Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2022	Sep. 30, 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	4,260,179	3,598,969
Decrease in estimates for claims occurring in prior years	54,632	1,552
Loss reserve discount accretion	24,835	23,109
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	4,339,646	3,623,630
Net payments for claims:
Current year	643,908	547,082
Prior years	2,499,319	2,161,877
Total	3,143,227	2,708,959
Foreign currency translation	(193,755)	(53,935)
Net reserves at end of period	13,851,026	12,481,129	$ 12,848,362
Ceded reserves at end of period	2,689,097	2,438,447
Gross reserves at end of period	16,540,123	$ 14,919,576	$ 15,390,888
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability recognized for best estimate net of reinsurance, incurred but not reported (IBNR)	5,000
Liability recognized for best estimate net of reinsurance, reported losses	$ 343,000