Mortgage-Backed Securities - Credit Ratings (Detail) - Non-agency MBS	Mar. 31, 2019	Dec. 31, 2018
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	1.00%	0.50%
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	0.30%	0.30%
BBB
Credit Derivatives [Line Items]
Percentage of RMBS portfolio at fair value, by credit rating	0.80%	0.40%
BB
Credit Derivatives [Line Items]
Percentage of RMBS portfolio at fair value, by credit rating	4.10%	3.60%
B
Credit Derivatives [Line Items]
Percentage of RMBS portfolio at fair value, by credit rating	1.40%	1.10%
Below B or not rated
Credit Derivatives [Line Items]
Percentage of RMBS portfolio at fair value, by credit rating	92.30%	94.00%