Mortgage-Backed Securities - Credit Ratings (Detail) - Non-Agency RMBS	Mar. 31, 2020	Dec. 31, 2019
Credit Derivatives [Line Items]
Percentage of RMBS portfolio at fair value, by credit rating	100.00%	100.00%
AAA
Credit Derivatives [Line Items]
Percentage of RMBS portfolio at fair value, by credit rating	0.70%	0.40%
AA
Credit Derivatives [Line Items]
Percentage of RMBS portfolio at fair value, by credit rating	0.10%	0.10%
A
Credit Derivatives [Line Items]
Percentage of RMBS portfolio at fair value, by credit rating	1.00%	0.90%
BBB
Credit Derivatives [Line Items]
Percentage of RMBS portfolio at fair value, by credit rating	1.70%	1.60%
BB
Credit Derivatives [Line Items]
Percentage of RMBS portfolio at fair value, by credit rating	3.90%	3.80%
B
Credit Derivatives [Line Items]
Percentage of RMBS portfolio at fair value, by credit rating	1.50%	1.60%
Below B or not rated
Credit Derivatives [Line Items]
Percentage of RMBS portfolio at fair value, by credit rating	91.10%	91.60%