Investments (Schedule of Fair Value of Fixed Maturity Investments Trading) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Investment [Line Items]
Fixed maturity investments trading, at fair value	$ 7,088,419	$ 6,765,005
U.S. treasuries
Investment [Line Items]
Fixed maturity investments trading, at fair value	2,564,635	2,064,944
Agencies
Investment [Line Items]
Fixed maturity investments trading, at fair value	120,761	137,976
Municipal
Investment [Line Items]
Fixed maturity investments trading, at fair value	550,062	583,282
Non-U.S. government (Sovereign Debt)
Investment [Line Items]
Fixed maturity investments trading, at fair value	313,560	334,981
Non-U.S. government-backed corporate
Investment [Line Items]
Fixed maturity investments trading, at fair value	129,423	138,994
Corporate
Investment [Line Items]
Fixed maturity investments trading, at fair value	1,916,092	2,055,323
Agency mortgage-backed
Investment [Line Items]
Fixed maturity investments trading, at fair value	521,851	504,368
Non-agency mortgage-backed
Investment [Line Items]
Fixed maturity investments trading, at fair value	276,438	262,235
Commercial mortgage-backed
Investment [Line Items]
Fixed maturity investments trading, at fair value	487,621	554,625
Asset-backed
Investment [Line Items]
Fixed maturity investments trading, at fair value	$ 207,976	$ 128,277