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Liability for Unpaid Losses and Loss Adjustment Expenses - Additional Information (Detail) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | $ (8,070) | $ 8,658 | $ (31,538) |
Unpaid losses and loss adjustment expense reserves | 832,404 | 759,904 | 662,811 | $ 630,181
Unpaid losses and loss adjustment expense reserves, net | 759,383 | 665,461 | 580,653 | 553,908
Unpaid losses and loss adjustment expense reserves | $ 21,647 | $ 21,985 | $ 28,679 | $ 29,033
Survival ratio on a gross basis for open A&E claims, periods | 16 years 10 months 24 days | 33 years 3 months 18 days | 35 years 6 months |
Survival ratio on net basis for open A&E claims, periods | 22 years 9 months 18 days | 63 years 6 months | 59 years 6 months |
Construction Defect |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Unpaid losses and loss adjustment expense reserves | $ 34,800 | $ 31,400 |  |
Unpaid losses and loss adjustment expense reserves, net | 33,400 | 30,200 |  |
IBNR Reserves | Asbestos |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Unpaid losses and loss adjustment expense reserves | 18,900 | 20,100 | $ 27,300 |
Case Reserves | Asbestos |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Unpaid losses and loss adjustment expense reserves | 2,800 | 1,900 | 1,400 |
Commercial Specialty |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (1,400) | 1,600 | (23,500) |
Commercial Specialty | General Liability |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | 100 | (20,400) |
Commercial Specialty | General Liability | Accident Years Prior to 2006 And 2006 Through 2021 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | 3,900 |  |  |
Commercial Specialty | General Liability | Construction Defect | Accident Years Prior to 2005 and 2005 through 2009, 2011 and 2014 and 2016 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | (2,900) |  |
Commercial Specialty | General Liability | Construction Defect | Accident Years 2005 through 2009, 2012, 2015 through 2017 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (6,600) |
Commercial Specialty | General Liability | Other General Liability | Accident Years Prior to 2005 and 2005 through 2009, and 2013 through 2020 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | 3,000 |  |
Commercial Specialty | General Liability | Other General Liability | Accident Years 2005 through 2015 and 2016 through 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (13,800) |
Commercial Specialty | Property Lines |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (100) |
Commercial Specialty | Property Lines | Accident Years 2015 And 2018 Through 2021 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (3,100) |  |  |
Commercial Specialty | Property Lines | Accident Years 2015, 2016, 2017 and 2018 through 2020 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | 1,900 |  |
Commercial Specialty | Professional | Accident Years 2008 Through 2010 and 2012 Through 2015 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (2,200) |  |  |
Commercial Specialty | Professional Liability | Accident years 2019 and 2020 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | (500) |  |
Commercial Specialty | Professional Liability | Accident Years 2006 through 2010 and 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (1,800) |
Commercial Specialty | Commercial Auto Liability | Accident Years 2010 and 2012 through 2016 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (1,000) |
Commercial Specialty | Workers Compensation | Accident Years 2012 and Accident Years Prior to 2005 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (200) |
Reinsurance Operations |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (1,500) | (1,100) | (1,700) |
Reinsurance Operations | General Liability | Accident Year 2019 Through 2021 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | 2,100 |  |  |
Reinsurance Operations | Professional | Accident Year 2016 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (3,600) |  |  |
Reinsurance Operations | Professional Liability | Accident Year 2015 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | (1,100) |  |
Reinsurance Operations | Professional Liability | Accident Years 2014 and 2015 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (1,700) |
Exited Lines Segment |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (5,200) | 8,200 | (6,300) |
Exited Lines Segment | General Liability | Accident Years 2015 through 2021 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (600) |  |  |
Exited Lines Segment | General Liability | Accident Years 2007 And 2015 through 2020 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | (2,600) |  |
Exited Lines Segment | General Liability | Accident Year 2007, 2009, 2013 and 2015 through 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (2,400) |
Exited Lines Segment | Property Lines | Accident Years 2016 and 2017 through 2021 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | $ (4,600) |  |  |
Exited Lines Segment | Property Lines | Accident Years 2015 through 2020 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | 12,400 |  |
Exited Lines Segment | Property Lines | Accident Years 2015 through 2017, 2018 and 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (600) |
Exited Lines Segment | Reinsurance |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (3,300) |
Exited Lines Segment | Reinsurance | Accident Years 2010 through 2012, 2015, 2017 through 2020 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | $ (1,600) |  |
Exited Lines Segment | Reinsurance | Accident Years 2009 through 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (2,900) |
Exited Lines Segment | Reinsurance | Liability & Workers Compensation Lines | Accident Years 2009 through 2012 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | $ (400) |

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