v3.26.3
Reserve for Loss and Loss Expense - Schedule of Rollforward of Reserve for Loss and Loss Expense (Details) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]    
Gross reserve for loss and loss expense, at beginning of period $ 7,225,398 $ 6,589,801
Less: reinsurance recoverable on unpaid loss and loss expense, at beginning of period 877,843 1,022,245
Net reserve for loss and loss expense, at beginning of period 6,347,555 5,567,556
Incurred loss and loss expense for claims occurring in the:    
Current year 2,417,011 2,319,158
Prior years (22,545) 70,096
Total incurred loss and loss expense 2,394,466 2,389,254
Paid loss and loss expense for claims occurring in the:    
Current year 610,602 594,811
Prior years 1,210,969 1,199,822
Total paid loss and loss expense 1,821,571 1,794,633
Net reserve for loss and loss expense, at end of period 6,920,450 6,162,177
Add: Reinsurance recoverable on unpaid loss and loss expense, at end of period 1,086,045 914,245
Gross reserve for loss and loss expense, at end of period $ 8,006,495 $ 7,076,422