Unpaid losses and loss expenses Unpaid losses and loss expenses (Reconciliation of reserve Balances to Liability for Unpaid Loss) (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Unpaid Loss and Allocated Loss Adjustment Expense	$ 64,201
Ceded unpaid loss and allocated loss adjustment expense	17,963
Unpaid unallocated loss adjustment expenses	1,840
Unpaid losses and loss expenses	84,004	$ 80,122	$ 75,747	$ 72,330
North America Commercial P&C Insurance - Workers' Compensation [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Unpaid Loss and Allocated Loss Adjustment Expense	10,195
Ceded unpaid loss and allocated loss adjustment expense	1,090
North America Commercial P&C Insurance - Liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Unpaid Loss and Allocated Loss Adjustment Expense	22,401
Ceded unpaid loss and allocated loss adjustment expense	7,564
North America Commercial P&C Insurance - Other Casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Unpaid Loss and Allocated Loss Adjustment Expense	2,764
Ceded unpaid loss and allocated loss adjustment expense	1,148
North America Commercial P&C Insurance - Non-Casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Unpaid Loss and Allocated Loss Adjustment Expense	3,572
Ceded unpaid loss and allocated loss adjustment expense	1,022
North America Personal P&C Insurance [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Unpaid Loss and Allocated Loss Adjustment Expense	4,182
Ceded unpaid loss and allocated loss adjustment expense	529
Overseas General Insurance - Casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Unpaid Loss and Allocated Loss Adjustment Expense	8,639
Ceded unpaid loss and allocated loss adjustment expense	2,986
Overseas General Insurance - Non-Casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Unpaid Loss and Allocated Loss Adjustment Expense	3,907
Ceded unpaid loss and allocated loss adjustment expense	1,880
Global Reinsurance - Casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Unpaid Loss and Allocated Loss Adjustment Expense	1,328
Ceded unpaid loss and allocated loss adjustment expense	137
Global Reinsurance - Non-Casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Unpaid Loss and Allocated Loss Adjustment Expense	540
Ceded unpaid loss and allocated loss adjustment expense	83
Other Segments [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Unpaid Loss and Allocated Loss Adjustment Expense	6,673
Ceded unpaid loss and allocated loss adjustment expense	$ 1,524