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Reserve for Loss and Loss Expense Reserve for Loss and Loss Expense (Historical Claims Duration) (Details)	Dec. 31, 2018
General Liability [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	6.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	12.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	15.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	17.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	14.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	9.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	6.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	4.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	2.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	1.10%
General Liability [Member] | E&S Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	5.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	11.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	17.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	21.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	15.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	8.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	5.40%
Workers Compensation [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	21.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	24.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	13.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	8.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	5.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	3.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	2.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	2.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	1.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	1.10%
Commercial Automobile [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	37.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	17.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	14.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	13.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	9.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	4.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	1.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.40%
Business Owners' Policies [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	47.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	19.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	8.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	8.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	7.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	4.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	1.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	1.10%
Commercial Property [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	70.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	25.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	2.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	1.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	0.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.00%
Personal Automobile [Member] | Standard Personal Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	57.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	18.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	10.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	7.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	4.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	1.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	0.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.00%
Homeowners [Member] | Standard Personal Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	72.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	20.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	3.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	1.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	1.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	0.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.10%

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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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
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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.

+ References

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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.

+ References

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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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