Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of period	$ 14,248,879	$ 12,848,362
Net provision for losses and loss expenses:
Claims occurring during the current year	4,694,554	4,260,179
Decrease in estimates for claims occurring in prior years	27,186	54,632
Loss reserve discount accretion	22,862	24,835
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	4,744,602	4,339,646
Net payments for claims:
Current year	743,416	643,908
Prior years	2,915,979	2,499,319
Total	3,659,395	3,143,227
Foreign currency translation	(48,162)	(193,755)
Net reserves at end of period	15,285,924	13,851,026	$ 14,248,879
Ceded reserves at end of period	2,987,386	2,689,097
Gross reserves at end of period	$ 18,273,310	$ 16,540,123	$ 17,011,223