Securitized Debt - Narrative (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Gains (losses) on extinguishment of debt	$ 9,926,000	$ 0	$ (608,000)	$ 0	$ 7,055,000	$ 9,263,000	$ 387,000	$ 9,670,000	$ 9,318,000	$ 26,376,000	$ (35,274,000)
Securitized Loans | Non-Agency RMBS
Debt Instrument [Line Items]
Principal balance	2,900,000								2,900,000		166,000
Acquired securitized debt collateral outstanding principal balance					0
Payment to acquire securitize debt									3,500,000		167,000
Gains (losses) on extinguishment of debt									(608,000)		$ (1,000)
Securitized Loans | Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure | Non-Agency RMBS
Debt Instrument [Line Items]
Principal balance	$ 152,000,000				$ 175,000,000				$ 152,000,000	$ 175,000,000
Weighted average cost of financing	6.50%				6.40%				6.50%	6.40%
Securitized Loans | Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure | Securitized debt
Debt Instrument [Line Items]
Principal balance	$ 8,200,000,000				$ 8,600,000,000				$ 8,200,000,000	$ 8,600,000,000
Weighted average cost of financing	4.20%				4.50%				4.20%	4.50%
Acquired securitized debt collateral outstanding principal balance									$ 324,000,000	$ 861,000,000
Payment to acquire securitize debt									314,000,000	835,000,000
Gains (losses) on extinguishment of debt									$ 10,000,000	$ 26,000,000