Loss and Loss Adjustment Expenses - Summary of Average Annual Percentage Payout Of Incurred Losses By Age Net of Reinsurance For Worker's Compensation and general Liability (Unaudited) (Detail)	Dec. 31, 2020
Shortduration Insurance Contracts Liability For Unpaid Claims And Allocated Claim Adjustment Expense Net [Abstract]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance, Year 1	25.60%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance, Year 2	30.20%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance, Year 3	14.10%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance, Year 4	7.60%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance, Year 5	4.30%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance, Year 6	2.10%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance, Year 7	1.60%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance, Year 8	0.90%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance, Year 9	0.70%
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance, Year 10	0.40%

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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 4G
-URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671329-158438

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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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