Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of period	$ 17,166,641	$ 15,661,820
Net provision for losses and loss expenses:
Claims occurring during the current year	3,822,512	3,411,888
Decrease in estimates for claims occurring in prior years	17,823	14,700
Loss reserve discount accretion	15,881	17,786
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	3,856,216	3,444,374
Net payments for claims:
Current year	431,981	383,639
Prior years	2,537,599	2,205,566
Total	2,969,580	2,589,205
Foreign currency translation	164,747	(56,143)
Net reserves at end of period	18,218,024	16,460,846	$ 17,166,641
Ceded reserves at end of period	3,278,099	3,106,344
Gross reserves at end of period	$ 21,496,123	$ 19,567,190	$ 20,368,030