Derivatives Offsetting Assets (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset		$ 2,940	$ 28,534
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		2,940	28,534
Derivative, Collateral, Obligation to Return Securities	[1]	(180)	(6,449)
Derivative, Collateral, Obligation to Return Cash		666	22,085	[1]
Derivative Asset, Fair Value, Amount Offset Against Collateral		2,094	0
Interest rate swaps
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset		791	28,534
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		791	28,534
Derivative, Collateral, Obligation to Return Securities	[1]	0	(6,449)
Derivative, Collateral, Obligation to Return Cash		0	(22,085)
Derivative Asset, Fair Value, Amount Offset Against Collateral		791	$ 0
Eurodollar futures
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset		666
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		666
Derivative, Collateral, Obligation to Return Securities	[1]	0
Derivative, Collateral, Obligation to Return Cash		(666)
Derivative Asset, Fair Value, Amount Offset Against Collateral		0
TBA securities
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset		1,483
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		1,483
Derivative, Collateral, Obligation to Return Securities	[1]	(180)
Derivative, Collateral, Obligation to Return Cash		0
Derivative Asset, Fair Value, Amount Offset Against Collateral		$ 1,303

[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.