Investments (Schedule of Fair Value of Fixed Maturity Investments Trading) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Fixed maturity investments trading	$ 8,088,870	$ 7,426,555
U.S. treasuries
Investment [Line Items]
Fixed maturity investments trading	3,331,411	3,168,763
Agencies
Investment [Line Items]
Fixed maturity investments trading	174,883	47,646
Municipal
Investment [Line Items]
Fixed maturity investments trading	6,854	509,802
Non-U.S. government
Investment [Line Items]
Fixed maturity investments trading	279,818	287,660
Non-U.S. government-backed corporate
Investment [Line Items]
Fixed maturity investments trading	160,063	163,651
Corporate
Investment [Line Items]
Fixed maturity investments trading	2,450,244	2,063,459
Agency mortgage-backed
Investment [Line Items]
Fixed maturity investments trading	817,880	500,456
Non-agency mortgage-backed
Investment [Line Items]
Fixed maturity investments trading	278,680	300,331
Commercial mortgage-backed
Investment [Line Items]
Fixed maturity investments trading	282,294	202,062
Asset-backed
Investment [Line Items]
Fixed maturity investments trading	$ 306,743	$ 182,725