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Claim and Claim Adjustment Expense Reserves (Historical Claims Duration) (Details) | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009 | Dec. 31, 2008
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Medical Professional Liability | Specialty |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year One |  |  |  |  |  |  |  |  |  | 3.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two |  |  |  |  |  |  |  |  | 18.90% |
Short-duration Insurance Contracts, Historical Claims Duration, Year Three |  |  |  |  |  |  |  | 23.50% |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Four |  |  |  |  |  |  | 19.80% |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Five |  |  |  |  |  | 12.70% |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Six |  |  |  |  | 7.20% |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven |  |  |  | 4.50% |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight |  |  | 3.30% |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine |  | 3.60% |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten | 0.70% |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Historical Claims Duration, Net | 97.50% |  |  |  |  |  |  |  |  |
Other Professional Liability and Management Liability | Specialty |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year One |  |  |  |  |  |  |  |  |  | 5.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two |  |  |  |  |  |  |  |  | 21.00% |
Short-duration Insurance Contracts, Historical Claims Duration, Year Three |  |  |  |  |  |  |  | 21.10% |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Four |  |  |  |  |  |  | 17.30% |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Five |  |  |  |  |  | 9.90% |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Six |  |  |  |  | 6.00% |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven |  |  |  | 5.60% |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight |  |  | 4.00% |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine |  | 2.60% |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten | 1.50% |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Historical Claims Duration, Net | 94.90% |  |  |  |  |  |  |  |  |
Surety | Specialty |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year One |  |  |  |  |  |  |  |  |  | 21.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two |  |  |  |  |  |  |  |  | 38.40% |
Short-duration Insurance Contracts, Historical Claims Duration, Year Three |  |  |  |  |  |  |  | 17.70% |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Four |  |  |  |  |  |  | 7.50% |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Five |  |  |  |  |  | 3.40% |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Six |  |  |  |  | 2.00% |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven |  |  |  | (2.30%) |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight |  |  | (1.10%) |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine |  | 0.00% |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten | 0.00% |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Historical Claims Duration, Net | 87.00% |  |  |  |  |  |  |  |  |
Commercial Auto | Commercial |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year One |  |  |  |  |  |  |  |  |  | 28.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two |  |  |  |  |  |  |  |  | 22.90% |
Short-duration Insurance Contracts, Historical Claims Duration, Year Three |  |  |  |  |  |  |  | 18.40% |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Four |  |  |  |  |  |  | 14.10% |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Five |  |  |  |  |  | 9.20% |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Six |  |  |  |  | 3.40% |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven |  |  |  | 0.90% |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight |  |  | 0.20% |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine |  | 0.40% |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten | 0.70% |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Historical Claims Duration, Net | 98.30% |  |  |  |  |  |  |  |  |
General Liability | Commercial |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  | 16.60% |
Short-duration Insurance Contracts, Historical Claims Duration, Year Three |  |  |  |  |  |  |  | 20.70% |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Four |  |  |  |  |  |  | 20.80% |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Five |  |  |  |  |  | 15.20% |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Six |  |  |  |  | 8.40% |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven |  |  |  | 4.20% |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight |  |  | 2.50% |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine |  | 1.50% |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten | 1.20% |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Historical Claims Duration, Net | 96.00% |  |  |  |  |  |  |  |  |
Workers' Compensation | Commercial |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year One |  |  |  |  |  |  |  |  |  | 13.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two |  |  |  |  |  |  |  |  | 21.20% |
Short-duration Insurance Contracts, Historical Claims Duration, Year Three |  |  |  |  |  |  |  | 14.40% |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Four |  |  |  |  |  |  | 10.70% |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Five |  |  |  |  |  | 7.80% |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Six |  |  |  |  | 5.50% |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven |  |  |  | 4.90% |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight |  |  | 3.70% |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine |  | 3.00% |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten | 3.20% |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Historical Claims Duration, Net | 87.90% |  |  |  |  |  |  |  |  |
Excluding Hardy | International |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year One |  |  |  |  |  |  |  |  |  | 19.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two |  |  |  |  |  |  |  |  | 25.60% |
Short-duration Insurance Contracts, Historical Claims Duration, Year Three |  |  |  |  |  |  |  | 12.20% |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Four |  |  |  |  |  |  | 6.80% |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Five |  |  |  |  |  | 5.90% |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Six |  |  |  |  | 4.70% |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven |  |  |  | 5.10% |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight |  |  | 3.10% |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine |  | 5.80% |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten | 0.40% |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Historical Claims Duration, Net | 89.20% |  |  |  |  |  |  |  |  |
Hardy | International |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year One |  |  |  |  |  |  |  |  |  | 24.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two |  |  |  |  |  |  |  |  | 39.00% |
Short-duration Insurance Contracts, Historical Claims Duration, Year Three |  |  |  |  |  |  |  | 14.30% |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Four |  |  |  |  |  |  | 5.00% |  |  |
Short-duration Insurance Contracts, Historical Claims Duration, Year Five |  |  |  |  |  | 2.80% |  |  |  |
Short-Duration Insurance Contracts, Historical Claims Duration, Net | 85.80% |  |  |  |  |  |  |  |  |

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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 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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