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Reserve for Losses and Loss Adjustment Expenses - Prior year development (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended | 9 Months Ended
--- | --- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Jun. 30, 2021 | Sep. 30, 2021 | Sep. 30, 2020
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) | $ 122,509 | $ 48,042 |  | $ 223,431 | $ 111,132
Operating segments | Insurance |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) | $ 5,100 | $ 2,300 |  | $ 13,100 | $ 5,900
Percentage of prior year development | 0.50% | 0.30% |  | 0.50% | 0.30%
Operating segments | Insurance | Short tailed lines |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) |  | $ 11,800 |  | $ 65,300 | $ 27,200
Operating segments | Insurance | Short tailed lines | Property excluding marine |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) | $ 5,400 | 8,000 |  | 27,000 | 17,500
Operating segments | Insurance | Short tailed lines | Travel and accident |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) | 5,100 | 3,400 |  | 14,400 | 6,200
Operating segments | Insurance | Short tailed lines | Lenders products |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) | 5,400 |  |  | 24,000 | 3,500
Operating segments | Insurance | Medium tailed lines |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) | (43,900) | (10,600) |  | (89,400) | (35,700)
Operating segments | Insurance | Medium tailed lines | Programs |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) |  | (7,100) |  | (6,900) | (13,500)
Operating segments | Insurance | Medium tailed lines | Professional liability |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) |  |  |  | (26,200) |
Operating segments | Insurance | Medium tailed lines | Contract binding |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) | (37,000) | (3,700) |  | (57,100) | (23,000)
Operating segments | Insurance | Long tailed lines |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) |  | 1,100 |  | 37,100 | 14,400
Operating segments | Insurance | Long tailed lines | Construction and national accounts |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) | 26,300 | 8,700 |  |  |
Operating segments | Insurance | Long tailed lines | Executive assurance and casualty |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) |  | (11,700) |  |  |
Operating segments | Insurance | Long tailed lines | Other product lines |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) | 6,700 | 4,200 |  |  | 11,700
Operating segments | Insurance | Short tailed and long tailed lines |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) | 49,000 | 12,900 |  | 102,500 | 41,600
Operating segments | Reinsurance |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) | $ 72,300 | $ 42,000 |  | $ 119,600 | $ 93,800
Percentage of prior year development | 10.70% | 7.60% |  | 5.80% | 5.90%
Operating segments | Reinsurance | Short tailed lines |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) | $ 65,400 |  |  | $ 132,600 | $ 101,800
Operating segments | Reinsurance | Short tailed lines | Property catastrophe and property other than property catastrophe |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) | 46,100 | $ 27,600 |  | 71,700 | 52,100
Operating segments | Reinsurance | Short tailed lines | Property catastrophe |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) |  |  |  | (36,300) |
Operating segments | Reinsurance | Short tailed lines | Other specialty lines |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) | 18,900 | 7,800 |  | 97,100 | 47,100
Operating segments | Reinsurance | Medium tailed lines | Marine and aviation |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) |  | 9,400 |  |  |
Operating segments | Reinsurance | Long tailed lines |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) |  | (3,600) |  | (20,100) | (19,200)
Operating segments | Reinsurance | Long tailed lines | Casualty |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) |  | (3,600) |  |  | (19,200)
Operating segments | Reinsurance | Short tailed and medium tailed lines |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) |  | 45,600 |  | 139,700 | 113,000
Operating segments | Reinsurance | Medium tailed and long tailed lines |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) | 6,900 |  |  |  |
Operating segments | Reinsurance | Medium tailed and long tailed lines | Casualty |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) | 4,700 |  |  |  |
Operating segments | Mortgage |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) | $ 45,100 | $ 4,500 |  | $ 99,100 | $ 10,800
Percentage of prior year development | 14.50% | 1.30% |  | 10.10% | 1.00%
Operating segments | Other |  |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior year development favorable (unfavorable) |  | $ (700) | $ (8,400) |  | $ 600

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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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