Reserve for Loss and Loss Expense - Schedule of Rollforward of Reserve for Loss and Loss Expense (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserve for loss and loss expense, at beginning of period	$ 5,336,911	$ 5,144,821
Less: reinsurance recoverable on unpaid loss and loss expense, at beginning of period	618,601	757,513
Net reserve for loss and loss expense, at beginning of period	4,718,310	4,387,308
Incurred loss and loss expense for claims occurring in the:
Current year	1,442,719	1,221,635
Prior years	187,121	(8,067)
Total incurred loss and loss expense	1,629,840	1,213,568
Paid loss and loss expense for claims occurring in the:
Current year	347,501	320,026
Prior years	753,658	720,314
Total paid loss and loss expense	1,101,159	1,040,340
Net reserve for loss and loss expense, at end of period	5,246,991	4,560,536
Add: Reinsurance recoverable on unpaid loss and loss expense, at end of period	656,534	616,487
Gross reserve for loss and loss expense, at end of period	$ 5,903,525	$ 5,177,023