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, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves as of beginning of period	$ 3,704,050	$ 3,493,778	$ 2,568,730
Net incurred related to:
Current year	2,123,876	1,390,767	1,902,424
Prior years	(26,855)	(270,749)	(40,996)
Total net incurred	2,097,021	1,120,018	1,861,428
Net paid related to:
Current year	265,649	391,061	450,527
Prior years	832,405	503,708	524,298
Total net paid	1,098,054	894,769	974,825
Amounts acquired	1,858,775	0	0
Foreign exchange	31,260	(14,977)	38,445
Net reserves as of end of period	6,593,052	3,704,050	3,493,778
Reinsurance recoverable as of end of period	2,791,297	2,372,221	1,586,630
Gross reserves as of end of period	$ 9,384,349	$ 6,076,271	$ 5,080,408