Claim and Claim Adjustment Expense Reserves - Schedule of Historical Claims Duration (Detail) - CNA Financial [Member]	Dec. 31, 2016
Medical Professional Liability [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Claims in Year 1	3.10%
Average Annual Percentage Payout of Claims in Year 2	18.20%
Average Annual Percentage Payout of Claims in Year 3	22.30%
Average Annual Percentage Payout of Claims in Year 4	19.60%
Average Annual Percentage Payout of Claims in Year 5	12.70%
Average Annual Percentage Payout of Claims in Year 6	8.00%
Average Annual Percentage Payout of Claims in Year 7	5.00%
Average Annual Percentage Payout of Claims in Year 8	4.80%
Average Annual Percentage Payout of Claims in Year 9	3.70%
Average Annual Percentage Payout of Claims in Year 10	0.30%
Total	97.70%
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	5.70%
Average Annual Percentage Payout of Claims in Year 2	20.60%
Average Annual Percentage Payout of Claims in Year 3	21.00%
Average Annual Percentage Payout of Claims in Year 4	17.00%
Average Annual Percentage Payout of Claims in Year 5	10.00%
Average Annual Percentage Payout of Claims in Year 6	6.30%
Average Annual Percentage Payout of Claims in Year 7	5.60%
Average Annual Percentage Payout of Claims in Year 8	3.80%
Average Annual Percentage Payout of Claims in Year 9	1.90%
Average Annual Percentage Payout of Claims in Year 10	3.20%
Total	95.10%
Surety [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Claims in Year 1	23.40%
Average Annual Percentage Payout of Claims in Year 2	32.80%
Average Annual Percentage Payout of Claims in Year 3	20.20%
Average Annual Percentage Payout of Claims in Year 4	8.90%
Average Annual Percentage Payout of Claims in Year 5	3.70%
Average Annual Percentage Payout of Claims in Year 6	1.50%
Average Annual Percentage Payout of Claims in Year 7	(0.70%)
Average Annual Percentage Payout of Claims in Year 8	2.00%
Average Annual Percentage Payout of Claims in Year 9	1.00%
Average Annual Percentage Payout of Claims in Year 10	2.00%
Total	94.80%
Commercial Auto [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Claims in Year 1	27.20%
Average Annual Percentage Payout of Claims in Year 2	23.10%
Average Annual Percentage Payout of Claims in Year 3	18.30%
Average Annual Percentage Payout of Claims in Year 4	13.90%
Average Annual Percentage Payout of Claims in Year 5	9.10%
Average Annual Percentage Payout of Claims in Year 6	3.90%
Average Annual Percentage Payout of Claims in Year 7	1.30%
Average Annual Percentage Payout of Claims in Year 8	0.20%
Average Annual Percentage Payout of Claims in Year 9	0.30%
Average Annual Percentage Payout of Claims in Year 10	0.30%
Total	97.60%
General Liability [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Claims in Year 1	5.00%
Average Annual Percentage Payout of Claims in Year 2	16.30%
Average Annual Percentage Payout of Claims in Year 3	20.50%
Average Annual Percentage Payout of Claims in Year 4	20.10%
Average Annual Percentage Payout of Claims in Year 5	15.30%
Average Annual Percentage Payout of Claims in Year 6	8.00%
Average Annual Percentage Payout of Claims in Year 7	4.20%
Average Annual Percentage Payout of Claims in Year 8	2.70%
Average Annual Percentage Payout of Claims in Year 9	1.10%
Average Annual Percentage Payout of Claims in Year 10	1.30%
Total	94.50%
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	21.40%
Average Annual Percentage Payout of Claims in Year 3	14.60%
Average Annual Percentage Payout of Claims in Year 4	10.50%
Average Annual Percentage Payout of Claims in Year 5	7.50%
Average Annual Percentage Payout of Claims in Year 6	5.70%
Average Annual Percentage Payout of Claims in Year 7	4.60%
Average Annual Percentage Payout of Claims in Year 8	3.00%
Average Annual Percentage Payout of Claims in Year 9	2.10%
Average Annual Percentage Payout of Claims in Year 10	0.30%
Total	83.20%

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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