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Reserve for Loss and Loss Expense (Historical Claims Duration) (Details)	Dec. 31, 2019
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	6.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	12.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	15.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	17.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	14.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	9.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	5.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	4.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	2.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	1.90%
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	4.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	12.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	18.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	22.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	14.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	9.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	6.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	3.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	2.00%
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	21.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	24.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	13.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	8.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	4.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	4.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	2.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	2.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	1.60%
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	37.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	17.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	14.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	12.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	9.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	4.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	1.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	1.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.10%
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	47.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	19.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	8.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	8.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	6.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	3.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	2.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.80%
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	70.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	25.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	2.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	0.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	0.30%
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	58.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	18.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	9.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	6.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	4.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	1.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	0.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.10%
Homeowners [Member] | Standard Personal Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	72.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	20.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	3.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	1.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	1.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	0.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	0.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.20%
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.

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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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