Claim and Claim Adjustment Expense Reserves (Historical Claims Duration) (Details)	Dec. 31, 2025
International
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	15.00%
Year 2	22.60%
Year 3	11.30%
Year 4	6.80%
Year 5	5.80%
Year 6	6.50%
Year 7	5.60%
Year 8	2.60%
Year 9	2.80%
Year 10	1.30%
Medical Professional Liability \| Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	3.60%
Year 2	16.60%
Year 3	21.40%
Year 4	16.70%
Year 5	13.20%
Year 6	9.70%
Year 7	6.10%
Year 8	2.90%
Year 9	1.80%
Year 10	0.40%
Other Professional Liability and Management Liability \| Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	6.40%
Year 2	19.50%
Year 3	19.80%
Year 4	15.00%
Year 5	11.50%
Year 6	7.30%
Year 7	5.80%
Year 8	4.70%
Year 9	3.50%
Year 10	0.70%
Surety \| Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	15.20%
Year 2	36.20%
Year 3	12.90%
Year 4	8.40%
Year 5	6.30%
Year 6	9.20%
Year 7	(0.90%)
Year 8	2.60%
Year 9	0.00%
Year 10	0.00%
Commercial Auto \| Commercial
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	22.60%
Year 2	22.50%
Year 3	19.10%
Year 4	15.20%
Year 5	10.70%
Year 6	5.50%
Year 7	2.60%
Year 8	0.90%
Year 9	0.70%
Year 10	0.00%
General Liability \| Commercial
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	3.40%
Year 2	12.10%
Year 3	15.50%
Year 4	16.60%
Year 5	14.50%
Year 6	9.80%
Year 7	8.40%
Year 8	5.40%
Year 9	5.40%
Year 10	4.00%
Workers' Compensation \| Commercial
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	17.60%
Year 2	24.20%
Year 3	14.50%
Year 4	8.80%
Year 5	5.80%
Year 6	3.80%
Year 7	2.30%
Year 8	1.20%
Year 9	0.80%
Year 10	0.00%

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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/exampleRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 9F
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480046/944-40-55-9F

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 4G
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-4G

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Percentage of average annual payout, after reinsurance, in the fifth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

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Percentage of average annual payout, after reinsurance, in the fourth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

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Percentage of average annual payout, after reinsurance, in the ninth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

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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.

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Percentage of average annual payout, after reinsurance, in the seventh year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

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Percentage of average annual payout, after reinsurance, in the tenth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

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Percentage of average annual payout, after reinsurance, in the third year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

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Percentage of average annual payout, after reinsurance, in the second year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

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