Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 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	1,916,140	1,926,394
Decrease in estimates for claims occurring in prior years	(246)	(6,459)
Loss reserve discount accretion	20,962	24,367
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	1,936,856	1,944,302
Net payments for claims:
Current year	316,567	315,437
Prior year	1,527,522	1,432,828
Total	1,844,089	1,748,265
Foreign currency translation	(72,351)	37,097
Net reserves at end of period	10,077,330	9,823,399
Ceded reserves at end of period	1,659,245	1,500,868
Gross reserves at end of period	$ 11,736,575	$ 11,324,267	$ 11,670,408