Derivatives Offsetting Assets (Details) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset		$ 2,612	$ 2,940
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		2,612	2,940
Derivative, Collateral, Obligation to Return Securities	[1]	(1,393)	(180)
Derivative, Collateral, Obligation to Return Cash		123	666
Derivative Asset, Fair Value, Amount Offset Against Collateral		1,096	2,094
Interest rate swaps
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset		694	791
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		694	791
Derivative, Collateral, Obligation to Return Securities	[1]	0	0
Derivative, Collateral, Obligation to Return Cash		0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral		694	791
Eurodollar futures
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset		0	666
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		0	666
Derivative, Collateral, Obligation to Return Securities	[1]	0	0
Derivative, Collateral, Obligation to Return Cash		0	(666)
Derivative Asset, Fair Value, Amount Offset Against Collateral		0	0
TBA securities
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset		871	1,483
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		871	1,483
Derivative, Collateral, Obligation to Return Securities	[1]	(346)	(180)
Derivative, Collateral, Obligation to Return Cash		(123)	0
Derivative Asset, Fair Value, Amount Offset Against Collateral		402	1,303
Options on U.S. Treasury futures
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset		1,047	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		1,047	0
Derivative, Collateral, Obligation to Return Securities	[1]	(1,047)	0
Derivative, Collateral, Obligation to Return Cash		0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral		$ 0	$ 0

[1] Amounts disclosed for collateral received by or posted to the same counterparty include cash and the fair value of MBS up to and not exceeding the net amount of the asset or liability presented in the balance sheet. The fair value of the total collateral received by or posted to the same counterparty may exceed the amounts presented.