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
		Mar. 31, 2019	Mar. 31, 2018
Mortgage Loans On Real Estate [Line Items]
Beginning balance		$ 22,336	$ 14,883
Provision (benefit) for loan losses		(583)	8,032
Charge-offs, net of recoveries		(1,026)	(9,183)	[1]
Ending balance		20,727	13,732
Commercial Mortgage Loans
Mortgage Loans On Real Estate [Line Items]
Beginning balance		11,912	9,019
Provision (benefit) for loan losses		(485)	5,248
Charge-offs, net of recoveries		(1,026)	(6,008)	[1]
Ending balance		10,401	8,259
Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Beginning balance		6,496	5,622
Provision (benefit) for loan losses			2,784
Charge-offs, net of recoveries	[1]		(3,175)
Ending balance		6,496	5,231
Preferred Equity Interests
Mortgage Loans On Real Estate [Line Items]
Beginning balance		3,928	242
Provision (benefit) for loan losses		(98)
Ending balance		$ 3,830	$ 242

[1] Includes $3,908 of charge-offs related to loans transferred to held for sale during the three months ended March 31, 2018.