Derivative Instruments, Realized Gain (Loss) Related to Derivatives (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative, Gain (Loss) on Derivative, Net [Abstract]
Realized gain (loss) on derivatives, net	$ (3,162)	$ (707)	$ (2,033)	$ 13	$ (1,260)	$ (1,480)	$ (1,797)	$ (1,017)	$ (5,889)	$ (5,554)	$ (7,963)
Not Designated as Hedging Instrument [Member] | Realized Gain (Loss) on Derivatives, Net [Member]
Derivative, Gain (Loss) on Derivative, Net [Abstract]
Realized gain (loss) on derivatives, net									(5,889)	(5,554)	(7,963)
Not Designated as Hedging Instrument [Member] | Realized Gain (Loss) on Derivatives, Net [Member] | Interest Rate Swaps [Member]
Derivative, Gain (Loss) on Derivative, Net [Abstract]
Realized gain (loss) on derivatives, net									(2,149)	(1,573)	(6,600)
Not Designated as Hedging Instrument [Member] | Realized Gain (Loss) on Derivatives, Net [Member] | Swaptions [Member]
Derivative, Gain (Loss) on Derivative, Net [Abstract]
Realized gain (loss) on derivatives, net									(1,451)	(680)	(884)
Not Designated as Hedging Instrument [Member] | Realized Gain (Loss) on Derivatives, Net [Member] | TBAs [Member]
Derivative, Gain (Loss) on Derivative, Net [Abstract]
Realized gain (loss) on derivatives, net									(2,315)	(1,339)	(316)
Not Designated as Hedging Instrument [Member] | Realized Gain (Loss) on Derivatives, Net [Member] | Treasury Futures [Member]
Derivative, Gain (Loss) on Derivative, Net [Abstract]
Realized gain (loss) on derivatives, net									$ 26	$ (1,962)	$ (163)