Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 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	1,926,394	1,878,458
Decrease in estimates for claims occurring in prior years	(6,458)	(15,511)
Loss reserve discount accretion	24,366	23,536
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	1,944,302	1,886,483
Net payments for claims:
Current year	315,437	480,601
Prior year	1,432,828	1,188,704
Total	1,748,265	1,669,305
Foreign currency translation	37,097	(19,768)
Net reserves at end of period	9,823,399	9,442,282
Ceded reserve at end of period	1,500,868	1,455,594
Gross reserves at end of period	$ 11,324,267	$ 10,897,876	$ 11,197,195