Securitized Debt - Narrative (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Debt Instrument [Line Items]
Gains (losses) on extinguishment of debt	$ (608,000)	$ 387,000	$ (608,000)	$ 10,057,000
Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure | Securitized debt
Debt Instrument [Line Items]
Principal balance of debt	$ 7,900,000,000		$ 7,900,000,000		$ 8,600,000,000
Weighted average cost of financing	4.60%		4.60%		4.60%
Repurchase of securitized debt borrowings, collateralized by Non-Agency RMBS		154,000,000		294,000,000
Acquired securitized debt collateral outstanding principal balance		155,000,000		304,000,000
Gains (losses) on extinguishment of debt		387,000		10,000,000
Securitized Loans | Non-agency MBS
Debt Instrument [Line Items]
Acquired securitized debt collateral outstanding principal balance	$ 2,900,000	$ 0	$ 3,500,000	$ 0
Gains (losses) on extinguishment of debt			608,000
Securitized Loans | Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure | Non-agency MBS
Debt Instrument [Line Items]
Principal balance of debt	$ 162,000,000		$ 162,000,000		$ 175,000,000
Weighted average cost of financing	6.50%		6.50%		6.40%