Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 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	2,308,309	2,222,671
Decrease in estimates for claims occurring in prior years	1,530	2,050
Loss reserve discount accretion	15,399	17,658
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	2,325,238	2,242,379
Net payments for claims:
Current year	279,703	289,154
Prior years	1,485,116	1,545,471
Total	1,764,819	1,834,625
Foreign currency translation	(16,153)	(45,572)
Net reserves at end of period	12,164,659	11,066,107	$ 11,620,393
Ceded reserves at end of period	2,316,288	2,022,797
Gross reserves at end of period	14,480,947	13,088,904	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