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Reserve for Loss and Loss Expense (Exposure to Environmental Claims) (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | --- | --- | ---
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Loss and Loss expense reserves, Gross | $ 28,415 | $ 29,962 | $ 30,411 | $ 30,653
Liability for Asbestos and Environmental Claims, Net | 21,212 | 22,716 | 23,161 | 22,994
Asbestos Claim [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Loss and Loss expense reserves, Gross | 7,600 |  |  |
Asbestos [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Loss and Loss expense reserves, Gross | 7,577 | 7,847 | 8,024 | 8,751
Liability for Asbestos and Environmental Claims, Net | 6,346 | $ 6,615 | $ 6,793 | $ 7,314
Landfill Sites Related Claims [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Loss and Loss expense reserves, Gross | 12,400 |  |  |
Liability for Asbestos and Environmental Claims, Net | 7,700 |  |  |
Leaking Underground Storage Tanks Claims [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Loss and Loss expense reserves, Gross | 8,400 |  |  |
Liability for Asbestos and Environmental Claims, Net | $ 7,200 |  |  |

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- Definition Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, before estimated recoveries from reinsurers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 + Details
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- Definition Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, net of estimated reinsurance recoveries. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 + Details
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