Fair Value Measurements - Financial Instruments Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
MBS
Derivative assets, at fair value	$ 91	$ 763
Derivative Liabilities	(3,431)	(4,833)
Agency MBS
MBS
Fair Value of MBS	4,399,466	4,054,424
Private-Label MBS
MBS
Fair Value of MBS	37	76
Fair Value, Measurements, Recurring
MBS
Fair Value of MBS	4,399,503	4,054,500
Derivative assets, at fair value	91	763
Derivative Liabilities	(3,431)	(4,833)
Other assets	6,133	439
Total	4,402,296	4,050,869
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1
MBS
Fair Value of MBS	0	0
Derivative assets, at fair value	0	0
Derivative Liabilities	(219)	(1,341)
Other assets	0	0
Total	(219)	(1,341)
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
MBS
Fair Value of MBS	4,399,466	4,054,424
Derivative assets, at fair value	91	763
Derivative Liabilities	(3,212)	(3,492)
Other assets	0	0
Total	4,396,345	4,051,695
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 3
MBS
Fair Value of MBS	37	76
Derivative assets, at fair value	0	0
Derivative Liabilities	0	0
Other assets	6,133	439
Total	6,170	515
Fair Value, Measurements, Recurring \| Agency MBS
MBS
Trading securities	4,399,466	4,054,424
Fair Value, Measurements, Recurring \| Agency MBS \| Fair Value, Inputs, Level 1
MBS
Trading securities	0	0
Fair Value, Measurements, Recurring \| Agency MBS \| Fair Value, Inputs, Level 2
MBS
Trading securities	4,399,466	4,054,424
Fair Value, Measurements, Recurring \| Agency MBS \| Fair Value, Inputs, Level 3
MBS
Trading securities	0	0
Fair Value, Measurements, Recurring \| Private-Label MBS
MBS
Trading securities	37	76
Fair Value, Measurements, Recurring \| Private-Label MBS \| Fair Value, Inputs, Level 1
MBS
Trading securities	0	0
Fair Value, Measurements, Recurring \| Private-Label MBS \| Fair Value, Inputs, Level 2
MBS
Trading securities	0	0
Fair Value, Measurements, Recurring \| Private-Label MBS \| Fair Value, Inputs, Level 3
MBS
Trading securities	$ 37	$ 76

MBS, at fair value

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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Fair value of asset after deduction of liability.

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- Definition

The fair value as of the balance sheet date of firm holdings in mortgages, mortgage backed securities (investment instruments that represent ownership of an undivided interest in a group of mortgages, where principal and interest from the individual mortgages are used to pay investors' principal and interest on the mortgage backed security) and asset-backed securities that are primarily paid from the cash flows of a discrete pool of receivables or other financial assets (such as credit-card receivables), either fixed or revolving, that by their terms convert into cash within a finite time period. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

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Fair value portion of other assets.

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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