Claim and Claim Adjustment Expense Reserves (Historical Claims Duration) (Details)	Dec. 31, 2019
Medical Professional Liability \| Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	3.80%
Year 2	20.00%
Year 3	24.30%
Year 4	19.40%
Year 5	12.40%
Year 6	7.80%
Year 7	5.10%
Year 8	3.10%
Year 9	1.10%
Year 10	0.80%
Total	97.80%
Other Professional Liability and Management Liability \| Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	6.60%
Year 2	22.60%
Year 3	21.70%
Year 4	16.90%
Year 5	10.50%
Year 6	6.00%
Year 7	4.90%
Year 8	3.30%
Year 9	3.80%
Year 10	0.70%
Total	97.00%
Surety \| Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	20.00%
Year 2	44.50%
Year 3	21.00%
Year 4	4.40%
Year 5	2.20%
Year 6	0.90%
Year 7	(2.50%)
Year 8	(1.10%)
Year 9	0.00%
Year 10	1.60%
Total	91.00%
Commercial Auto \| Commercial
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	28.60%
Year 2	24.00%
Year 3	18.60%
Year 4	14.30%
Year 5	9.40%
Year 6	3.20%
Year 7	1.50%
Year 8	0.40%
Year 9	0.00%
Year 10	0.00%
Total	100.00%
General Liability \| Commercial
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	4.30%
Year 2	15.80%
Year 3	19.00%
Year 4	20.10%
Year 5	15.70%
Year 6	8.70%
Year 7	6.00%
Year 8	2.50%
Year 9	1.50%
Year 10	1.60%
Total	95.20%
Workers' Compensation \| Commercial
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	14.10%
Year 2	21.40%
Year 3	13.90%
Year 4	10.30%
Year 5	7.00%
Year 6	3.70%
Year 7	2.90%
Year 8	2.70%
Year 9	1.40%
Year 10	0.80%
Total	78.20%
Excluding Hardy \| International
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	20.80%
Year 2	25.70%
Year 3	11.00%
Year 4	7.30%
Year 5	6.80%
Year 6	5.30%
Year 7	5.00%
Year 8	1.70%
Year 9	1.20%
Year 10	2.10%
Total	86.90%
Hardy \| International
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	23.30%
Year 2	39.60%
Year 3	8.70%
Year 4	5.60%
Year 5	4.50%
Year 6	2.50%
Year 7	2.80%
Total	87.00%

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.

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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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