Variable Interest Entities - Summary of Assets and Liabilities of Variable Interest Entities (Detail) - USD ($) $ in Thousands		Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Variable Interest Entity [Line Items]
Residential mortgage loans held-for-investment through consolidated securitization trusts, net for allowance of credit losses of $147 at June 30, 2020 and $175 at December 31, 2019(1)	[1]	$ 367,539		$ 458,348
Accrued interest receivable		7,856		16,398
Total Assets		2,715,498		4,938,631
Accrued interest payable		5,187		16,757
Asset-backed securities issued by securitization trusts	[1]	358,854		448,987
Total Liabilities		2,315,251		4,366,679
Residential Mortgage Backed Securities
Variable Interest Entity [Line Items]
Residential mortgage loans held-for-investment through consolidated securitization trusts, net for allowance of credit losses of $147 at June 30, 2020 and $175 at December 31, 2019(1)		131,110	$ 141,645	152,922			$ 11,660
Variable Interest Entities Primary Beneficiary
Variable Interest Entity [Line Items]
Accrued interest receivable		1,300		1,500
Total Assets		369,000		460,000
Accrued interest payable		1,200		1,400
Asset-backed securities issued by securitization trusts		352,800		449,000
Total Liabilities		360,000		450,000
Variable Interest Entities Primary Beneficiary | Residential Mortgage Backed Securities
Variable Interest Entity [Line Items]
Residential mortgage loans held-for-investment through consolidated securitization trusts, net for allowance of credit losses of $147 at June 30, 2020 and $175 at December 31, 2019(1)		367,539	$ 416,571	458,348	$ 514,749	$ 535,077	$ 549,016
Accrued interest receivable		1,259		1,495
Total Assets		368,798		459,843
Accrued interest payable		1,214		1,448
Asset-backed securities issued by securitization trusts		358,854		448,987
Total Liabilities		$ 360,068		$ 450,435

[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.