Reserves for Losses and Loss Adjustment Expenses - Schedule of Supplementary Unaudited Information About Annual Percentage Payout of Incurred Losses and ALAE, Net of Reinsurance (Detail)	Dec. 31, 2024
Casualty Lines
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	2.80%
Year 2	8.20%
Year 3	12.50%
Year 4	14.90%
Year 5	14.00%
Year 6	11.00%
Year 7	11.10%
Year 8	7.40%
Year 9	5.40%
Year 10	3.90%
Remainder	8.80%
Specialty Lines
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	30.40%
Year 2	24.30%
Year 3	14.20%
Year 4	8.50%
Year 5	8.10%
Year 6	5.10%
Year 7	3.10%
Year 8	2.00%
Year 9	1.40%
Year 10	0.90%
Remainder	2.00%
Run-off Lines
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	15.00%
Year 2	18.00%
Year 3	14.80%
Year 4	13.10%
Year 5	11.10%
Year 6	8.80%
Year 7	5.40%
Year 8	3.90%
Year 9	2.80%
Year 10	2.00%
Remainder	5.10%

Short-duration Insurance Contracts, Historical Claims Duration, Remainder

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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 sixth 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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