Investments (Schedule of Contractual Maturities of Fixed Maturity Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Trading Securities [Abstract]
Amortized Cost, Due in less than one year	$ 385,371
Amortized Cost, Due after one through five years	4,482,713
Amortized Cost, Due after five through ten years	917,280
Amortized Cost, Due after ten years	88,229
Amortized Cost	7,499,997	$ 7,434,870
Fair Value, Due in less than one year	383,353
Fair Value, Due after one through five years	4,421,536
Fair Value, Due after five through ten years	904,070
Fair Value, Due after ten years	85,790
Fair Value	7,420,778	$ 7,426,555
Mortgage-backed
Trading Securities [Abstract]
Amortized Cost	1,310,933
Fair Value	1,310,978
Asset-backed
Trading Securities [Abstract]
Amortized Cost	315,471
Fair Value	$ 315,051