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Reserve for Claims and Claim Expenses (Prior Year Development of the Reserve for Net Claims and Claim Expenses) (Details) - USD ($) | 3 Months Ended
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Mar. 31, 2021 | Mar. 31, 2020
Liability for Catastrophe Claims [Line Items] |  |
Net claims and claim expenses incurred – prior accident years | $ (9,032,000) | $ 13,900,000
Property |  |
Liability for Catastrophe Claims [Line Items] |  |
Net claims and claim expenses incurred – prior accident years | (5,162,000) | 13,901,000
Property | Actuarial methods - actual reported claims less than expected claims |  |
Liability for Catastrophe Claims [Line Items] |  |
Net claims and claim expenses incurred – prior accident years |  | 54,000,000.0
Property | Large and Small Catastrophe Events and Attritional Loss Movements |  |
Liability for Catastrophe Claims [Line Items] |  |
Net claims and claim expenses incurred – prior accident years | (5,162,000) |
Property | Large catastrophe events |  |
Liability for Catastrophe Claims [Line Items] |  |
Net claims and claim expenses incurred – prior accident years | (14,192,000) | (32,091,000)
Property | 2019 Large Loss Events |  |
Liability for Catastrophe Claims [Line Items] |  |
Net claims and claim expenses incurred – prior accident years | (4,742,000) | (19,681,000)
Property | 2018 Large Loss Events |  |
Liability for Catastrophe Claims [Line Items] |  |
Net claims and claim expenses incurred – prior accident years | (3,263,000) | (14,200,000)
Property | 2017 Large Loss Events |  |
Liability for Catastrophe Claims [Line Items] |  |
Net claims and claim expenses incurred – prior accident years | (5,508,000) | (6,237,000)
Property | Other |  |
Liability for Catastrophe Claims [Line Items] |  |
Net claims and claim expenses incurred – prior accident years | (679,000) | 8,027,000
Property | Small catastrophe events and attritional loss movements |  |
Liability for Catastrophe Claims [Line Items] |  |
Net claims and claim expenses incurred – prior accident years | 9,030,000 | 45,992,000
Property | Other small catastrophe events and attritional loss movements |  |
Liability for Catastrophe Claims [Line Items] |  |
Net claims and claim expenses incurred – prior accident years | 9,030,000 | 45,992,000
Property | Operating Segments |  |
Liability for Catastrophe Claims [Line Items] |  |
Net claims and claim expenses incurred – prior accident years | (5,162,000) | 13,901,000
Casualty and Specialty |  |
Liability for Catastrophe Claims [Line Items] |  |
Net claims and claim expenses incurred – prior accident years | (3,870,000) | (1,000.0)
Casualty and Specialty | Actuarial methods - actual reported claims less than expected claims |  |
Liability for Catastrophe Claims [Line Items] |  |
Net claims and claim expenses incurred – prior accident years | (3,870,000) | (1,000)
Casualty and Specialty | Operating Segments |  |
Liability for Catastrophe Claims [Line Items] |  |
Net claims and claim expenses incurred – prior accident years | $ (3,870,000) | $ (1,000)

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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 Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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