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, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net		$ 45,847
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		14,857
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claim Adjustment Expense, Aggregate Reconciling Items	[1]	831
Unpaid unallocated loss adjustment expenses		1,425
Unpaid losses and loss expenses		62,960	$ 63,179	$ 60,540	$ 37,303
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		9,183
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		1,766
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		16,485
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		4,812
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		1,884
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		544
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		1,871
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		1,531
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		2,319
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		895
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		5,833
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		2,070
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		2,265
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		1,208
Global Reinsurance - Casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net		1,218
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		58
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		390
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		99
Other Short-duration Insurance Product Line [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net		4,399
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments		$ 1,874

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