Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of year	$ 9,590,265	$ 9,244,872
Net provision for losses and loss expenses:
Claims occurring during the current year	2,998,687	2,838,777
Decrease in estimates for claims occurring in prior years	(7,648)	(23,518)
Loss reserve discount accretion	34,436	37,080
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	3,025,475	2,852,339
Net payments for claims:
Current year	628,078	612,615
Prior year	1,996,977	1,931,454
Total	2,625,055	2,544,069
Foreign currency translation	57,789	(6,266)
Net reserves at end of period	10,048,474	9,546,876
Ceded reserve at end of period	1,605,872	1,550,954
Gross reserves at end of period	$ 11,654,346	$ 11,097,830	$ 11,197,195