Unpaid losses and loss expenses Unpaid losses and loss expenses (Average Annual Payout) (Details)	Dec. 31, 2018
North America Commercial P&C Insurance - Workers' Compensation [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	10.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	16.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	10.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	7.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	5.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	4.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	3.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	3.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	2.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	2.00%
North America Commercial P&C Insurance - Liability [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	5.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	14.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	17.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	16.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	12.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	9.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	6.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	3.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	3.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	1.00%
North America Commercial P&C Insurance - Other Casualty [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	14.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	25.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	18.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	13.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	8.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	6.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	2.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	2.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	1.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	1.00%
North America Commercial P&C Insurance - Non-Casualty [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	47.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	38.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	8.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	3.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	1.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	1.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.00%
North America Personal P&C Insurance [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	59.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	24.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	7.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	5.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	3.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	1.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	1.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.00%
Overseas General Insurance - Casualty [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	8.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	15.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	15.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	12.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	9.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	8.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	6.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	6.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	3.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	3.00%
Overseas General Insurance - Non-Casualty [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	44.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	35.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	11.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	4.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	2.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	1.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	1.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.00%
Global Reinsurance - Casualty [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	20.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	22.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	13.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	10.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	7.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	6.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	5.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	4.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	3.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	3.00%
Global Reinsurance - Non-Casualty [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	34.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	37.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	13.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	7.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	4.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	3.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	1.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	1.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.00%