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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, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) | $ 161,452 | $ 163,585 | $ 272,712
Other |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) | 700 | (23,800) | 2,200
Underwriting segments | Insurance |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) | $ 7,800 | $ 15,800 | $ 24,400
Prior year reserve development (points) | 0.30% | 0.70% | 1.10%
Underwriting 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) | $ 33,600 | $ 54,900 | $ 48,400
Underwriting segments | Insurance | Short-tailed lines | Property |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) | 21,600 |  | 50,100
Underwriting segments | Insurance | Short-tailed lines | Travel, accident and health |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) | 8,400 |  | (5,000)
Underwriting 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) | (75,200) |  | (50,300)
Underwriting 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) |  |
Underwriting 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) |  |
Underwriting 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) | (23,100) |  | (50,300)
Underwriting 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) | 49,400 |  | 26,300
Underwriting 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) |  |  | 1,400
Underwriting segments | Insurance | Long-tailed lines | Executive assurance |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) |  |  | 19,700
Underwriting 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) | 38,800 |  |
Underwriting 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) | 9,300 |  |
Underwriting 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) |  | (39,100) |
Underwriting segments | Insurance | Medium-tailed and long-tailed lines | Programs |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) |  | (30,100) |
Underwriting segments | Insurance | Medium-tailed and long-tailed lines | Professional liability |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) |  | 19,300 |
Underwriting segments | Insurance | Medium-tailed and long-tailed lines | Surety |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) |  | 15,800 |
Underwriting segments | Insurance | Medium-tailed and long-tailed lines | Contract binding |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) |  | (33,600) |
Underwriting 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) | 83,000 |  |
Underwriting segments | Reinsurance |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) | $ 134,000 | $ 46,400 | $ 138,500
Prior year reserve development (points) | 6.20% | 3.20% | 11.00%
Underwriting 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) | $ 144,000 | $ 70,500 | $ 110,400
Underwriting 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 | 33,700 | 80,800
Underwriting 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) | 53,600 | 40,800 |
Underwriting segments | Reinsurance | Medium-tailed lines |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) |  | 16,000 |
Underwriting 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) | (21,900) | (40,100) |
Underwriting 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) |  | (44,500) |
Underwriting 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) |  |  | 28,100
Underwriting 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) |  |  | 12,500
Underwriting segments | Reinsurance | Medium-tailed and long-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) |  |  | 15,600
Underwriting 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) | 155,900 |  |
Underwriting segments | Mortgage |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior year development favorable (unfavorable) | $ 19,000 | $ 125,200 | $ 107,600
Prior year reserve development (points) | 1.40% | 9.20% | 9.10%
Underwriting 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 | $ 117,100 | $ 103,400

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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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