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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)