Financing Arrangements, Portfolio Investments - Assets Pledged as Collateral (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Financial Instruments Owned and Pledged as Collateral [Line Items]
Outstanding Borrowings	$ 577,413	$ 651,965
Fair Value of Collateral Pledged	639,683	702,684
Amortized Cost Of Collateral Pledged	669,825	725,322
Agency ARMs
Financial Instruments Owned and Pledged as Collateral [Line Items]
Outstanding Borrowings	227,609	171,852
Fair Value of Collateral Pledged	141,585	181,694
Amortized Cost Of Collateral Pledged	143,754	183,342
Agency Fixed Rate
Financial Instruments Owned and Pledged as Collateral [Line Items]
Outstanding Borrowings	261,644	413,199
Fair Value of Collateral Pledged	374,691	437,002
Amortized Cost Of Collateral Pledged	386,853	446,851
Agency IOs/U.S. Treasury Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Outstanding Borrowings	88,160	66,914
Fair Value of Collateral Pledged	123,407	83,988
Amortized Cost Of Collateral Pledged	$ 139,218	$ 95,129