Residential Mortgage Loans Held-For-Securitization - Summary of Reconciliation of Carrying Value of Residential Mortgage Loans (Detail) - USD ($) $ in Thousands		3 Months Ended	6 Months Ended	12 Months Ended
		Jun. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Mar. 31, 2020
Balance at beginning of period	[1]		$ 458,348
Deductions during period:
Balance at end of period	[1]	$ 367,539	367,539	$ 458,348
Residential Mortgage Backed Securities
Balance at beginning of period		141,645	152,922	11,660
Additions during period:
Loan acquisitions				168,850
Premium and deferred transaction costs on new loans				3,702
Deductions during period:
Collections of principal		(10,129)	(20,178)	(30,992)
Amortization of premium and costs		(406)	(823)	(298)
Allowance for loan losses		(56)	(56)		$ (56)
Other			(755)
Balance at end of period		$ 131,110	$ 131,110	$ 152,922

[1] The consolidated balance sheets include assets of consolidated variable interest entities, or VIEs, that can only be used to settle obligations and liabilities of the VIEs for which creditors do not have recourse to the Company. At June 30, 2020 and December 31, 2019, total assets of the consolidated VIEs were $369 million and $460 million (including accrued interest receivable of $1.3 million and $1.5 million), respectively (which is recorded above in the line item entitled “Interest receivable”), and total liabilities were $360 million and $450 million (including accrued interest payable of $1.2 million and $1.4 million), respectively (which is recorded in the line item above entitled “Accrued interest payable”). Please refer to Note 5, “Variable Interest Entities,” for further discussion.