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, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net		$ 46,062
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		14,331
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claim Adjustment Expense, Aggregate Reconciling Items	[1]	873
Unpaid unallocated loss adjustment expenses		1,424
Unpaid losses and loss expenses		62,690	$ 62,960	$ 63,179	$ 60,540
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		9,414
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		1,657
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		16,447
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		5,400
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		1,913
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		546
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		1,759
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		1,150
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		2,525
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		603
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		5,977
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		2,113
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		2,377
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		1,263
Global Reinsurance - Casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net		1,177
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		35
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		255
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		107
Short-Duration Insurance, Other [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net		4,218
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		$ 1,457

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