Unpaid losses and loss expenses Unpaid losses and loss expenses (Reconciliation of reserve Balances to Liability for Unpaid Loss)(Details) (Details) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net		$ 50,492
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		14,687
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claim Adjustment Expense, Aggregate Reconciling Items	[1]	1,109
Unpaid unallocated loss adjustment expenses		1,523
Unpaid losses and loss expenses		67,811	$ 62,690	$ 62,960	$ 63,179
North America Commercial P&C Insurance - Workers' Compensation [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net		9,600
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		1,461
North America Commercial P&C Insurance - Liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net		17,441
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		5,743
North America Commercial P&C Insurance - Other Casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net		2,124
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		564
North America Commercial P&C Insurance - Non-Casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net		2,491
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		1,380
North America Personal P&C Insurance [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net		3,025
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		141
Overseas General Insurance - Casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net		6,832
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		2,426
Overseas General Insurance - Non-Casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net		2,725
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		1,686
Global Reinsurance - Casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net		1,200
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		29
Global Reinsurance - Non-Casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net		312
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		104
Other Segments [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net		4,742
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		$ 1,153

[1] Primarily includes the claims reserve of our International A&H business and Life Insurance segment reserves.