Unpaid losses and loss expenses Unpaid losses and loss expenses (Details) (Supplementary PPD) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)
North America Commercial P&C Insurance - Workers' Compensation [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	$ (288)
North America Commercial P&C Insurance - Workers' Compensation [Member] | prior to 2010 [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(93)
North America Commercial P&C Insurance - Liability [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(273)
North America Commercial P&C Insurance - Liability [Member] | prior to 2010 [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(49)
North America Commercial P&C Insurance - Other Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(36)
North America Commercial P&C Insurance - Other Casualty [Member] | prior to 2010 [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	5
North America Commercial P&C Insurance - Non-Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	32
North America Commercial P&C Insurance - Non-Casualty [Member] | prior to 2010 [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(6)
North America Personal P&C Insurance [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(86)
North America Personal P&C Insurance [Member] | prior to 2010 [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(1)
Overseas General Insurance - Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(61)
Overseas General Insurance - Casualty [Member] | prior to 2010 [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(18)
Overseas General Insurance - Non-Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	1
Overseas General Insurance - Non-Casualty [Member] | prior to 2010 [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	1
Global Reinsurance - Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(58)
Global Reinsurance - Casualty [Member] | prior to 2010 [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(50)
Global Reinsurance - Non-Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	30
Global Reinsurance - Non-Casualty [Member] | prior to 2010 [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	$ (4)