Mortgage-Backed Securities - Credit Ratings (Detail) - Non-agency MBS	Mar. 31, 2018	Dec. 31, 2017
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	0.30%	0.30%
AA
Credit Derivatives [Line Items]
Percentage of RMBS portfolio at fair value, by credit rating	0.50%	0.30%
A
Credit Derivatives [Line Items]
Percentage of RMBS portfolio at fair value, by credit rating	0.50%	0.60%
BBB
Credit Derivatives [Line Items]
Percentage of RMBS portfolio at fair value, by credit rating	1.70%	1.90%
BB
Credit Derivatives [Line Items]
Percentage of RMBS portfolio at fair value, by credit rating	2.20%	2.50%
B
Credit Derivatives [Line Items]
Percentage of RMBS portfolio at fair value, by credit rating	2.30%	2.30%
Below B
Credit Derivatives [Line Items]
Percentage of RMBS portfolio at fair value, by credit rating	58.20%	59.60%
Not Rated
Credit Derivatives [Line Items]
Percentage of RMBS portfolio at fair value, by credit rating	34.30%	32.50%