Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of period	$ 10,697,998	$ 10,248,883
Net provision for losses and loss expenses:
Claims occurring during the current year	1,097,523	971,806
Decrease in estimates for claims occurring in prior years	433	6,481
Loss reserve discount accretion	9,297	10,363
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	1,107,253	988,650
Net payments for claims:
Current year	84,310	295,873
Prior years	864,064	567,756
Total	948,374	863,629
Foreign currency translation	(77,167)	(163)
Net reserves at end of period	10,697,998	10,248,883	$ 10,785,637	$ 10,697,998	$ 10,373,741
Ceded reserves at end of period			1,946,878		1,719,369
Gross reserves at end of period			$ 12,732,515	12,583,249	$ 12,093,110
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	5,927	0
Net payments for claims:
Net reserves at end of period	5,927	0		5,927
Cumulative Effect, Period Of Adoption, Adjusted Balance
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of period	10,703,925	10,248,883
Net payments for claims:
Net reserves at end of period	$ 10,703,925	$ 10,248,883		$ 10,703,925