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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, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | $ (11,711) | $ (6,950) | $ (31,617) | $ (23,043)
Commercial Specialty |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (3,500) | (5,200) | (17,800) | (12,100)
Commercial Specialty | General Liability |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (2,500) | (4,400) | (20,500) | (9,600)
Commercial Specialty | General Liability | Accident Years 2005 through 2009 and 2012 | Construction Defect |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (1,900) |  |  |
Commercial Specialty | General Liability | Accident Year 1995, 2005, 2007 through 2012 and 2015 and 2016 through 2018 | Other General Liability |  |  |  |
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 2004 Through 2012 and 2016 | Construction Defect |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | (500) |  | (1,000)
Commercial Specialty | General Liability | Accident Years 1999 through 2014 and 2016 and 2017 | Other General Liability |  |  |  |
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 | Accident Years 2005 through 2009, 2012, 2015 through 2017 | Construction Defect |  |  |  |
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 | Accident Years 2005 through 2016 and 2019 | Other General Liability |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (13,900) |
Commercial Specialty | General Liability | Accident Years 1999 Through 2018 | Other General Liability |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | (8,600)
Commercial Specialty | Property Lines | Accident Years 2017 through 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | 100 |  | 5,800 |
Commercial Specialty | Property Lines | Accident Years 2010, 2012 Through 2018 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | (900)
Commercial Specialty | Professional Liability | Accident Years 2006 through 2010 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (1,100) |  |  |
Commercial Specialty | Professional Liability | Accident Years 2007 through 2010 and 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (1,900) |
Commercial Specialty | Professional Liability | Accident Years 2009 and 2010 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | (1,100)
Commercial Specialty | Commercial Auto Liability | Accident Years 2012,2013 and 2016 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | (600) |  |
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 | Commercial Auto Liability | Accident Years 2000 Through 2002, 2010, 2012, and 2013 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | (1,400)
Commercial Specialty | Workers Compensation | Accident Years 2012 and prior to 2005 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (200) |
Commercial Specialty | Reinsurance |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | 1,000
Reinsurance Operations |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (4,900) | 700 | (5,100) | 3,500
Reinsurance Operations | Property Lines |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | 3,800
Reinsurance Operations | Property Lines | Accident years 2009 through 2012 and 2014 through 2018 and 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (3,200) |  |  |
Reinsurance Operations | Property Lines | Accident Years 2018 | Typhoon Jebi |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | 7,600
Reinsurance Operations | Property Lines | Accident Years 2011, 2012, and 2014 through 2018 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | 700 |  |
Reinsurance Operations | Property Lines | Accident years 2009 through 2010 and 2012 through 2017 and 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (3,400) |
Reinsurance Operations | Property Lines | Accident Years 2010 Through 2017 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | (4,100)
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) |  | (1,700) |
Reinsurance Operations | Professional Liability | Accident Years 2007, 2008 and 2010 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | (300)
Specialty Property |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (2,000) | (1,300) | (6,600) | (10,500)
Specialty Property | General Liability | Accident Years 2017 and 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (200) |  |  |
Specialty Property | General Liability | Accident Years 2010, 2014 through 2018 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | (400) |  |
Specialty Property | General Liability | Accident Years 2015 through 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (2,000) |
Specialty Property | General Liability | Accident Years 2010 and 2015 through 2017 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | (400)
Specialty Property | Property Lines |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | (900) | (4,600) | (10,100)
Specialty Property | Property Lines | Accident Years 2017 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | (200) |  |
Specialty Property | Property Lines | Accident Years 2017 | Catastrophe |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (1,800) |  | (1,800) |
Specialty Property | Property Lines | Accident Years 2018 | Catastrophe |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | (600) |  | (8,900)
Specialty Property | Property Lines | Accident Years 2015 through 2018 and 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (2,800) |
Specialty Property | Property Lines | Accident Years 2016 and 2017 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | (1,200)
Farm, Ranch & Stable |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | (1,300) | (1,200) | (2,100) | (4,000)
Farm, Ranch & Stable | General Liability | Accident years 2013 and 2015 through 2016 and 2018 through 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | (100) |
Farm, Ranch & Stable | Property Lines |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | (2,200)
Farm, Ranch & Stable | Property Lines | Accident Years 2017 through 2019 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve | $ (1,300) |  |  |
Farm, Ranch & Stable | Property Lines | Accident Years 2018 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  | $ (1,100) |  | (2,000)
Farm, Ranch & Stable | Property Lines | Accident Years 2018 | Catastrophe |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | (1,100)
Farm, Ranch & Stable | Property Lines | Accident Years 2018 | Expected Claims Frequency And Severity |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | (900)
Farm, Ranch & Stable | Property Lines | Accident Year 2016 Through 2019 | Catastrophe |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  | $ (2,000) |
Farm, Ranch & Stable | Liability Lines | Accident Years 2013 through 2017 |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Changes in prior year reserve |  |  |  | $ (1,700)

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