Fair Value Measurements - Carrying and Fair Values (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securitized debt, collateralized by Non-Agency RMBS	$ 303,389	$ 334,124
Carrying Amount | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Repurchase agreements	5,851,204	5,600,903
Carrying Amount | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securitized debt, collateralized by Non-Agency RMBS	303,389	334,124
Fair Value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Repurchase agreements	5,873,452	5,619,385
Fair Value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securitized debt, collateralized by Non-Agency RMBS	$ 292,285	$ 324,261