Investments (Schedule of Contractual Maturities of Fixed Maturity Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Trading Securities [Abstract]
Trading Debt Securities, Amortized Cost, Due in less than one year	$ 273,579
Trading Debt Securities, Amortized Cost, Due after one through five years	4,136,965
Trading Debt Securities, Amortized Cost, Due after five through ten years	1,026,303
Trading Debt Securities, Amortized Cost, Due after ten years	171,187
Trading Debt Securities, Amortized Cost	7,007,201
Trading Debt Securities, Fair Value, Due in less than one year	266,356
Trading Debt Securities, Fair Value, Due after one through five years	4,163,134
Trading Debt Securities, Fair Value, Due after five through ten years	1,049,939
Trading Debt Securities, Fair Value, Due after ten years	180,339
Trading Debt Securities, Fair Value	7,073,129
Available-for-sale Securities [Abstract]
Available for Sale, Amortized Cost, Due in less than one year	0
Available for Sale, Amortized Cost, Due after one through five years	0
Available for Sale, Amortized Cost, Due after five through ten years	0
Available for Sale, Amortized Cost, Due after ten years	0
Available for Sale, Amortized Cost	10,923	$ 15,943
Available for Sale, Fair Value, Due in less than one year	0
Available for Sale, Fair Value, Due after one through five years	0
Available for Sale, Fair Value, Due after five through ten years	0
Available for Sale, Fair Value, Due after ten years	0
Available for Sale, Fair Value	12,434	$ 17,813
Fixed Maturity Investments [Abstract]
Debt Securities, Amortized Cost, Due in less than one year	273,579
Debt Securities, Amortized Cost, Due after one through five years	4,136,965
Debt Securities, Amortized Cost, Due after five through ten years	1,026,303
Debt Securities, Amortized Cost, Due after ten years	171,187
Debt Securities, Amortized Cost	7,018,124
Debt Securities, Fair Value, Due in less than one year	266,356
Debt Securities, Fair Value, Due after one through five years	4,163,134
Debt Securities, Fair Value, Due after five through ten years	1,049,939
Debt Securities, Fair Value, Due after ten years	180,339
Debt Securities, Fair Value	7,085,563
Mortgage-backed
Trading Securities [Abstract]
Trading Debt Securities, Amortized Cost	1,244,139
Trading Debt Securities, Fair Value	1,263,214
Available-for-sale Securities [Abstract]
Available for Sale, Amortized Cost	10,923
Available for Sale, Fair Value	12,434
Fixed Maturity Investments [Abstract]
Debt Securities, Amortized Cost	1,255,062
Debt Securities, Fair Value	1,275,648
Asset-backed
Trading Securities [Abstract]
Trading Debt Securities, Amortized Cost	155,028
Trading Debt Securities, Fair Value	150,147
Available-for-sale Securities [Abstract]
Available for Sale, Amortized Cost	0
Available for Sale, Fair Value	0
Fixed Maturity Investments [Abstract]
Debt Securities, Amortized Cost	155,028
Debt Securities, Fair Value	$ 150,147

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