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Mortgage-Backed Securities - Investment Holdings - Narrative (Detail)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Interest-only      
Investment Holdings [Line Items]      
Interest-only RMBS, fair value $ 323,000 $ 410,000  
Interest-only RMBS, amortized cost 382,000 458,000  
Interest-only RMBS, net unrealized gain (loss) (59,000) (48,000)  
Agency RMBS - Pass-through      
Investment Holdings [Line Items]      
Gross unrealized loss 41,000 100,000  
Non-agency MBS      
Investment Holdings [Line Items]      
Gross unrealized loss $ 283 $ 485  
Non-Agency RMBS Deemed To Be Equivalent To Alt-A Quality      
Investment Holdings [Line Items]      
Percentage in Alt-A collateral 66.00% 68.00%  
Non Agency Residential MBS Deemed To Be Equivalent To Prime Quality      
Investment Holdings [Line Items]      
Percentage in prime collateral 13.00% 14.00%  
Re-REMIC Security      
Investment Holdings [Line Items]      
Fair value of securities exchanged $ 63,000 $ 65,000 $ 95,000
Gain (loss) on sale of investments $ 9,000 $ 13,000 $ 24,000
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%