Reserve for Claims and Claim Expenses (Schedule of Liability for Unpaid Claims and Claims Adjustment Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves as of January 1	$ 3,493,778	$ 2,568,730	$ 2,632,519
Net incurred related to:
Current year	1,390,767	1,902,424	694,957
Prior years	(270,749)	(40,996)	(164,126)
Total net incurred	1,120,018	1,861,428	530,831
Net paid related to:
Current year	391,061	450,527	83,015
Prior years	503,708	524,298	506,279
Total net paid	894,769	974,825	589,294
Foreign exchange	(14,977)	38,445	(5,326)
Net reserves as of December 31	3,704,050	3,493,778	2,568,730
Reinsurance recoverable as of December 31	2,372,221	1,586,630	279,564
Gross reserves as of December 31	$ 6,076,271	$ 5,080,408	$ 2,848,294