Claim and Claim Adjustment Expense Reserves - Schedule of Historical Claims Duration (Detail) - CNA Financial [Member]	Dec. 31, 2018
Medical Professional Liability [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Claims in Year 1	3.80%
Average Annual Percentage Payout of Claims in Year 2	19.70%
Average Annual Percentage Payout of Claims in Year 3	24.30%
Average Annual Percentage Payout of Claims in Year 4	20.10%
Average Annual Percentage Payout of Claims in Year 5	12.10%
Average Annual Percentage Payout of Claims in Year 6	7.20%
Average Annual Percentage Payout of Claims in Year 7	4.50%
Average Annual Percentage Payout of Claims in Year 8	2.50%
Average Annual Percentage Payout of Claims in Year 9	1.00%
Average Annual Percentage Payout of Claims in Year 10	2.90%
Total	98.10%
Other Professional Liability and Management Liability [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Claims in Year 1	6.20%
Average Annual Percentage Payout of Claims in Year 2	21.50%
Average Annual Percentage Payout of Claims in Year 3	21.30%
Average Annual Percentage Payout of Claims in Year 4	17.40%
Average Annual Percentage Payout of Claims in Year 5	10.70%
Average Annual Percentage Payout of Claims in Year 6	6.40%
Average Annual Percentage Payout of Claims in Year 7	5.80%
Average Annual Percentage Payout of Claims in Year 8	2.90%
Average Annual Percentage Payout of Claims in Year 9	3.30%
Average Annual Percentage Payout of Claims in Year 10	2.10%
Total	97.60%
Surety [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Claims in Year 1	21.60%
Average Annual Percentage Payout of Claims in Year 2	37.90%
Average Annual Percentage Payout of Claims in Year 3	21.00%
Average Annual Percentage Payout of Claims in Year 4	7.90%
Average Annual Percentage Payout of Claims in Year 5	2.10%
Average Annual Percentage Payout of Claims in Year 6	1.60%
Average Annual Percentage Payout of Claims in Year 7	(1.70%)
Average Annual Percentage Payout of Claims in Year 8	(0.50%)
Total	89.90%
Commercial Auto [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Claims in Year 1	28.00%
Average Annual Percentage Payout of Claims in Year 2	23.00%
Average Annual Percentage Payout of Claims in Year 3	18.50%
Average Annual Percentage Payout of Claims in Year 4	14.20%
Average Annual Percentage Payout of Claims in Year 5	9.30%
Average Annual Percentage Payout of Claims in Year 6	3.10%
Average Annual Percentage Payout of Claims in Year 7	1.30%
Average Annual Percentage Payout of Claims in Year 8	0.30%
Average Annual Percentage Payout of Claims in Year 9	0.20%
Average Annual Percentage Payout of Claims in Year 10	0.70%
Total	98.60%
General Liability [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Claims in Year 1	4.30%
Average Annual Percentage Payout of Claims in Year 2	15.90%
Average Annual Percentage Payout of Claims in Year 3	19.50%
Average Annual Percentage Payout of Claims in Year 4	20.50%
Average Annual Percentage Payout of Claims in Year 5	15.70%
Average Annual Percentage Payout of Claims in Year 6	8.00%
Average Annual Percentage Payout of Claims in Year 7	5.60%
Average Annual Percentage Payout of Claims in Year 8	3.20%
Average Annual Percentage Payout of Claims in Year 9	1.60%
Average Annual Percentage Payout of Claims in Year 10	0.80%
Total	95.10%
Workers' Compensation [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Claims in Year 1	13.70%
Average Annual Percentage Payout of Claims in Year 2	21.10%
Average Annual Percentage Payout of Claims in Year 3	13.90%
Average Annual Percentage Payout of Claims in Year 4	10.60%
Average Annual Percentage Payout of Claims in Year 5	7.50%
Average Annual Percentage Payout of Claims in Year 6	4.40%
Average Annual Percentage Payout of Claims in Year 7	4.40%
Average Annual Percentage Payout of Claims in Year 8	3.20%
Average Annual Percentage Payout of Claims in Year 9	2.50%
Average Annual Percentage Payout of Claims in Year 10	1.20%
Total	82.50%

Short-duration insurance contracts, historical claims duration, net.

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Short-duration insurance contracts net, historical claim duration, year eight.

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Short-duration insurance contracts net historical claim duration year seven.

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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.

+ References

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=99384497&loc=SL65671329-158438

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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 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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