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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