Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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	956,181	958,684
Decrease in estimates for claims occurring in prior years	(3,582)	8,727
Loss reserve discount accretion	10,620	12,192
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	963,219	979,603
Net payments for claims:
Current year	111,030	97,461
Prior year	810,709	773,571
Total	921,739	871,032
Foreign currency translation	2,501	23,204
Net reserves at end of period	10,100,895	9,722,040
Ceded reserve at end of period	1,684,000	1,502,284
Gross reserves at end of period	$ 11,784,895	$ 11,224,324	$ 11,670,408