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Reserve for Loss and Loss Expense (Exposure to Environmental Claims) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and Loss expense reserves, Gross	$ 28,701	$ 30,020	$ 28,415	$ 29,962
Liability for Asbestos and Environmental Claims, Net	21,589	$ 22,783	$ 21,212	$ 22,716
Asbestos Claim [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and Loss expense reserves, Gross	6,300
Liability for Asbestos and Environmental Claims, Net	5,100
Landfill Sites Related Claims [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and Loss expense reserves, Gross	12,100
Liability for Asbestos and Environmental Claims, Net	7,300
Leaking Underground Storage Tanks Claims [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and Loss expense reserves, Gross	10,300
Liability for Asbestos and Environmental Claims, Net	$ 9,200

X

- 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (cc)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

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-Publisher FASB
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-Section 50
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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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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+ References

No definition available.

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