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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, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Changes in prior year reserve $ (44) $ (34) $ (39) $ (115)
Maximum        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Changes in prior year reserve (100) (100) (100)  
Belmont Core        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Changes in prior year reserve 2,600   4,000 (400)
Belmont Core | Maximum        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Changes in prior year reserve   100    
Belmont Core | Property Lines        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Changes in prior year reserve (1,600) (3,700) (6,100) (4,000)
Belmont Core | Casualty Lines        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Changes in prior year reserve 4,200 3,700 10,100 3,600
Belmont Non-Core        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Changes in prior year reserve (2,700)   (4,000) $ 300
Belmont Non-Core | Maximum        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Changes in prior year reserve   $ (100)    
Belmont Non-Core | Property Lines        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Changes in prior year reserve (1,200)   (3,400)  
Belmont Non-Core | Casualty Lines        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Changes in prior year reserve $ (1,500)   $ (600)