Derivative Instruments and Hedging Activities - Components of Realized and Unrealized Gains and Losses (Details) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gains (Losses)		$ 1,002	$ (2,631)	$ 4,716	$ (5,728)	$ (3,620)	$ (1,020)	$ (1,291)	$ (1,736)	$ (2,641)	$ (7,667)	$ 5,464
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Realized Gains (Losses)										(6,708)	2,990	(10,566)
Unrealized Gains (Losses)										(2,700)	(1,627)	5,756
TBA securities | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Realized Gains (Losses)										5,244	13,708	(12,393)
Unrealized Gains (Losses)										$ (2,253)	2,472	(629)
Eurodollar Future
Derivative Instruments, Gain (Loss) [Line Items]
Number of derivatives held		2,769								2,769
Eurodollar Future | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Realized Gains (Losses)	[1]									$ (2,321)	(2,146)	(3,591)
Unrealized Gains (Losses)	[1]									(342)	533	2,366
Interest Rate Swap | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Realized Gains (Losses)										0	259
Unrealized Gains (Losses)										(26)	(232)
Swaption | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gains (Losses)										(658)	(1,068)	1,153
Options on US Treasury Futures | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Realized Gains (Losses)										(9,631)	(8,831)	5,418
Unrealized Gains (Losses)										$ 579	$ (3,332)	$ 2,866

[1] At December 31, 2015, the Eurodollar futures consist of 2,769 contracts with expiration dates ranging between March 2016 and September 2017.