Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 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	1,502,817	1,327,695
Decrease in estimates for claims occurring in prior years	28,205	3,761
Loss reserve discount accretion	7,733	7,796
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	1,538,755	1,339,252
Net payments for claims:
Current year	110,274	84,598
Prior years	1,106,481	933,656
Total	1,216,755	1,018,254
Foreign currency translation	1,154	9,983
Net reserves at end of period	14,572,033	13,179,343	$ 14,248,879
Ceded reserves at end of period	2,859,602	2,543,546
Gross reserves at end of period	$ 17,431,635	$ 15,722,889	$ 17,011,223