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Derivative Instruments and Hedging Activities - Narrative (Details) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
TBA purchases | $ 2,000 | $ 23,000 |
Restricted cash | 808,000 | $ 70,525,000 | $ 20,300,000
Collateral for Hedging Instruments |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Restricted cash | 100,000 |  | 9,900,000
Not Designated as Hedging Instrument | TBA securities |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
TBA purchases | $ 0 |  | $ 0

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