Fair Value Measurements on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Liabilities:
Derivative instruments at fair value	$ 52,197	$ 15,901
Fair Value, Measurements, Recurring | Derivative Financial Instruments, Liabilities
Liabilities:
Derivative instruments at fair value	52,197	15,901
Fair Value, Measurements, Recurring | Derivative Financial Instruments, Assets
Assets:
Asset fair value measurement	5,833	46,207
Agency MBS | Fair Value, Measurements, Recurring
Assets:
Asset fair value measurement	3,510,051	3,548,719
Non-Agency MBS | Fair Value, Measurements, Recurring
Assets:
Asset fair value measurement	643,610	795,203
Level 2 | Fair Value, Measurements, Recurring | Derivative Financial Instruments, Liabilities
Liabilities:
Derivative instruments at fair value	52,197	15,901
Level 2 | Fair Value, Measurements, Recurring | Derivative Financial Instruments, Assets
Assets:
Asset fair value measurement	5,833	46,207
Level 2 | Agency MBS | Fair Value, Measurements, Recurring
Assets:
Asset fair value measurement	3,510,051	3,548,719
Level 2 | Non-Agency MBS | Fair Value, Measurements, Recurring
Assets:
Asset fair value measurement	$ 643,610	$ 795,203