Securitized Debt - Narrative (Detail) - Securitized Loans - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Non-Agency RMBS
Debt Instrument [Line Items]
Acquired securitized debt collateral outstanding principal balance	$ 0	$ 0
Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure | Non-Agency RMBS
Debt Instrument [Line Items]
Principal balance	$ 147,000,000		$ 152,000,000
Weighted average cost of financing	6.50%		6.50%
Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure | Securitized debt
Debt Instrument [Line Items]
Principal balance	$ 8,500,000,000		$ 8,200,000
Weighted average cost of financing	4.00%		3.30%