Investments (Schedule of Fair Value of Fixed Maturity Investments Trading) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Fixed maturity investments	$ 11,171,655	$ 8,088,870
U.S. treasuries
Investment [Line Items]
Fixed maturity investments	4,467,345	3,331,411
Agencies
Investment [Line Items]
Fixed maturity investments	343,031	174,883
Municipal
Investment [Line Items]
Fixed maturity investments	0	6,854
Non-U.S. government
Investment [Line Items]
Fixed maturity investments	497,392	279,818
Non-U.S. government-backed corporate
Investment [Line Items]
Fixed maturity investments	321,356	160,063
Corporate
Investment [Line Items]
Fixed maturity investments	3,075,660	2,450,244
Agency mortgage-backed
Investment [Line Items]
Fixed maturity investments	1,148,499	817,880
Non-agency mortgage-backed
Investment [Line Items]
Fixed maturity investments	294,604	278,680
Commercial mortgage-backed
Investment [Line Items]
Fixed maturity investments	468,698	282,294
Asset-backed
Investment [Line Items]
Fixed maturity investments	$ 555,070	$ 306,743