Investments (Schedule of Fair Value of Fixed Maturity Investments Trading) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Fixed maturity investments trading, fair value	$ 7,426,555	$ 6,891,244
U.S. treasuries
Investment [Line Items]
Fixed maturity investments trading, fair value	3,168,763	2,617,894
Agencies
Investment [Line Items]
Fixed maturity investments trading, fair value	47,646	90,972
Municipal
Investment [Line Items]
Fixed maturity investments trading, fair value	509,802	519,069
Non-U.S. government (Sovereign debt)
Investment [Line Items]
Fixed maturity investments trading, fair value	287,660	333,224
Non-U.S. government-backed corporate
Investment [Line Items]
Fixed maturity investments trading, fair value	163,651	133,300
Corporate
Investment [Line Items]
Fixed maturity investments trading, fair value	2,063,459	1,877,243
Agency mortgage-backed
Investment [Line Items]
Fixed maturity investments trading, fair value	500,456	462,493
Non-agency mortgage-backed
Investment [Line Items]
Fixed maturity investments trading, fair value	300,331	258,944
Commercial mortgage-backed
Investment [Line Items]
Fixed maturity investments trading, fair value	202,062	409,747
Asset-backed
Investment [Line Items]
Fixed maturity investments trading, fair value	$ 182,725	$ 188,358