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Liability for Unpaid Losses and Loss Adjustment Expenses - Additional Information (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | $ (51) | $ (2,992) | $ 0 | $ (9,626)
Commercial Specialty |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | 14,500 | (1,200) | 21,000 | (1,600)
Commercial Specialty | Property Lines | Accident Years 2020 and 2021 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | (1,800) |  |
Commercial Specialty | Property Lines | Accident Years 2019 through 2021 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | (3,300)
Commercial Specialty | Property Lines | Accident Years 2015 and 2018 through 2022 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | 2,200 |  | 1,400 |
Commercial Specialty | General Liability | Accident Years Prior to 2005 through 2018 2019 through 2022 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | 12,400 |  |  |
Commercial Specialty | General Liability | Accident Years Prior to 2005 through 2018 | Construction Defect Segments |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (6,500) |  |  |
Commercial Specialty | General Liability | Accident Years Prior to 2005, 2005, 2013, 2014, 2016 through 2020 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | 500 |  |
Commercial Specialty | General Liability | Accident Years 2013 and 2019 through 2022 prior to 2013 and 2014 through 2018 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | 19,700 |
Commercial Specialty | General Liability | Accident Years prior to 2013 and 2014 through 2018 | Construction Defect Segments |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (8,600) |
Commercial Specialty | General Liability | Accident Years Prior to 2005, 2006, 2007, 2011 through 2015, and 2017 through 2021 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | 1,700
Commercial Specialty | Professional | Accident Years prior to 2016 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (100) |  |  |
Commercial Specialty | Professional | Accident Years prior to 2017 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (100) |
Reinsurance Operations |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (2,700) | (1,200) | (1,700) | (2,500)
Reinsurance Operations | General Liability |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (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 |  | (1,200) |  | (2,400)
Reinsurance Operations | Professional | Accident Years 2020 2017 2016 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (2,600) |  |  |
Reinsurance Operations | Professional | Accident Years 2016, 2022, 2017, and 2020 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (1,500) |
Exited Lines |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (11,900) | (600) | (19,300) | (5,500)
Exited Lines | Property Lines | Accident Years 2013 through 2020 2022 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (10,000) |  |  |
Exited Lines | Property Lines | Accident Years 2016, 2020 and 2021 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | 2,200 |  |
Exited Lines | Property Lines | Accident Years 2013 through 2020, 2021 and 2022 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (15,900) |
Exited Lines | Property Lines | Accident Years 2016, 2017, 2020 and 2021 2018 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | 2,000
Exited Lines | General Liability | Accident Years 2017, 2018, 2019 and 2021 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | (600)
Exited Lines | General Liability | Accident Years 2014, 2016 through 2019, 2021 and 2022 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | $ (1,800) |  |  |
Exited Lines | General Liability | Accident Years 2012 through 2019, 2021 and 2022 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | $ (3,400) |
Exited Lines | Reinsurance | Accident Years 2017 through 2021 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | $ (7,000)
Exited Lines | Reinsurance | Accident Years 2016 through 2021 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | $ (2,700) |  |

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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 -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-3 + Details
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