Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves at beginning of period	$ 10,248,883	$ 10,056,914
Net provision for losses and loss expenses:
Claims occurring during the current year	1,980,138	1,916,140
Decrease in estimates for claims occurring in prior years	16,764	(246)
Loss reserve discount accretion	20,577	20,962
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	2,017,479	1,936,856
Net payments for claims:
Current year	548,589	316,567
Prior years	1,200,906	1,527,522
Total	1,749,495	1,844,089
Foreign currency translation	(2,228)	(72,351)
Net reserves at end of period	$ 10,248,883	$ 10,056,914	$ 10,514,639	$ 10,248,883	$ 10,077,330
Ceded reserves at end of period			1,805,639		1,659,245
Gross reserves at end of period			$ 12,320,278	$ 11,966,448	$ 11,736,575