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RESERVES FOR LOSSES AND LOSS SETTLEMENT EXPENSES - Changes in Property and Casualty Losses and Loss Settlement Expense Reserves (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2026
Sep. 30, 2026
Dec. 31, 2025
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]      
Gross liability for losses and loss settlement expenses at beginning of year   $ 1,924,826 $ 1,796,782
Ceded losses and loss settlement expenses   (213,633) (198,083)
Net liability for losses and loss settlement expenses at beginning of year   1,711,193 1,598,699
Losses and loss settlement expenses incurred for claims occurring during      
Current year   641,454 778,480
Prior years $ 900 (4,691) (14,078)
Total incurred   636,763 764,402
Losses and loss settlement expense payments for claims occurring during      
Current year   143,442 194,149
Prior years   363,135 457,759
Total paid   506,577 651,908
Net liability for losses and loss settlement expenses at end of period 1,841,379 1,841,379 1,711,193
Ceded losses and loss settlement expenses 220,768 220,768 213,633
Gross liability for losses and loss settlement expenses at end of period $ 2,062,147 $ 2,062,147 $ 1,924,826