Fair Value Measurements - Carrying and Fair Values (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securitized debt, collateralized by Non-Agency RMBS	$ 133,557	$ 159,955
Carrying Amount | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Repurchase agreements	13,427,545	14,030,465
Carrying Amount | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securitized debt, collateralized by Non-Agency RMBS	133,557	159,955
Fair Value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Repurchase agreements	13,450,193	14,085,164
Fair Value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securitized debt, collateralized by Non-Agency RMBS	$ 117,552	$ 140,711