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, 2020	Mar. 31, 2019	Dec. 31, 2019
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves as of beginning of period	$ 6,593,052	$ 3,704,050
Net incurred related to:
Current year	557,054	231,341
Prior years	13,900	(4,306)
Total net incurred	570,954	227,035
Net paid related to:
Current year	31,201	8,149
Prior years	436,286	300,120
Total net paid	467,487	308,269
Amounts acquired	0	1,858,775
Foreign exchange	(55,395)	1,550
Net reserves as of end of period	6,641,124	5,483,141
Reinsurance recoverable	2,765,583	2,908,343	$ 2,791,297
Gross reserves as of end of period	$ 9,406,707	$ 8,391,484