Unpaid losses and loss expenses Unpaid losses and loss expenses (Supplementary PPD) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($)
North America Commercial P&C Insurance - Workers' Compensation [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	$ (373)
North America Commercial P&C Insurance - Workers' Compensation [Member] | Accident years prior to 2015
Supplementary PPD [Line Items]
Short-Duration PPD	(151)
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	2,968
North America Commercial P&C Insurance - Workers' Compensation [Member] | Accident years 2015 - 2024
Supplementary PPD [Line Items]
Short-Duration PPD	(222)
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	7,227
North America Commercial P&C Insurance - Liability [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	415
North America Commercial P&C Insurance - Liability [Member] | Accident years prior to 2015
Supplementary PPD [Line Items]
Short-Duration PPD	69
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	2,007
North America Commercial P&C Insurance - Liability [Member] | Accident years 2015 - 2024
Supplementary PPD [Line Items]
Short-Duration PPD	346
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	20,394
North America Commercial P&C Insurance - Other Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	99
North America Commercial P&C Insurance - Other Casualty [Member] | Accident years prior to 2015
Supplementary PPD [Line Items]
Short-Duration PPD	(4)
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	210
North America Commercial P&C Insurance - Other Casualty [Member] | Accident years 2015 - 2024
Supplementary PPD [Line Items]
Short-Duration PPD	103
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	2,554
North America Commercial P&C Insurance - Non-Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(434)
North America Commercial P&C Insurance - Non-Casualty [Member] | Accident years prior to 2015
Supplementary PPD [Line Items]
Short-Duration PPD	18
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	5
North America Commercial P&C Insurance - Non-Casualty [Member] | Accident years 2015 - 2024
Supplementary PPD [Line Items]
Short-Duration PPD	(452)
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	3,567
North America Personal P&C Insurance [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(301)
North America Personal P&C Insurance [Member] | Accident years prior to 2015
Supplementary PPD [Line Items]
Short-Duration PPD	(11)
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	36
North America Personal P&C Insurance [Member] | Accident years 2015 - 2024
Supplementary PPD [Line Items]
Short-Duration PPD	(290)
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	4,146
Overseas General Insurance - Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(27)
Overseas General Insurance - Casualty [Member] | Accident years prior to 2015
Supplementary PPD [Line Items]
Short-Duration PPD	75
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	644
Overseas General Insurance - Casualty [Member] | Accident years 2015 - 2024
Supplementary PPD [Line Items]
Short-Duration PPD	(102)
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	7,995
Overseas General Insurance - Non-Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(261)
Overseas General Insurance - Non-Casualty [Member] | Accident years prior to 2015
Supplementary PPD [Line Items]
Short-Duration PPD	(11)
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	129
Overseas General Insurance - Non-Casualty [Member] | Accident years 2015 - 2024
Supplementary PPD [Line Items]
Short-Duration PPD	(250)
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	3,778
Global Reinsurance - Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	1
Global Reinsurance - Casualty [Member] | Accident years prior to 2015
Supplementary PPD [Line Items]
Short-Duration PPD	(7)
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	267
Global Reinsurance - Casualty [Member] | Accident years 2015 - 2024
Supplementary PPD [Line Items]
Short-Duration PPD	8
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	1,061
Global Reinsurance - Non-Casualty [Member]
Supplementary PPD [Line Items]
Short-Duration PPD	(28)
Global Reinsurance - Non-Casualty [Member] | Accident years prior to 2015
Supplementary PPD [Line Items]
Short-Duration PPD	0
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	14
Global Reinsurance - Non-Casualty [Member] | Accident years 2015 - 2024
Supplementary PPD [Line Items]
Short-Duration PPD	(28)
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	$ 526