Investments (Schedule of Contractual Maturities of Fixed Maturity Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Trading Securities [Abstract]
Trading Debt Securities, Amortized Cost, Due in less than one year	$ 280,939
Trading Debt Securities, Amortized Cost, Due after one through five years	4,083,141
Trading Debt Securities, Amortized Cost, Due after five through ten years	1,014,691
Trading Debt Securities, Amortized Cost, Due after ten years	159,118
Trading Debt Securities, Amortized Cost	7,014,564
Trading Debt Securities, Fair Value, Due in less than one year	280,850
Trading Debt Securities, Fair Value, Due after one through five years	4,102,622
Trading Debt Securities, Fair Value, Due after five through ten years	1,044,737
Trading Debt Securities, Fair Value, Due after ten years	166,324
Trading Debt Securities, Fair Value	7,088,419
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	10,323	$ 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	11,721	$ 17,813
Fixed Maturity Investments [Abstract]
Debt Securities, Amortized Cost, Due in less than one year	280,939
Debt Securities, Amortized Cost, Due after one through five years	4,083,141
Debt Securities, Amortized Cost, Due after five through ten years	1,014,691
Debt Securities, Amortized Cost, Due after ten years	159,118
Debt Securities, Amortized Cost	7,024,887
Debt Securities, Fair Value, Due in less than one year	280,850
Debt Securities, Fair Value, Due after one through five years	4,102,622
Debt Securities, Fair Value, Due after five through ten years	1,044,737
Debt Securities, Fair Value, Due after ten years	166,324
Debt Securities, Fair Value	7,100,140
Mortgage-backed
Trading Securities [Abstract]
Trading Debt Securities, Amortized Cost	1,269,868
Trading Debt Securities, Fair Value	1,285,910
Available-for-sale Securities [Abstract]
Available for Sale, Amortized Cost	10,323
Available for Sale, Fair Value	11,721
Fixed Maturity Investments [Abstract]
Debt Securities, Amortized Cost	1,280,191
Debt Securities, Fair Value	1,297,631
Asset-backed
Trading Securities [Abstract]
Trading Debt Securities, Amortized Cost	206,807
Trading Debt Securities, Fair Value	207,976
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	206,807
Debt Securities, Fair Value	$ 207,976

Fixed Maturity Investments [Abstract]

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