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Reserve for Claims and Claim Expenses (Prior Year Development of the Reserve for Net Claims and Claim Expenses) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Liability for Catastrophe Claims [Line Items] |  |  |
Net claims and claim expenses incurred – prior accident years | $ (183,812) | $ (26,855) | $ (270,749)
Actuarial assumption changes |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Net claims and claim expenses incurred – prior accident years |  | 9,245 |
Other |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Net claims and claim expenses incurred – prior accident years | 212 | (40) | (197)
Property |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Net claims and claim expenses incurred – prior accident years | (157,261) | (2,933) | (221,290)
Property | Actuarial assumption changes |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Net claims and claim expenses incurred – prior accident years | 5,445 | 9,200 | (5,682)
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 | (162,706) | (12,178) | (215,608)
Property | Large catastrophe events |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Net claims and claim expenses incurred – prior accident years | (120,905) | (17,557) | (182,029)
Property | 2019 Large Loss Events |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Net claims and claim expenses incurred – prior accident years | (44,389) |  |
Property | 2018 Large Loss Events |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Net claims and claim expenses incurred – prior accident years | (43,991) | 81,555 |
Property | 2017 Large Loss Events |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Net claims and claim expenses incurred – prior accident years | (32,649) | (101,572) | (172,512)
Property | Other |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Net claims and claim expenses incurred – prior accident years | 124 | (31,916) | (9,517)
Property | New Zealand Earthquake (2011) |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Net claims and claim expenses incurred – prior accident years |  | (7,497) |
Property | Tohoku Earthquake and Tsunami (2011) |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Net claims and claim expenses incurred – prior accident years |  | (5,198) |
Property | New Zealand Earthquake (2010) |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Net claims and claim expenses incurred – prior accident years |  | 47,071 |
Property | Small catastrophe events and attritional loss movements |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Net claims and claim expenses incurred – prior accident years | (41,801) | 5,379 | (33,579)
Property | Other small catastrophe events and attritional loss movements |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Net claims and claim expenses incurred – prior accident years | (41,801) | 5,379 | (33,579)
Property | Operating Segments |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Net claims and claim expenses incurred – prior accident years | (157,261) | (2,933) | (221,290)
Casualty and Specialty |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Net claims and claim expenses incurred – prior accident years | (26,763) | (23,882) | (49,262)
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 | (29,280) | (52,796) | (41,476)
Casualty and Specialty | Actuarial assumption changes |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Net claims and claim expenses incurred – prior accident years | 2,517 | 28,914 | (7,786)
Casualty and Specialty | Operating Segments |  |  |
Liability for Catastrophe Claims [Line Items] |  |  |
Net claims and claim expenses incurred – prior accident years | $ (26,763) | $ (23,882) | $ (49,262)

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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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