Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of year	$ 10,056,914	$ 9,590,265
Net provision for losses and loss expenses:
Claims occurring during the current year	2,917,231	2,998,687
Decrease in estimates for claims occurring in prior years	5,262	(7,648)
Loss reserve discount accretion	32,082	34,436
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	2,954,575	3,025,475
Net payments for claims:
Current year	597,859	628,078
Prior year	2,106,394	1,996,977
Total	2,704,253	2,625,055
Foreign currency translation	(101,071)	57,789
Net reserves at end of period	10,206,165	10,048,474
Ceded reserves at end of period	1,665,997	1,605,872
Gross reserves at end of period	$ 11,872,162	$ 11,654,346	$ 11,670,408