Claim and Claim Adjustment Expense Reserves - Schedule of Historical Claims Duration (Detail) - CNA Financial [Member]	Dec. 31, 2017
Medical Professional Liability [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Claims in Year 1	3.30%
Average Annual Percentage Payout of Claims in Year 2	18.90%
Average Annual Percentage Payout of Claims in Year 3	23.50%
Average Annual Percentage Payout of Claims in Year 4	19.80%
Average Annual Percentage Payout of Claims in Year 5	12.70%
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	3.30%
Average Annual Percentage Payout of Claims in Year 9	3.60%
Average Annual Percentage Payout of Claims in Year 10	0.70%
Total	97.50%
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	5.90%
Average Annual Percentage Payout of Claims in Year 2	21.00%
Average Annual Percentage Payout of Claims in Year 3	21.10%
Average Annual Percentage Payout of Claims in Year 4	17.30%
Average Annual Percentage Payout of Claims in Year 5	9.90%
Average Annual Percentage Payout of Claims in Year 6	6.00%
Average Annual Percentage Payout of Claims in Year 7	5.60%
Average Annual Percentage Payout of Claims in Year 8	4.00%
Average Annual Percentage Payout of Claims in Year 9	2.60%
Average Annual Percentage Payout of Claims in Year 10	1.50%
Total	94.90%
Surety [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Claims in Year 1	21.40%
Average Annual Percentage Payout of Claims in Year 2	38.40%
Average Annual Percentage Payout of Claims in Year 3	17.70%
Average Annual Percentage Payout of Claims in Year 4	7.50%
Average Annual Percentage Payout of Claims in Year 5	3.40%
Average Annual Percentage Payout of Claims in Year 6	2.00%
Average Annual Percentage Payout of Claims in Year 7	(2.30%)
Average Annual Percentage Payout of Claims in Year 8	(1.10%)
Total	87.00%
Commercial Auto [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Claims in Year 1	28.10%
Average Annual Percentage Payout of Claims in Year 2	22.90%
Average Annual Percentage Payout of Claims in Year 3	18.40%
Average Annual Percentage Payout of Claims in Year 4	14.10%
Average Annual Percentage Payout of Claims in Year 5	9.20%
Average Annual Percentage Payout of Claims in Year 6	3.40%
Average Annual Percentage Payout of Claims in Year 7	0.90%
Average Annual Percentage Payout of Claims in Year 8	0.20%
Average Annual Percentage Payout of Claims in Year 9	0.40%
Average Annual Percentage Payout of Claims in Year 10	0.70%
Total	98.30%
General Liability [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Claims in Year 1	4.90%
Average Annual Percentage Payout of Claims in Year 2	16.60%
Average Annual Percentage Payout of Claims in Year 3	20.70%
Average Annual Percentage Payout of Claims in Year 4	20.80%
Average Annual Percentage Payout of Claims in Year 5	15.20%
Average Annual Percentage Payout of Claims in Year 6	8.40%
Average Annual Percentage Payout of Claims in Year 7	4.20%
Average Annual Percentage Payout of Claims in Year 8	2.50%
Average Annual Percentage Payout of Claims in Year 9	1.50%
Average Annual Percentage Payout of Claims in Year 10	1.20%
Total	96.00%
Workers' Compensation [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Claims in Year 1	13.50%
Average Annual Percentage Payout of Claims in Year 2	21.20%
Average Annual Percentage Payout of Claims in Year 3	14.40%
Average Annual Percentage Payout of Claims in Year 4	10.70%
Average Annual Percentage Payout of Claims in Year 5	7.80%
Average Annual Percentage Payout of Claims in Year 6	5.50%
Average Annual Percentage Payout of Claims in Year 7	4.90%
Average Annual Percentage Payout of Claims in Year 8	3.70%
Average Annual Percentage Payout of Claims in Year 9	3.00%
Average Annual Percentage Payout of Claims in Year 10	3.20%
Total	87.90%

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

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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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 4G
-URI http://asc.fasb.org/extlink&oid=65670685&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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