Investments (Schedule of Fair Value of Fixed Maturity Investments Trading) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Total fixed maturity investments trading	$ 11,045,801	$ 11,171,655
U.S. treasuries
Schedule of Investments [Line Items]
Total fixed maturity investments trading	3,915,130	4,467,345
Agencies
Schedule of Investments [Line Items]
Total fixed maturity investments trading	537,490	343,031
Non-U.S. government
Schedule of Investments [Line Items]
Total fixed maturity investments trading	635,282	497,392
Non-U.S. government-backed corporate
Schedule of Investments [Line Items]
Total fixed maturity investments trading	283,577	321,356
Corporate
Schedule of Investments [Line Items]
Total fixed maturity investments trading	3,259,780	3,075,660
Agency mortgage-backed
Schedule of Investments [Line Items]
Total fixed maturity investments trading	1,056,272	1,148,499
Non-agency mortgage-backed
Schedule of Investments [Line Items]
Total fixed maturity investments trading	275,026	294,604
Commercial mortgage-backed
Schedule of Investments [Line Items]
Total fixed maturity investments trading	540,502	468,698
Asset-backed
Schedule of Investments [Line Items]
Total fixed maturity investments trading	$ 542,742	$ 555,070