Carrying Amount and Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Liabilities
Junior subordinated notes	$ 5,500	$ 6,110
Carrying Amount
Assets
Total investment in mortgage loans, held for investment, net	417,517	480,061
Investment in mortgage loans, held for sale	5,131	4,873
Cash and cash equivalents	39,139	42,453
Restricted cash	65,000	63,067
Carrying Amount | CMBS securitizations
Liabilities
Notes payable	281,126	318,008
Carrying Amount | 7.625% Senior Notes due 2024
Liabilities
Indebtedness	55,158	55,100
Carrying Amount | 7.125% Senior Notes due 2019
Liabilities
Indebtedness	65,131	64,997
Carrying Amount | Junior subordinated notes, at amortized cost
Liabilities
Junior subordinated notes	25,100	25,100
Carrying Amount | CDO notes payable, at amortized cost
Liabilities
Notes payable	102,918	116,102
Carrying Amount | Loan Payable On Real Estate
Liabilities
Loans payable on real estate	40,330	40,499
Carrying Amount | Recourse Indebtedness | Junior subordinated notes, at fair value
Liabilities
Junior subordinated notes	5,500	6,110
Carrying Amount | Recourse Indebtedness | 7.625% Senior Notes due 2024
Liabilities
Indebtedness	55,158	55,100
Carrying Amount | Recourse Indebtedness | 7.125% Senior Notes due 2019
Liabilities
Indebtedness	65,131	64,997
Carrying Amount | Recourse Indebtedness | Junior subordinated notes, at amortized cost
Liabilities
Junior subordinated notes	25,100	25,100
Carrying Amount | Non Recourse Indebtedness | CMBS securitizations
Liabilities
Notes payable	281,126	318,008
Carrying Amount | Non Recourse Indebtedness | CDO notes payable, at amortized cost
Liabilities
Notes payable	102,918	116,102
Carrying Amount | Non Recourse Indebtedness | Loan Payable On Real Estate
Liabilities
Loans payable on real estate	40,330	40,499
Estimated Fair Value
Assets
Total investment in mortgage loans, held for investment, net	380,819	444,962
Investment in mortgage loans, held for sale	5,131	4,873
Cash and cash equivalents	39,139	42,453
Restricted cash	65,000	63,067
Estimated Fair Value | Recourse Indebtedness | Junior subordinated notes, at fair value
Liabilities
Junior subordinated notes	5,500	6,110
Estimated Fair Value | Recourse Indebtedness | 7.625% Senior Notes due 2024
Liabilities
Indebtedness	41,117	41,229
Estimated Fair Value | Recourse Indebtedness | 7.125% Senior Notes due 2019
Liabilities
Indebtedness	60,128	58,010
Estimated Fair Value | Recourse Indebtedness | Junior subordinated notes, at amortized cost
Liabilities
Junior subordinated notes	5,780	6,550
Estimated Fair Value | Non Recourse Indebtedness | CMBS securitizations
Liabilities
Notes payable	282,856	320,711
Estimated Fair Value | Non Recourse Indebtedness | CDO notes payable, at amortized cost
Liabilities
Notes payable	83,595	94,513
Estimated Fair Value | Non Recourse Indebtedness | Loan Payable On Real Estate
Liabilities
Loans payable on real estate	$ 40,698	$ 40,608