Investments (Schedule of Fair Value of Fixed Maturity Investments Trading) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Fixed maturity investments trading, Fair value	$ 6,891,244	$ 6,765,005
U.S. treasuries
Investment [Line Items]
Fixed maturity investments trading, Fair value	2,617,894	2,064,944
Agencies
Investment [Line Items]
Fixed maturity investments trading, Fair value	90,972	137,976
Municipal
Investment [Line Items]
Fixed maturity investments trading, Fair value	519,069	583,282
Non-U.S. government (Sovereign Debt)
Investment [Line Items]
Fixed maturity investments trading, Fair value	333,224	334,981
Non-U.S. government-backed corporate
Investment [Line Items]
Fixed maturity investments trading, Fair value	133,300	138,994
Corporate
Investment [Line Items]
Fixed maturity investments trading, Fair value	1,877,243	2,055,323
Agency mortgage-backed
Investment [Line Items]
Fixed maturity investments trading, Fair value	462,493	504,368
Non-agency mortgage-backed
Investment [Line Items]
Fixed maturity investments trading, Fair value	258,944	262,235
Commercial mortgage-backed
Investment [Line Items]
Fixed maturity investments trading, Fair value	409,747	554,625
Asset-backed
Investment [Line Items]
Fixed maturity investments trading, Fair value	$ 188,358	$ 128,277