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Unpaid losses and loss expenses Unpaid losses and loss expenses (Average Annual Payout) (Details) | Dec. 31, 2017
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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 | 4.00%
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 | 15.00%
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 | 4.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine | 2.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten | 3.00%
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 | 15.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two | 26.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three | 17.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four | 12.00%
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 | 1.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine | 1.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten | 0.00%
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 | 46.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two | 37.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three | 7.00%
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 | 58.00%
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 | 14.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four | 12.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five | 10.00%
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 | 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%
Global Reinsurance - Casualty [Member] |
Short-duration Insurance Contracts, Historical Claims Duration [Line Items] |
Short-duration Insurance Contracts, Historical Claims Duration, Year One | 19.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two | 20.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three | 12.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four | 11.00%
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 | 5.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight | 4.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine | 4.00%
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 | 38.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three | 14.00%
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 | 2.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven | 0.00%
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%

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- Definition Percentage of average annual payout, after reinsurance, in the eighth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4G -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671329-158438 + Details
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- Definition Percentage of average annual payout, after reinsurance, in the first year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4G -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671329-158438 + Details
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