred stock, beginning	[1]				5,470
Recovery through conversion rate adjustment					0	0
Preferred stock, ending	[1]		$ 5,464		5,464		$ 5,470
Right to recover for covered losses, beginning	[1]				7
VE territory covered losses incurred			71	$ 1	162	4
Right to recover for covered losses, ending	[1]		$ 163		163		7
U.K.& I preferred stock
Class of Stock [Line Items]
Recovery through conversion rate adjustment					$ (5)		$ (35)
Preferred stock, conversion ratio		12.9390	12.9390		12.9390		12.9550
Preferred Stock and Right to Recover for Covered Losses [Roll Forward]
Preferred stock, beginning	[1]				$ 2,291
Recovery through conversion rate adjustment					(5)		$ (35)
Preferred stock, ending	[1]		$ 2,286		2,286		2,291
VE territory covered losses incurred					0
Europe preferred stock
Class of Stock [Line Items]
Recovery through conversion rate adjustment					$ (1)		$ (21)
Preferred stock, conversion ratio		13.8860	13.8860		13.8860		13.8880
Preferred Stock and Right to Recover for Covered Losses [Roll Forward]
Preferred stock, beginning	[1]				$ 3,179
Recovery through conversion rate adjustment					(1)		$ (21)
Preferred stock, ending	[1]		$ 3,178		3,178		$ 3,179
VE territory covered losses incurred					0
Right to Recover for Covered Losses
Class of Stock [Line Items]
Recovery through conversion rate adjustment					(6)	(50)
Preferred Stock and Right to Recover for Covered Losses [Roll Forward]
Recovery through conversion rate adjustment					(6)	(50)
VE territory covered losses incurred			$ 71	$ 1	$ 162	$ 4

[1] Figures in the table may not recalculate exactly due to rounding. As-converted and book values are based on unrounded numbers.