Reserves for Loss and Loss Expenses (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Dec. 31, 2018	Sep. 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	3,007,364	2,917,231
Decrease in estimates for claims occurring in prior years	22,340	5,262
Loss reserve discount accretion	29,246	32,082
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	3,058,950	2,954,575
Net payments for claims:
Current year	767,945	597,859
Prior years	1,907,979	2,106,394
Total	2,675,924	2,704,253
Foreign currency translation	(49,455)	(101,071)
Net reserves at end of period	$ 10,248,883	$ 10,056,914	$ 10,582,454	$ 10,248,883	$ 10,206,165
Ceded reserves at end of period			1,814,501		1,665,997
Gross reserves at end of period			$ 12,396,955	$ 11,966,448	$ 11,872,162