Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of period	$ 12,848,362	$ 11,620,393
Net provision for losses and loss expenses:
Claims occurring during the current year	1,327,695	1,115,173
Decrease in estimates for claims occurring in prior years	3,761	(859)
Loss reserve discount accretion	7,796	7,278
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	1,339,252	1,121,592
Net payments for claims:
Current year	84,598	97,586
Prior years	933,656	794,472
Total	1,018,254	892,058
Foreign currency translation	9,983	(14,779)
Net reserves at end of period	13,179,343	11,835,148	$ 12,848,362
Ceded reserves at end of period	2,543,546	2,245,380
Gross reserves at end of period	$ 15,722,889	$ 14,080,528	$ 15,390,888