Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of period	$ 11,620,393	$ 10,697,998
Net provision for losses and loss expenses:
Claims occurring during the current year	3,598,969	3,328,827
Decrease in estimates for claims occurring in prior years	1,552	849
Loss reserve discount accretion	23,109	27,335
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	3,623,630	3,357,011
Net payments for claims:
Current year	547,082	570,924
Prior years	2,161,877	2,077,945
Total	2,708,959	2,648,869
Foreign currency translation	(53,935)	(21,003)
Net reserves at end of period	12,481,129	11,391,064	$ 11,620,393
Ceded reserves at end of period	2,438,447	2,068,295
Gross reserves at end of period	14,919,576	13,459,359	13,784,430
Cumulative effect adjustment resulting from changes in accounting principles
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of period	0	5,927
Net payments for claims:
Net reserves at end of period			0
Cumulative Effect, Period Of Adoption, Adjusted Balance
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of period	$ 11,620,393	$ 10,703,925
Net payments for claims:
Net reserves at end of period			$ 11,620,393