Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 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	1,883,708	1,647,674
Decrease in estimates for claims occurring in prior years	9,604	7,367
Loss reserve discount accretion	7,480	8,737
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	1,900,792	1,663,778
Net payments for claims:
Current year	127,193	107,434
Prior years	1,304,763	1,158,864
Total	1,431,956	1,266,298
Foreign currency translation	43,780	(52,944)
Net reserves at end of period	17,679,257	16,006,356	$ 17,166,641
Ceded reserves at end of period	3,242,730	3,093,272
Gross reserves at end of period	$ 20,921,987	$ 19,099,628	$ 20,368,030