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Reserve for Losses and Loss Adjustment Expenses - Prior years reserve development (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) | $ 771,011 | $ 356,184 | $ 161,452
Operating segments | Insurance |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) | $ 25,300 | $ 16,200 | $ 7,800
Prior year reserve development (points) | 0.60% | 0.40% | 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) | $ 55,300 | $ 81,700 | $ 33,600
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) |  | 38,900 | 21,600
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) | 37,400 | 26,700 |
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) | 14,800 | 16,200 | 8,400
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) |  | (93,500) | (75,200)
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) |  |  | (16,000)
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) | (24,700) | (30,800) |
Operating segments | Insurance | Medium-tailed lines | Surety |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) |  |  | (37,900)
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) | (6,400) | (57,600) | (23,100)
Operating segments | Insurance | Medium-tailed lines | Marine |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) | 12,900 |  |
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) |  | 28,100 | 49,400
Operating segments | Insurance | Long-tailed lines | Casualty |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) | (18,500) |  |
Operating segments | Insurance | Long-tailed lines | Other business |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) | 21,500 |  | 38,800
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) | (7,300) |  |
Operating segments | Insurance | Long-tailed lines | Construction Product Line |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) |  |  | 9,300
Operating segments | Insurance | 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) | (29,900) |  |
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) |  | 109,800 | 83,000
Operating segments | Reinsurance |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) | $ 191,600 | $ 178,800 | $ 134,000
Prior year reserve development (points) | 4.80% | 6.30% | 6.20%
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) |  | $ 175,500 | $ 144,000
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) |  |  | 87,700
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) | $ 24,000 | (36,400) |
Operating segments | Reinsurance | Short-tailed lines | Property excluding property catastrophe |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) | 109,400 | 88,600 |
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) | 35,000 | 123,300 | 53,600
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) | 28,000 |  |
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) | (4,600) | (5,300) | (21,900)
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) | (4,600) |  |
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) | 196,200 | 184,100 | 155,900
Operating segments | Mortgage |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) | $ 554,100 | $ 169,600 | $ 19,000
Prior year reserve development (points) | 47.80% | 13.20% | 1.40%
Operating segments | Mortgage | Primary mortgage business |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) |  |  | $ 16,200
Operating segments | Other |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) |  | $ (8,400) | $ 700

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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 https://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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