Residential Mortgage Loans Held-For-Securitization - Summary of Reconciliation of Carrying Value of Residential Mortgage Loans (Detail) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2020	Dec. 31, 2019
Balance at beginning of period	[1]	$ 458,348
Deductions during period:
Balance at end of period	[1]	416,571	$ 458,348
Residential Mortgage Backed Securities
Balance at beginning of period		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		(10,049)	(30,992)
Amortization of premium and costs		(417)	(298)
Allowance for loan losses		(56)
Other		(755)
Balance at end of period		$ 141,645	$ 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 March 31, 2020 and December 31, 2019, total assets of the consolidated VIEs were $418 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 $409 million and $450 million (including accrued interest payable of $1.3 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.See accompanying notes to unaudited consolidated financial statements.