Variable Interest Entities - 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, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Variable Interest Entity [Line Items]
Balance at beginning of period	[1]			$ 458,348
Deductions during period:
Provision for loan losses		$ (564)		(620)
Balance at end of period	[1]	367,539		367,539		$ 458,348
Residential Mortgage Backed Securities
Variable Interest Entity [Line Items]
Balance at beginning of period		141,645		152,922	$ 11,660	11,660
Loan acquisitions						168,850
Deductions during period:
Collections of principal		(10,129)		(20,178)		(30,992)
Provision for loan losses				(26)
Balance at end of period		131,110		131,110		152,922
Variable Interest Entities Primary Beneficiary | Residential Mortgage Backed Securities
Variable Interest Entity [Line Items]
Balance at beginning of period		416,571	$ 535,077	458,348	549,016	549,016
Deductions during period:
Collections of principal		(48,736)	(19,925)	(90,184)	(33,445)	(89,113)
Principal paydowns and other deductions		(316)	(403)	(653)	(833)	(1,566)
Provision for loan losses		(564)		(594)
Charge-offs, net		584		622	11	11
Balance at end of period		$ 367,539	$ 514,749	$ 367,539	$ 514,749	$ 458,348

[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.