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Supplementary Information of Average Historical Claims (Detail)	Dec. 31, 2022
Commercial Specialty | Property Insurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average annual percentage payout of incurred claims by age, net of reinsurance year one	61.00%
Average annual percentage payout of incurred claims by age, net of reinsurance year two	28.90%
Average annual percentage payout of incurred claims by age, net of reinsurance year three	4.70%
Commercial Specialty | Casualty Insurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average annual percentage payout of incurred claims by age, net of reinsurance year one	7.70%
Average annual percentage payout of incurred claims by age, net of reinsurance year two	17.40%
Average annual percentage payout of incurred claims by age, net of reinsurance year three	19.30%
Average annual percentage payout of incurred claims by age, net of reinsurance year four	19.70%
Average annual percentage payout of incurred claims by age, net of reinsurance year five	12.20%
Average annual percentage payout of incurred claims by age, net of reinsurance year six	7.60%
Average annual percentage payout of incurred claims by age, net of reinsurance year seven	4.30%
Average annual percentage payout of incurred claims by age, net of reinsurance year eight	1.70%
Average annual percentage payout of incurred claims by age, net of reinsurance year nine	0.60%
Average annual percentage payout of incurred claims by age, net of reinsurance year ten	0.10%
Reinsurance Operations | Casualty Insurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average annual percentage payout of incurred claims by age, net of reinsurance year one	0.60%	[1]
Average annual percentage payout of incurred claims by age, net of reinsurance year two	1.90%	[1]
Average annual percentage payout of incurred claims by age, net of reinsurance year three	1.10%	[1]
Average annual percentage payout of incurred claims by age, net of reinsurance year four	0.90%	[1]
Average annual percentage payout of incurred claims by age, net of reinsurance year five	0.00%	[1]
Average annual percentage payout of incurred claims by age, net of reinsurance year six	0.00%	[1]
Average annual percentage payout of incurred claims by age, net of reinsurance year seven	0.00%	[1]
Average annual percentage payout of incurred claims by age, net of reinsurance year eight	0.00%	[1]
Average annual percentage payout of incurred claims by age, net of reinsurance year nine	0.00%	[1]
Average annual percentage payout of incurred claims by age, net of reinsurance year ten	0.00%	[1]
Exited Lines Segment | Property Insurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average annual percentage payout of incurred claims by age, net of reinsurance year one	73.90%
Average annual percentage payout of incurred claims by age, net of reinsurance year two	15.60%
Average annual percentage payout of incurred claims by age, net of reinsurance year three	5.30%
Average annual percentage payout of incurred claims by age, net of reinsurance year four	3.10%
Exited Lines Segment | Casualty Insurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average annual percentage payout of incurred claims by age, net of reinsurance year one	13.90%
Average annual percentage payout of incurred claims by age, net of reinsurance year two	20.30%
Average annual percentage payout of incurred claims by age, net of reinsurance year three	21.50%
Average annual percentage payout of incurred claims by age, net of reinsurance year four	17.30%
Average annual percentage payout of incurred claims by age, net of reinsurance year five	7.20%
Average annual percentage payout of incurred claims by age, net of reinsurance year six	5.80%
Average annual percentage payout of incurred claims by age, net of reinsurance year seven	1.90%
Average annual percentage payout of incurred claims by age, net of reinsurance year eight	0.10%
[1] May not be indicative of future average annual percentage payout of incurred claims due to a change in mix of business

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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.

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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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