Roll-Forwards of Allowance for Loan Losses for Commercial Mortgage Loans, Mezzanine Loans and Preferred Equity Interests Held for Investment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017
Mortgage Loans On Real Estate [Line Items]
Beginning balance	$ 16,699	$ 23,514	$ 14,883		$ 12,354
Provision (benefit) for loan losses	7,489	5,516	30,269		27,914
Charge-offs, net of recoveries	(571)	(3,890)	(21,535)	[1]	(15,128)
Ending balance	23,617	25,140	23,617		25,140
Commercial Mortgage Loans
Mortgage Loans On Real Estate [Line Items]
Beginning balance	11,226	12,722	9,019		10,640
Provision (benefit) for loan losses	5,506	3,368	25,502		16,690
Charge-offs, net of recoveries	(571)		(18,360)	[1]	(11,240)
Ending balance	16,161	16,090	16,161		16,090
Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Beginning balance	5,231	9,092	5,622
Provision (benefit) for loan losses	1,258	2,148	4,042		11,238
Charge-offs, net of recoveries		(3,890)	(3,175)	[1]	(3,888)
Ending balance	6,489	7,350	6,489		7,350
Preferred Equity Interests
Mortgage Loans On Real Estate [Line Items]
Beginning balance	242	1,700	242		1,714
Provision (benefit) for loan losses	725		725		(14)
Ending balance	$ 967	$ 1,700	$ 967		$ 1,700

[1] Includes $5,331 of charge-offs related to loans transferred to held for sale during the nine months ended September 30, 2018.