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Reserve for Loss and Loss Expense Reserve for Loss and Loss Expense (Historical Claims Duration) (Details) | Dec. 31, 2017
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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 | 6.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two | 12.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three | 15.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four | 16.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five | 14.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six | 10.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven | 6.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight | 4.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine | 2.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten | 1.30%
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 | 4.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two | 12.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three | 18.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four | 20.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five | 15.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six | 8.30%
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 | 20.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two | 23.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three | 13.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four | 8.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five | 5.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six | 3.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven | 2.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight | 2.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine | 1.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten | 1.90%
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 | 38.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two | 17.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three | 14.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four | 12.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five | 9.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six | 4.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven | 1.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight | 1.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine | 0.70%
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 | 46.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two | 20.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three | 8.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four | 7.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five | 7.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six | 4.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven | 2.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight | 1.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine | 0.10%
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 | 70.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two | 25.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three | 2.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four | 0.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five | 0.40%
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 | 56.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two | 18.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three | 10.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four | 7.40%
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 | 0.40%
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 | 71.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two | 21.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three | 3.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four | 2.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five | 1.60%
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 | 0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine | 0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten | 0.10%

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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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://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 + 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://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 + 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://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 + 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://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 + 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://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 + Details
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- Definition 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. + 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 + Details
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- Definition 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. + 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 + Details
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- Definition 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. + 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 + Details
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