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, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net	$ 45,304
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	12,840
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claim Adjustment Expense, Aggregate Reconciling Items	741
Unpaid unallocated loss adjustment expenses	1,655
Unpaid losses and loss expenses	60,540	$ 37,303	$ 38,315	$ 37,443
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	7,584
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	1,872
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	17,312
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	4,273
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	2,250
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	555
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	1,771
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	437
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	2,021
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	(168)
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	2,102
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	968
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	5,681
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	2,617
Global Reinsurance Property [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net	154
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	11
Global Reinsurance Non-Property [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid Loss and Allocated Loss Adjustment Expenses, Net	1,592
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	(114)
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	4,837
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	$ 1,825