Financing Arrangements, Portfolio Investments - Assets Pledged as Collateral (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial Instruments Owned and Pledged as Collateral [Line Items]
Outstanding Borrowings	$ 1,276,918	$ 773,142
Fair Value of Collateral Pledged	1,453,699	945,143
Amortized Cost Of Collateral Pledged	1,368,115	889,327
Agency ARMs RMBS
Financial Instruments Owned and Pledged as Collateral [Line Items]
Outstanding Borrowings	86,349	102,088
Fair Value of Collateral Pledged	90,343	109,552
Amortized Cost Of Collateral Pledged	92,586	110,903
Agency Fixed-rate RMBS
Financial Instruments Owned and Pledged as Collateral [Line Items]
Outstanding Borrowings	842,474	289,619
Fair Value of Collateral Pledged	890,359	308,411
Amortized Cost Of Collateral Pledged	902,744	318,544
Agency IOs/U.S. Treasury Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Outstanding Borrowings	0	60,862
Fair Value of Collateral Pledged	0	82,153
Amortized Cost Of Collateral Pledged	0	93,819
Non-Agency RMBS
Financial Instruments Owned and Pledged as Collateral [Line Items]
Outstanding Borrowings	38,160	113,749
Fair Value of Collateral Pledged	51,841	150,944
Amortized Cost Of Collateral Pledged	50,693	149,969
CMBS
Financial Instruments Owned and Pledged as Collateral [Line Items]
Outstanding Borrowings	309,935	206,824
Fair Value of Collateral Pledged	421,156	294,083
Amortized Cost Of Collateral Pledged	$ 322,092	$ 216,092