Derivative Instruments and Hedging Activities - Fair Value of Derivative Instruments Not Designated as Hedging Instruments (Details) - USD ($) $ in Thousands		Mar. 31, 2016	Dec. 31, 2015
TBA securities
Derivative [Line Items]
Derivatives Not Designated As Hedging Instruments - Assets		$ 288,455	$ 226,929
Options on U.S. Treasury futures
Derivative [Line Items]
Derivatives Not Designated As Hedging Instruments - Assets		79	15
Interest rate swap futures
Derivative [Line Items]
Derivatives Not Designated As Hedging Instruments - Assets		0	706
Derivatives Not Designated As Hedging Instruments - Liabilities		682	0
Swaptions
Derivative [Line Items]
Derivatives Not Designated As Hedging Instruments - Assets		391	821
Eurodollar futures
Derivative [Line Items]
Derivatives Not Designated As Hedging Instruments - Liabilities		3,277	1,242
U.S. Treasury futures
Derivative [Line Items]
Derivatives Not Designated As Hedging Instruments - Liabilities		64	0
Interest rate swaps
Derivative [Line Items]
Derivatives Not Designated As Hedging Instruments - Liabilities	[1]	377	$ 258
Not Designated as Hedging Instrument | Interest rate swaps
Derivative [Line Items]
Derivative liabilities	[1]	500
Derivative assets		$ 100

[1] Includes interest rate swaps in our Agency IO portfolio. Contracts in a liability position of $0.5 million have been netted against the asset position of $0.1 million in the accompanying condensed consolidated balance sheets at March 31, 2016. There was no netting of interest rate swaps at December 31, 2015.