Roll-Forwards of Allowance for Loan Losses for Commercial Mortgage Loans, Mezzanine Loans and Preferred Equity Interests (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Mortgage Loans On Real Estate [Line Items]
Beginning balance	$ 17,097	$ 9,218
Provision for loan losses	1,325	2,000
Charge-offs, net of recoveries	(257)	(421)
Ending balance	18,165	10,797
Commercial mortgages
Mortgage Loans On Real Estate [Line Items]
Beginning balance	3,154
Provision for loan losses	174
Ending balance	3,328
Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Beginning balance	12,139	7,892
Provision for loan losses	158	2,000
Charge-offs, net of recoveries	(257)	(421)
Ending balance	12,040	9,471
Preferred Equity Interests
Mortgage Loans On Real Estate [Line Items]
Beginning balance	1,804	1,326
Provision for loan losses	993
Ending balance	$ 2,797	$ 1,326