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Mortgage-Backed Securities - Investment Holdings - Narrative (Detail)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
USD ($)
Jun. 30, 2019
USD ($)
Dec. 31, 2018
USD ($)
Agency RMBS - Pass-through      
Investment Holdings [Line Items]      
Gross unrealized loss $ 7,000 $ 7,000 $ 95,000
Non-agency MBS      
Investment Holdings [Line Items]      
Gross unrealized loss $ 505 $ 505 $ 601
Non-Agency RMBS Deemed To Be Equivalent To Alt-A Quality      
Investment Holdings [Line Items]      
Percentage in Alt-A collateral 73.00% 73.00% 71.00%
Non Agency Residential MBS Deemed To Be Equivalent To Prime Quality      
Investment Holdings [Line Items]      
Percentage in prime collateral 9.00% 9.00% 12.00%
Re Remic Security      
Investment Holdings [Line Items]      
Fair Value Of Securities Exchanged During Period $ 22,000 $ 22,000  
Gain (Loss) on Sale of Investments $ 2,000 $ 2,000  
Minimum | Alt-A      
Investment Holdings [Line Items]      
FICO Score   680  
Maximum | Alt-A      
Investment Holdings [Line Items]      
FICO Score   720  
Alt-A securities underlying collateral of full documented loans   30.00%