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			6 Months Ended
	Jun. 30, 2018		Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017
Mortgage Loans On Real Estate [Line Items]
Beginning balance	$ 13,732		$ 13,531	$ 14,883		$ 12,354
Provision (benefit) for loan losses	14,748		20,863	22,780		22,398
Charge-offs, net of recoveries	(11,781)	[1]	(10,880)	(20,964)	[2]	(11,238)
Ending balance	16,699		23,514	16,699		23,514
Commercial Mortgage Loans
Mortgage Loans On Real Estate [Line Items]
Beginning balance	8,259		10,979	9,019		10,640
Provision (benefit) for loan losses	14,748		12,623	19,996		13,322
Charge-offs, net of recoveries	(11,781)	[1]	(10,880)	(17,789)	[2]	(11,240)
Ending balance	11,226		12,722	11,226		12,722
Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Beginning balance	5,231		852	5,622
Provision (benefit) for loan losses			8,240	2,784		9,090
Charge-offs, net of recoveries				(3,175)	[2]	2
Ending balance	5,231		9,092	5,231		9,092
Preferred Equity Interests
Mortgage Loans On Real Estate [Line Items]
Beginning balance	242		1,700	242		1,714
Provision (benefit) for loan losses						(14)
Ending balance	$ 242		$ 1,700	$ 242		$ 1,700

[1] Includes $1,423 of charge-offs related to loans transferred to held for sale during the three months ended June 30, 2018.

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