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, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Unpaid Loss and Allocated Loss Adjustment Expense	$ 60,209
Ceded unpaid loss and allocated loss adjustment expense	18,156
Unpaid unallocated loss adjustment expenses	1,757
Unpaid losses and loss expenses	80,122	$ 75,747	$ 72,330	$ 67,192
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	10,159
Ceded unpaid loss and allocated loss adjustment expense	1,190
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	21,092
Ceded unpaid loss and allocated loss adjustment expense	7,427
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	2,596
Ceded unpaid loss and allocated loss adjustment expense	1,064
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	3,203
Ceded unpaid loss and allocated loss adjustment expense	1,673
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	3,550
Ceded unpaid loss and allocated loss adjustment expense	577
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	8,157
Ceded unpaid loss and allocated loss adjustment expense	2,527
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	3,515
Ceded unpaid loss and allocated loss adjustment expense	1,736
Global Reinsurance - Casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Unpaid Loss and Allocated Loss Adjustment Expense	1,240
Ceded unpaid loss and allocated loss adjustment expense	80
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	411
Ceded unpaid loss and allocated loss adjustment expense	101
Other Segments [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net Unpaid Loss and Allocated Loss Adjustment Expense	6,286
Ceded unpaid loss and allocated loss adjustment expense	$ 1,781