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Mortgage-Backed Securities - Accretable Yield (Detail) - Available-for-sale Securities - Non-Agency RMBS Having Deteriorated Credit When Acquired - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Accretable Yield Movement Schedule [Roll Forward]    
Balance at beginning of period $ 1,248,309 $ 1,303,589
Purchases 0 43,866
Yield income earned (228,208) (231,205)
Reclassification (to) from non-accretable difference 59,957 157,117
Sales and deconsolidation (26,378) (25,058)
Balance at end of period $ 1,053,680 $ 1,248,309