Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2023	Jun. 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	3,064,046	2,748,725
Decrease in estimates for claims occurring in prior years	28,853	9,765
Loss reserve discount accretion	15,510	16,579
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	3,108,409	2,775,069
Net payments for claims:
Current year	375,365	318,129
Prior years	2,019,371	1,688,843
Total	2,394,736	2,006,972
Foreign currency translation	5,929	(94,526)
Net reserves at end of period	14,968,481	13,521,933	$ 14,248,879
Ceded reserves at end of period	2,951,515	2,623,888
Gross reserves at end of period	$ 17,919,996	$ 16,145,821	$ 17,011,223