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Reserve for Loss and Loss Expense Reserve for Loss and Loss Expense (Historical Claims Duration) (Details) | Dec. 31, 2018
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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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- Definition 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 -Paragraph 4G -URI http://asc.fasb.org/extlink&oid=99384497&loc=SL65671329-158438 + Details
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- Definition 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. + References 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=99384497&loc=SL65671329-158438 + Details
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- Definition 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. + References 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=99384497&loc=SL65671329-158438 + Details
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- Definition 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 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=99384497&loc=SL65671329-158438 + Details
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- Definition 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 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=99384497&loc=SL65671329-158438 + Details
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