Investments (Schedule of Fair Value of Fixed Maturity Investments Trading) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Total fixed maturity investments trading	$ 7,259,851	$ 6,891,244
U.S. treasuries
Investment [Line Items]
Total fixed maturity investments trading	2,635,800	2,617,894
Agencies
Investment [Line Items]
Total fixed maturity investments trading	86,373	90,972
Municipal
Investment [Line Items]
Total fixed maturity investments trading	506,109	519,069
Non-U.S. government (Sovereign debt)
Investment [Line Items]
Total fixed maturity investments trading	341,725	333,224
Non-U.S. government-backed corporate
Investment [Line Items]
Total fixed maturity investments trading	118,092	133,300
Corporate
Investment [Line Items]
Total fixed maturity investments trading	2,205,442	1,877,243
Agency mortgage-backed
Investment [Line Items]
Total fixed maturity investments trading	505,829	462,493
Non-agency mortgage-backed
Investment [Line Items]
Total fixed maturity investments trading	270,983	258,944
Commercial mortgage-backed
Investment [Line Items]
Total fixed maturity investments trading	382,874	409,747
Asset-backed
Investment [Line Items]
Total fixed maturity investments trading	$ 206,624	$ 188,358