Mortgage-Backed and Credit Risk Transfer Securities - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	$ 72,596	$ 211,220
Agency RMBS, Agency CMBS and CMO
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	51,500	159,300
Agency IO, Non-Agency RMBS and Non-Agency CMBS
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	$ 21,100	$ 51,900