Securitized Debt - Narrative (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Debt Instrument [Line Items]
Gains (losses) on extinguishment of debt	$ 459,000	$ (608,000)	$ 459,000	$ (608,000)
Securitized Loans | Non-Agency RMBS
Debt Instrument [Line Items]
Principal balance		2,900,000		2,900,000
Acquired securitized debt collateral outstanding principal balance		0	0	0
Repurchase of non retained secured debt		3,500,000		3,500,000
Gains (losses) on extinguishment of debt		$ 608,000		$ 608,000
Variable Interest Entities, Primary Beneficiary | Securitized Loans | Non-Agency RMBS
Debt Instrument [Line Items]
Principal balance	$ 143,000,000		$ 143,000,000		$ 152,000,000
Weighted average cost of financing	6.50%		6.50%		6.50%
Acquired securitized debt collateral outstanding principal balance	$ 147,000,000		$ 147,100,000
Repurchase of non retained secured debt			146,700,000
Gains (losses) on extinguishment of debt	459,000		459,000
Variable Interest Entities, Primary Beneficiary | Securitized Loans | Securitized debt
Debt Instrument [Line Items]
Principal balance	$ 8,700,000,000		$ 8,700,000,000		$ 8,200,000
Weighted average cost of financing	3.50%		3.50%		3.30%