U.S. and Europe Retrospective Responsibility Plans - Preferred Stock Rollforward (Details) € in Millions, $ in Millions		3 Months Ended		12 Months Ended
		Dec. 31, 2018 EUR (€)	Dec. 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
Preferred Stock and Right to Recover for Covered Losses [Roll Forward]
Preferred stock, beginning	[1]		5,470
Recovery through conversion rate adjustment			0
Preferred stock, ending	[1]		5,464	$ 5,470
Right to recover for covered losses, beginning	[1]		7
VE territory covered losses incurred			91
Right to recover for covered losses, ending	[1]		92	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	12.9550
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	2,291
Europe preferred stock
Class of Stock [Line Items]
Recovery through conversion rate adjustment			$ (1)	$ (21)
Preferred stock, conversion ratio		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	$ 3,179
Right to Recover for Covered Losses
Class of Stock [Line Items]
Recovery through conversion rate adjustment			(6)
Preferred Stock and Right to Recover for Covered Losses [Roll Forward]
Recovery through conversion rate adjustment			(6)
VE territory covered losses incurred			$ 91

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