Reserve for Claims and Claim Expenses (Schedule of Liability for Unpaid Claims and Claims Adjustment Expense) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves as of January 1	$ 3,493,778	$ 2,568,730
Net incurred related to:
Current year	203,038	164,075
Prior years	(31,335)	29,006
Total net incurred	171,703	193,081
Net paid related to:
Current year	9,050	7,729
Prior years	328,129	147,500
Total net paid	337,179	155,229
Foreign exchange	12,104	2,287
Net reserves as of March 31	3,340,406	2,608,869
Reinsurance recoverable as of March 31	1,572,321	325,819
Gross reserves as of March 31	$ 4,912,727	$ 2,934,688