Investments (Schedule of Fair Value of Fixed Maturity Investments Trading) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Investment [Line Items]
Total fixed maturity investments trading	$ 7,092,969	$ 6,891,244
U.S. treasuries
Investment [Line Items]
Total fixed maturity investments trading	2,956,952	2,617,894
Agencies
Investment [Line Items]
Total fixed maturity investments trading	41,109	90,972
Municipal
Investment [Line Items]
Total fixed maturity investments trading	521,220	519,069
Non-U.S. government (Sovereign debt)
Investment [Line Items]
Total fixed maturity investments trading	177,855	333,224
Non-U.S. government-backed corporate
Investment [Line Items]
Total fixed maturity investments trading	121,892	133,300
Corporate
Investment [Line Items]
Total fixed maturity investments trading	2,028,750	1,877,243
Agency mortgage-backed
Investment [Line Items]
Total fixed maturity investments trading	499,310	462,493
Non-agency mortgage-backed
Investment [Line Items]
Total fixed maturity investments trading	299,530	258,944
Commercial mortgage-backed
Investment [Line Items]
Total fixed maturity investments trading	263,029	409,747
Asset-backed
Investment [Line Items]
Total fixed maturity investments trading	$ 183,322	$ 188,358