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Mortgage-Backed Securities - Investment Holdings - Narrative (Detail)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
USD ($)
Jun. 30, 2016
USD ($)
Jun. 30, 2017
USD ($)
Jun. 30, 2016
USD ($)
Dec. 31, 2016
USD ($)
Interest-Only RMBS          
Investment Holdings [Line Items]          
Interest-only RMBS, net unrealized gain (loss) $ (65,000)   $ (65,000)   $ (48,000)
Interest-only RMBS, amortized cost 420,000   420,000   458,000
Interest-only RMBS, fair value 354,000   354,000   410,000
Residential Mortgage-Backed Securities          
Investment Holdings [Line Items]          
Gross unrealized loss 141,361   141,361   169,675
Interest-only RMBS, net unrealized gain (loss) 716,388   716,388   672,292
Interest-only RMBS, amortized cost 6,390,109   6,390,109   6,825,525
Agency RMBS - Pass-through          
Investment Holdings [Line Items]          
Gross unrealized loss 59,000   59,000   100,000
Non-agency MBS          
Investment Holdings [Line Items]          
Gross unrealized loss $ 2,000   $ 2,000   $ 1,000
Non-Agency RMBS Deemed To Be Equivalent To Alt-A Quality          
Investment Holdings [Line Items]          
Percentage in Alt-A collateral 68.00%   68.00%   68.00%
Non Agency Residential MBS Deemed To Be Equivalent To Prime Quality          
Investment Holdings [Line Items]          
Percentage in prime collateral 11.00%   11.00%   14.00%
Re-REMIC Security          
Investment Holdings [Line Items]          
Fair value of securities exchanged $ 8,000 $ 17,000 $ 28,000 $ 17,000  
Gain (loss) on sale of investments $ 256 $ (259) $ 5,000 $ (259)  
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%