Unpaid losses and loss expenses Unpaid losses and loss expenses (Details) (Supplementary PPD) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)
North America Commercial P&C Insurance - Workers' Compensation [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	$ (350)
North America Commercial P&C Insurance - Workers' Compensation [Member] | prior to 2011
Supplementary PPD [Line Items]
Short-Duration PPD	(94)
North America Commercial P&C Insurance - Workers' Compensation [Member] | 2011 - 2020
Supplementary PPD [Line Items]
Short-Duration PPD	(256)
North America Commercial P&C Insurance - Liability [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(293)
North America Commercial P&C Insurance - Liability [Member] | prior to 2011
Supplementary PPD [Line Items]
Short-Duration PPD	(176)
North America Commercial P&C Insurance - Liability [Member] | 2011 - 2020
Supplementary PPD [Line Items]
Short-Duration PPD	(117)
North America Commercial P&C Insurance - Other Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	42
North America Commercial P&C Insurance - Other Casualty [Member] | prior to 2011
Supplementary PPD [Line Items]
Short-Duration PPD	23
North America Commercial P&C Insurance - Other Casualty [Member] | 2011 - 2020
Supplementary PPD [Line Items]
Short-Duration PPD	19
North America Commercial P&C Insurance - Non-Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(35)
North America Commercial P&C Insurance - Non-Casualty [Member] | prior to 2011
Supplementary PPD [Line Items]
Short-Duration PPD	2
North America Commercial P&C Insurance - Non-Casualty [Member] | 2011 - 2020
Supplementary PPD [Line Items]
Short-Duration PPD	(37)
North America Personal P&C Insurance [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	84
North America Personal P&C Insurance [Member] | prior to 2011
Supplementary PPD [Line Items]
Short-Duration PPD	(1)
North America Personal P&C Insurance [Member] | 2011 - 2020
Supplementary PPD [Line Items]
Short-Duration PPD	85
Overseas General Insurance - Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(50)
Overseas General Insurance - Casualty [Member] | prior to 2011
Supplementary PPD [Line Items]
Short-Duration PPD	(59)
Overseas General Insurance - Casualty [Member] | 2011 - 2020
Supplementary PPD [Line Items]
Short-Duration PPD	9
Overseas General Insurance - Non-Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(77)
Overseas General Insurance - Non-Casualty [Member] | prior to 2011
Supplementary PPD [Line Items]
Short-Duration PPD	(8)
Overseas General Insurance - Non-Casualty [Member] | 2011 - 2020
Supplementary PPD [Line Items]
Short-Duration PPD	(69)
Global Reinsurance - Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(22)
Global Reinsurance - Casualty [Member] | prior to 2011
Supplementary PPD [Line Items]
Short-Duration PPD	(10)
Global Reinsurance - Casualty [Member] | 2011 - 2020
Supplementary PPD [Line Items]
Short-Duration PPD	(12)
Global Reinsurance - Non-Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(2)
Global Reinsurance - Non-Casualty [Member] | prior to 2011
Supplementary PPD [Line Items]
Short-Duration PPD	0
Global Reinsurance - Non-Casualty [Member] | 2011 - 2020
Supplementary PPD [Line Items]
Short-Duration PPD	$ (2)