Unpaid losses and loss expenses Unpaid losses and loss expenses (Supplementary PPD) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($)
North America Commercial P&C Insurance - Workers' Compensation [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	$ (304)
North America Commercial P&C Insurance - Workers' Compensation [Member] | Accident years prior to 2014
Supplementary PPD [Line Items]
Short-Duration PPD	(113)
North America Commercial P&C Insurance - Workers' Compensation [Member] | Accident years 2014 - 2023
Supplementary PPD [Line Items]
Short-Duration PPD	(191)
North America Commercial P&C Insurance - Liability [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	222
North America Commercial P&C Insurance - Liability [Member] | Accident years prior to 2014
Supplementary PPD [Line Items]
Short-Duration PPD	47
North America Commercial P&C Insurance - Liability [Member] | Accident years 2014 - 2023
Supplementary PPD [Line Items]
Short-Duration PPD	175
North America Commercial P&C Insurance - Other Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	93
North America Commercial P&C Insurance - Other Casualty [Member] | Accident years prior to 2014
Supplementary PPD [Line Items]
Short-Duration PPD	(6)
North America Commercial P&C Insurance - Other Casualty [Member] | Accident years 2014 - 2023
Supplementary PPD [Line Items]
Short-Duration PPD	99
North America Commercial P&C Insurance - Non-Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(381)
North America Commercial P&C Insurance - Non-Casualty [Member] | Accident years prior to 2014
Supplementary PPD [Line Items]
Short-Duration PPD	(4)
North America Commercial P&C Insurance - Non-Casualty [Member] | Accident years 2014 - 2023
Supplementary PPD [Line Items]
Short-Duration PPD	(377)
North America Personal P&C Insurance [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(128)
North America Personal P&C Insurance [Member] | Accident years prior to 2014
Supplementary PPD [Line Items]
Short-Duration PPD	3
North America Personal P&C Insurance [Member] | Accident years 2014 - 2023
Supplementary PPD [Line Items]
Short-Duration PPD	(131)
Overseas General Insurance - Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(48)
Overseas General Insurance - Casualty [Member] | Accident years prior to 2014
Supplementary PPD [Line Items]
Short-Duration PPD	(22)
Overseas General Insurance - Casualty [Member] | Accident years 2014 - 2023
Supplementary PPD [Line Items]
Short-Duration PPD	(26)
Overseas General Insurance - Non-Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(268)
Overseas General Insurance - Non-Casualty [Member] | Accident years prior to 2014
Supplementary PPD [Line Items]
Short-Duration PPD	(4)
Overseas General Insurance - Non-Casualty [Member] | Accident years 2014 - 2023
Supplementary PPD [Line Items]
Short-Duration PPD	(264)
Global Reinsurance - Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	6
Global Reinsurance - Casualty [Member] | Accident years prior to 2014
Supplementary PPD [Line Items]
Short-Duration PPD	(16)
Global Reinsurance - Casualty [Member] | Accident years 2014 - 2023
Supplementary PPD [Line Items]
Short-Duration PPD	22
Global Reinsurance - Non-Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(42)
Global Reinsurance - Non-Casualty [Member] | Accident years prior to 2014
Supplementary PPD [Line Items]
Short-Duration PPD	0
Global Reinsurance - Non-Casualty [Member] | Accident years 2014 - 2023
Supplementary PPD [Line Items]
Short-Duration PPD	$ (42)