Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 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	3,328,827	3,007,364
Decrease in estimates for claims occurring in prior years	849	22,340
Loss reserve discount accretion	27,335	29,246
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	3,357,011	3,058,950
Net payments for claims:
Current year	570,924	767,945
Prior years	2,077,945	1,907,979
Total	2,648,869	2,675,924
Foreign currency translation	(21,003)	(49,455)
Net reserves at end of period	10,697,998	10,248,883	$ 11,391,064	$ 10,697,998	$ 10,582,454
Ceded reserves at end of period			2,068,295		1,814,501
Gross reserves at end of period			$ 13,459,359	12,583,249	$ 12,396,955
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