Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of period	$ 15,661,820	$ 14,248,879
Net provision for losses and loss expenses:
Claims occurring during the current year	5,229,468	4,694,554
Decrease in estimates for claims occurring in prior years	15,279	27,186
Loss reserve discount accretion	25,587	22,862
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	5,270,334	4,744,602
Net payments for claims:
Current year	795,792	743,416
Prior years	3,164,720	2,915,979
Total	3,960,512	3,659,395
Foreign currency translation	19,659	(48,162)
Net reserves at end of period	16,991,301	15,285,924	$ 15,661,820
Ceded reserves at end of period	3,164,009	2,987,386
Gross reserves at end of period	$ 20,155,310	$ 18,273,310	$ 18,739,652