Loans Held for Investment (Detail) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2019 USD ($) Loan		Dec. 31, 2018 USD ($) Loan		Mar. 31, 2018 USD ($)
Mortgage Loans On Real Estate [Line Items]
Unpaid Principal Balance						$ 90,260
Carrying Amount		$ 6,967
Commercial Portfolio Segment
Mortgage Loans On Real Estate [Line Items]
Unpaid Principal Balance		438,542	[1]	$ 502,974	[2]
Total investment in loans, Unamortized (Discounts) Premiums		99	[1]	97	[2]
Carrying Amount		$ 438,641	[1]	$ 503,071	[2]
Number of Loans | Loan		46	[1]	51	[2]
Weighted-Average Coupon	[3]	7.20%	[1]	7.20%	[2]
Deferred fees and costs, net		$ (397)	[4]	$ (674)	[5]
Total investments in loans, Unpaid Principal Balance		438,145		502,300
Commercial mortgages, mezzanine loans and other loans, Carrying Amount		438,244		502,397
Commercial mortgage loans | Commercial Portfolio Segment
Mortgage Loans On Real Estate [Line Items]
Unpaid Principal Balance		388,913		453,283
Total investment in loans, Unamortized (Discounts) Premiums		(64)		(66)
Carrying Amount		$ 388,849		$ 453,217
Number of Loans | Loan		30		35
Weighted-Average Coupon	[3]	6.90%		6.90%
Range of Maturity Dates, Minimum		Apr. 30, 2019		Feb. 28, 2019
Range of Maturity Dates, Maximum		Jun. 30, 2025		Jun. 30, 2025
Mezzanine Loans | Commercial Portfolio Segment
Mortgage Loans On Real Estate [Line Items]
Unpaid Principal Balance		$ 21,114		$ 21,114
Total investment in loans, Unamortized (Discounts) Premiums		164		164
Carrying Amount		$ 21,278		$ 21,278
Number of Loans | Loan		3		3
Weighted-Average Coupon	[3]	13.30%		13.30%
Range of Maturity Dates, Minimum		Jun. 30, 2020		Jun. 30, 2020
Range of Maturity Dates, Maximum		Mar. 31, 2023		Mar. 31, 2023
Preferred Equity Interests | Commercial Portfolio Segment
Mortgage Loans On Real Estate [Line Items]
Unpaid Principal Balance		$ 28,515		$ 28,577
Total investment in loans, Unamortized (Discounts) Premiums		(1)		(1)
Carrying Amount		$ 28,514		$ 28,576
Number of Loans | Loan		13		13
Weighted-Average Coupon	[3]	6.00%		6.00%
Range of Maturity Dates, Minimum		Mar. 31, 2023		Mar. 31, 2023
Range of Maturity Dates, Maximum		Jun. 30, 2029		Jun. 30, 2029

[1] Includes $53,128 of cash flow loans, of which $7,148 are commercial mortgage loans, $21,114 are mezzanine loans and $24,866 are preferred equity interests. See Note 2: Summary of Significant Accounting Policies, (j) Revenue Recognition, for further discussion of our cash flow loans.

[2] Includes $54,621 of cash flow loans, of which $8,579 are commercial mortgage loans, $21,114 are mezzanine loans and $24,928 are preferred equity interests. See Note 2: Summary of Significant Accounting Policies, (j) Revenue Recognition, for further discussion of our cash flow loans.

[3] Weighted-average coupon is calculated on the unpaid principal balance, which does not necessarily correspond to the carrying amount.

[4] Includes $1,703 of deferred fees, net of $1,306 of deferred costs.

[5] Includes $2,558 of deferred fees, net of $1,884 of deferred costs.