Investments (Schedule of Fair Value of Fixed Maturity Investments Trading) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Investment [Line Items]
Fixed maturity investments trading, at fair value	$ 7,073,129	$ 6,765,005
U.S. treasuries
Investment [Line Items]
Fixed maturity investments trading, at fair value	2,660,051	2,064,944
Agencies
Investment [Line Items]
Fixed maturity investments trading, at fair value	126,549	137,976
Municipal
Investment [Line Items]
Fixed maturity investments trading, at fair value	561,456	583,282
Non-U.S. government (Sovereign Debt)
Investment [Line Items]
Fixed maturity investments trading, at fair value	313,699	334,981
Non-U.S. government-backed corporate
Investment [Line Items]
Fixed maturity investments trading, at fair value	157,606	138,994
Corporate
Investment [Line Items]
Fixed maturity investments trading, at fair value	1,840,407	2,055,323
Agency mortgage-backed
Investment [Line Items]
Fixed maturity investments trading, at fair value	513,517	504,368
Non-agency mortgage-backed
Investment [Line Items]
Fixed maturity investments trading, at fair value	251,015	262,235
Commercial mortgage-backed
Investment [Line Items]
Fixed maturity investments trading, at fair value	498,682	554,625
Asset-backed
Investment [Line Items]
Fixed maturity investments trading, at fair value	$ 150,147	$ 128,277