Investments (Schedule of Fair Value of Fixed Maturity Investments Trading) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Total fixed maturity investments trading	$ 7,404,761	$ 7,426,555
U.S. treasuries
Investment [Line Items]
Total fixed maturity investments trading	2,834,487	3,168,763
Agencies
Investment [Line Items]
Total fixed maturity investments trading	45,365	47,646
Municipal
Investment [Line Items]
Total fixed maturity investments trading	506,470	509,802
Non-U.S. government (Sovereign debt)
Investment [Line Items]
Total fixed maturity investments trading	303,056	287,660
Non-U.S. government-backed corporate
Investment [Line Items]
Total fixed maturity investments trading	191,202	163,651
Corporate
Investment [Line Items]
Total fixed maturity investments trading	2,147,578	2,063,459
Agency mortgage-backed
Investment [Line Items]
Total fixed maturity investments trading	693,377	500,456
Non-agency mortgage-backed
Investment [Line Items]
Total fixed maturity investments trading	302,638	300,331
Commercial mortgage-backed
Investment [Line Items]
Total fixed maturity investments trading	197,126	202,062
Asset-backed
Investment [Line Items]
Total fixed maturity investments trading	$ 183,462	$ 182,725