Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of period	$ 10,248,883	$ 10,056,914
Net provision for losses and loss expenses:
Claims occurring during the current year	971,806	956,181
Decrease in estimates for claims occurring in prior years	6,481	(3,582)
Loss reserve discount accretion	10,363	10,620
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	988,650	963,219
Net payments for claims:
Current year	295,873	111,030
Prior years	567,756	810,709
Total	863,629	921,739
Foreign currency translation	(163)	2,501
Net reserves at end of period	10,373,741	10,100,895
Ceded reserves at end of period	1,719,369	1,684,000
Gross reserves at end of period	$ 12,093,110	$ 11,784,895	$ 11,966,448